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# Chapter 3: Soils And Foundations
3.1
General
The Soils and Foundations Chapter of the Code is divided into the following three
distinct Divisions:
Division A:
Site Investigations, Soil Classifications, Materials and Foundation
Types
Division B:
Service Load Design Method of Foundations
Division C:
Additional Considerations in Planning, Design and Construction of
Building Foundations
Division A (Site Investigations, Soil Classifications, Materials and Foundation
Types) consists of the following Sections:
Site Investigations
Identification, Classification and Description of Soils
Materials
Types of Foundation
Division B (Service Load Design Method of Foundations) has the sections as under:
Shallow Foundations
Geotechnical Design of Shallow Foundations
Geotechnical Design of Deep Foundations
Field Tests for Driven Piles and Drilled Shafts
Division C (Additional Considerations in Planning, Design and Construction of
Building Foundations) deals with the following sections:
Excavation
Dewatering
Slope Stability of Adjoining Building
Fills
Retaining Walls for Foundations
Waterproofing and Damp-proofing
Foundation on Slopes
Foundations on Fill and Problematic Soils
Foundation Design for Dynamic Forces
Geo-hazards for Buildings
3.2
Scope
The provisions of this Chapter shall be applicable to the design and construction of
foundations of buildings and structures for the safe support of dead and
superimposed loads without exceeding the allowable bearing stresses, permissible
settlements and design capability. Because of uncertainties and randomness involved
in sub-soil characteristics, Geotechnical Engineering requires a high degree of
engineering judgment. As such the Code provisions of this Chapter provided here
under, are kept elaborative for better understanding of the readers. Provisions that are
ংঃধঃবফ রহ রসঢ়বৎধঃরাব ভড়ৎস ঁংরহম ুংযধষষচ্ ধৎব সধহফধঃড়ৎু. ঙঃযবৎ ঢ়ৎড়ারংরড়হং ড়ভ:যরং
Chapter should be followed using sound Geotechnical Engineering judgment.
3.3
Definitions and Symbols
3.3.1
Definitions
For the terms used in this Chapter, the following definitions shall apply.
ALLOWABLE
BEARING CAPACITY
It is the minimum of the safe bearing capacity and
safe settlement pressure, so that the foundation/
structure is safe and stable under both shear failure
and settlement criteria. It may be denoted by
symbol X'ÐÐ>. The lateral dimensions of the
foundation (width or diameter and the length) are
designed on the basis of allowable bearing capacity.
Also known as Allowable Bearing Pressure.
ALLOWABLE LOAD
The maximum load that may be safely applied to a
foundation unit, considering both the strength and
settlement of the soil, under expected loading and
soil conditions.
ANGULAR
DISTORTION
Angle between the horizontal and any two
foundations or two points in a single foundation.
AUGUR PILE
Same as SCREW PILE.
BATTER PILE
The pile which is installed at an angle to the vertical
in order to carry lateral loads along with the vertical
loads. This is also known as RAKER PILE.
BEARING CAPACITY
The general term used to describe the load carrying
capacity of foundation soil or rock in terms of
average pressure that enables it to bear and transmit
loads from a structure.
BEARING SURFACE
The contact surface between a foundation unit and
the soil or rock upon which the foundation rests.
BORED PILE
A pile formed into a preformed hole of ground,
usually of reinforced concrete having a diameter
smaller than 600 mm.
BOULDER
Particles of rock that will not pass a 12 inch. (300
mm) square opening.
CAISSON
A deep foundation unit, relatively large section, sunk
down (not driven) to the ground. This is also called
WELL FOUNDATION.
CAST IN-SITU PILE
Same as BORED PILE.
CLAY
A
natural
aggregate
of
microscopic
and
submicroscopic mineral grains less than 0.002 mm in
size and plastic in moderate to wide range of water
contents.
CLAY MINERAL
A small group of minerals, commonly known as clay
minerals,
essentially
composed
of
hydrous
aluminium silicates with magnesium or iron
replacing wholly or in part some of the aluminium.
CLAY SOIL
Same as CLAY.
COBBLE
Particles of rock that will pass a 12-in. (300-mm)
square opening and be retained on a 3-in. (75-mm)
sieve.
COLLAPSIBLE SOIL
Consists predominant of sand and silt size particles
arranged in a loose honeycomb structure. These soils
are dry and strong in their natural state and
consolidate or collapse quickly if they become wet.
CONSOLIDATION
SETTLEMENT
A time dependent settlement resulting from gradual
reduction of volume of saturated soils because of
squeezing out of water from the pores due to
increase in effective stress and hence pore water
pressure. It is also known as primary consolidation
settlement. It is thus a time related process involving
compression, stress transfer and water drainage.
DEEP FOUNDATION
A foundation unit that provides support for a
structure transferring loads by end bearing and/or by
shaft resistance at considerable depth below the
ground. Generally, the depth is at least five times the
least dimension of the foundation.
DESIGN BEARING
CAPACITY
The maximum net average pressure applied to a soil
or rock by a foundation unit that the foundation soil
or rock will safely carry without the risk of both
shear failure and exceedance of permissible
settlement. It is equal to the least of the two values of
net allowable bearing capacity and safe bearing
pressure. This may also be called ALLOWABLE
BEARING PRESSURE.
DESIGN LOAD
The expected un-factored load to a foundation unit.
DIFFERENTIAL
SETTEMENT
The difference in the total settlements between two
foundations or two points in the same foundation.
DISPERSIVE SOIL
Soils that are structurally unstable and disperse in
water into basic particles i.e. sand, silt and clay.
Dispersible soils tend to be highly erodible.
Dispersive soils usually have a high Exchangeable
Sodium Percentage (ESP).
DISPLACEMENT PILE
Same as DRIVEN PILE.
DISTORTION
SETTLEMENT
Same as ELASTIC SETTLEMENT.
DOWNDRAG
The transfer of load (drag load) to a deep foundation,
when soil settles in relation to the foundation. This is
also known as NEGATIVE SKIN FRICTION.
DRILLED PIER
A deep foundation generally of large diameter shaft
usually more than 600 mm and constructed by
drilling and excavating into the soil.
DRILLED SHAFT
Same as DRILLED PIER.
DRIVEN PILE
A pile foundation pre-manufactured and placed in
ground by driving, jacking, jetting or screwing.
EFFECTIVE STRESS
The pressure transmitted through grain to grain at the
contact point through a soil mass is termed as
effective stress or effective pressure.
ELASTIC SETTLEMENT
It is attributed due to lateral spreading or elastic
deformation of dry, moist or saturated soil without a
change in the water content and volume.
END BEARING
The load being transmitted to the toe of a deep
foundation and resisted by the bearing capacity of
the soil beneath the toe.
EXCAVATION
The space created by the removal of soil or rock for
the purpose of construction.
EXPANSIVE SOIL
These are clay soils expand when they become
wetted and contract when dried. These are formed of
clay minerals like montmorillonite and illite.
FACTOR OF SAFETY
The ratio of ultimate capacity to design (working)
capacity of the foundation unit.
FILL
Man-made deposits of natural earth materials (soil,
rock) and/or waste materials.
FOOTING
A foundation constructed of masonry, concrete or
other material under the base of a wall or one or
more columns for the purpose of spreading the load
over a larger area at shallower depth of ground
surface.
FOUNDATION
Lower part of the structure which is in direct contact
with the soil and transmits loads to the ground.
FOUNDATION
ENGINEER
A graduate Engineer with at least five years of
experience in civil engineering particularly in
foundation design or construction.
GEOTECHNICAL
ENGINEER
ঊহমরহববৎ রিঃয গধংঃবৎ্থং ফবমৎবব রহ মবড়ঃবপযহরপধষ
engineering having at least 2 (two) years of
experience in geotechnical design/construction or
graduate in civil engineering/engineering geology
having 10 (ten) years of experience in geotechnical
design/construction.
GRAVEL
Particles of rock that will pass a 3-in. (75-mm) sieve
and be retained on a No. 4 (4.75-mm) sieve.
GROSS PRESSURE
The total pressure at the base of a footing due to the
weight of the superstructure and the original
overburden pressure.
GROSS ALLOWABLE
BEARING PRESSURE
The maximum gross average pressure of loading that
the soil can safely carry with a factor of safety
considering risk of shear failure. This may be
calculated by dividing gross ultimate bearing
capacity with a factor of safety.
GROUND WATER
TABLE
The level of water at which porewater pressure is
equal to atmospheric pressure. It is the top surface of
a free body of water (piezometric water level) in the
ground.
IMMEDIATE
SETTLEMENT
This vertical compression occurs immediately after
the application of loading either on account of elastic
behaviour that produces distortion at constant
volume and on account of compression of air void.
For sands, even the consolidation component is
immediate.
INORGANIC SOIL
Soil of mineral origin having small amount usually
less than 5 percent of organic matter content.
LATERALLY LOADED
PILE
A pile that is installed vertically to carry mainly the
lateral loads.
MAT FOUNDATION
See RAFT.
NEGATIVE SKIN
FRICTION
See DOWNDRAG.
NET PRESSURE
The gross pressure minus the surcharge pressure i.e.
the overburden pressure of the soil at the foundation
level.
NET SAFE BEARING
CAPACITY
The maximum net pressure that can be safely applied
from the foundation on the soil at its base, and at
which the shear failure of the soil is avoided with a
suitable factor of safety (\&C). It is denoted by
symbol X). Thus, X) 5
>
> ? .
NET ULTIMATE
BEARING CAPACITY
The minimum net pressure at the base of the
foundation, excluding the weight of the overburden,
at which the soil fails in shear due to the load on the
foundation from superstructure. It is denoted by the
symbol X)D. Thus, X)D 5 XDÐ" −X′ where, X′ is the
effective stress at foundation level due to overburden
soil.
ORGANIC SOIL
Soil having appreciable/significant amount of
organic matter content to influence the soil
properties.
OVERCONSOLIDATION
RATIO (OCR)
The ratio of the preconsolidation pressure (maximum
past pressure) to the existing effective overburden
pressure of the soil.
PEAT SOIL
An organic soil with high organic content, usually
more than 75% by weight, composed primarily of
vegetable tissue in various stages of decomposition
usually with an organic odor, a dark brown to black
color, a spongy consistency, and a texture ranging
from fibrous to amorphous. Fully decomposed
organic soils are known as MUCK.
PILE
A slender deep foundation unit made of materials
such as steel, concrete, wood, or combination thereof
that transmits the load to the ground by skin friction,
end bearing and lateral soil resistance.
PILE CAP
A pile cap is a special footing needed to transmit the
column load to a group or cluster of piles.
PILE HEAD
The upper small length of a pile. Also known as pile
top.
PILE SHOE
A separate reinforcement or steel form attached to
the bottom end (pile toe) of a pile to facilitate
driving, to protect the pile toe, and/or to improve the
toe resistance of the pile.
PILE TOE
The bottom end of a pile. Also known as pile tip.
PORE WATER
PRESSURE
The pressure induced in the water or vapour and
water filling the pores of soil. This is also known as
neutral stress.
PRESUMPTIVE
BEARING CAPACITY
The net
approximate
pressure
prescribed
as
appropriate for the particular type of ground to be
used in preliminary designs of foundations
RAFT
A relatively large spread foundation supporting an
arrangement of columns or walls in a regular or
irregular layout transmitting the loads to the soil by
means of a continuous slab and/or beams, with or
without depressions or openings. This is also known
as MAT FOUNDATION.
RAKER PILE
See BATTER PILE.
ROCK
A natural aggregate of one or more minerals that are
connected by strong and permanent cohesive forces.
ROTATION
It is the angle between the horizontal and any two
foundations or two points in a single foundation.
RELATIVE ROTATION
Same as ANGULAR DISTORTION
REPLACEMENT PILE
Same as BORED PILE.
SAFE BEARING
CAPACITY
It is the maximum gross pressure that can carry
safely, without shear failure. It is denoted by
symbol X'\<. Thus, X'\< 5 X) + X′. When the
excavation for foundation is backfilled, X'\< 5 X).
SAFE SETTLEMENT
PRESSURE
The maximum pressure that can be applied from the
foundation on the soil at its base such that the
settlement of the foundation/structure is less than or
equal to the permissible settlement. It may be
denoted by symbol X!.
SAND
Aggregates of rounded, sub-rounded, angular, sub-
angular or flat fragments of more or less unaltered
rock or minerals which is larger than 75 μm and
smaller than 4.75 mm in size.
SCREW PILE
A pre-manufactured pile consisting of steel helical
blades and a shaft placed into ground by screwing.
SECONDARY
CONSOLDATION
SETTLEMENT
This is the settlement speculated to be due to the
plastic deformation of the soil as a result of some
complex colloidal-chemical processes or creep under
imposed long term loading.
SERVICE LOAD
The expected un-factored load to a foundation unit.
SETTLEMENT
The downward vertical movement of foundation
under load. When settlement occurs over a large
area, it is sometimes called subsidence.
SHAFT RESISTANCE
The resistance mobilized on the shaft (side) of a
deep foundation. Upward resistance is called
positive shaft resistance. Downward force on the
shaft is called negative shaft resistance.
SHALLOW
FOUNDATION
A foundation unit that provides support for a
structure transferring loads at a small depth below
the ground. Generally, the depth is less than two
times the least dimension of the foundation.
SILT
Soil passing a No. 200 (75-μm) sieve either non-
plastic or plastic.
SOIL
A loose or soft deposit of particles of mineral and/or
organic origin that can be separated by such gentle
mechanical means as agitation in water.
SOIL PARTICLE SIZE
The sizes of particles that make up soil varying over
a wide range. Soil particles are generally gravel,
sand, silt and clay, though the terms boulder and
cobble can be used to describe larger sizes of gravel.
TILT
Rotation of the entire superstructure or at least a
well-defined part of it.
TOTAL SETTLEMENT
The total downward vertical displacement of a
foundation base under load from its as-constructed
position. It is the summation of immediate
settlement, consolidation settlement and secondary
consolidation settlement of the soil.
ULTIMATE BEARING
CAPACITY
The minimum gross pressure at the base of the
foundation at which the soil fails in shear due to the
load on the foundation from superstructure. It is
denoted by the symbol XDÐ" and obtained from
bearing capacity equation containing soil/ground
properties,
depth
of
foundation,
foundation
dimensions and shapes, and loading conditions. Also
known as Gross Ultimate Bearing Capacity.
3.3.2
Symbols and Notation
Every symbol used in this Chapter is explained where it first appears in the text.
However, for convenience of the reader, a list of main symbols and notation is
provided as under. Other common symbols and notation like those of soil
classifications are not included in this list.
\= Cross sectional area of pile
4
\= End bearing area of pile
\= Skin friction area (perimeter area) of pile
\= Width of footing/foundation (Sec 3.9.6, Sec 3.20.2)
\= Smallest dimension of pile group (Sec 3.10.5)
Bp
\= Width of plate
\= Reference width (300 mm) for computation of pile settlement
%
\= Cation exchange capacity
@@
\= Cyclic resistance ratio
C@
\= Cyclic stress ratio
\+
\= Compression index of soil
!
\= Empirical coefficient used for pile settlement computation
D
\= Uniformity coefficient
/
\= Coefficient of curvature
#
\= Diameter or width of pile
\#4
\= Diameter of pile at base
\#+
\= Critical depth of soil layer
\#1=
\= Effective grain size; the size of soil particle from which 10 percent
of the soil is finer
\#2=
\= The size of soil particle from which 30 percent of the soil is finer
\#º=
\= The size of soil particle from which 60 percent of the soil is finer
%.
\= Flexural rigidity of footing
%5;
\= Exchangeable magnesium percentage
Ep
\= Modulus of elasticity of pile material
%
\= Modulus of elasticity of soil
%C;
\= Exchangeable sodium percentage
\&B
\= Factor of safety against liquefaction
\&C
\= Factor of safety
,
\= Modulus of rigidity
----------------------
## = Height of wall from foundation footing (Sec 3.9.4)
## = Layer thickness (Sec 3.10.5)
\= Thickness of sample (Sec 3.5.6)
-′
\= Final thickness of sample (Sec 3.5.6)
.p
\= Plasticity index
.D4
\= Relative subsidence
\= Coefficient of earth pressure
0
\= Coefficient of earth pressure at rest
\= Length of pile (Sec 3.10)
\= Length of deflected part of wall/raft or centre to centre distance
between columns. (Sec 3.9.4)
Liquid limit
:
\= Standard penetration test value (SPT)
:º=
\= Corrected SPT value for field procedures
:\_º=
\= Average SPT :º= value
(:1)º=
\= Corrected SPT value for overburden pressure (for sandy soil)
Nc, Nq, Nγ = Bearing capacity factors
@ @
\= Overconsolidation ratio
;.
\= Plasticity index; same as .!
?'ÐÐ>
\= Allowable load
Qp
\= End bearing at the base or tip of the pile
Qp
\= Load transferred to the soil at pile tip level
?
\= Skin friction or shaft friction or side shear
?DÐ"
\= Ultimate bearing/load carrying capacity
@
\= Group settlement ratio of pile group
C'
\= Settlement due to axial deformation
Sg
\= Settlement of pile group
Spt
\= Settlement at pile tip
C
\= Settlement of pile due to skin friction
C7
\= Degree of saturation
C"( )Ð\<)
\= Total settlement of a single pile
I
\= Weight of the pile
I;.
\= Weighted plasticity index
L'
\= Peak horizontal acceleration on the ground surface
O
\= Apparent cohesion of soil
OD
\= Undrained cohesion of soil
dp
\= Diameter of pile
P
\= Void ratio
P+
\= Critical void ratio
PB
\= Void ratio at liquid limit
PA
\= Void ratio at plastic limit
P
\= Initial void ratio
P
\= Initial void ratio; same as P
B4
\= End bearing resistance on unit tip area of pile
B)
\= Natural frequency
B
\= Skin frictional resistance on unit surface area of pile
B
\= Adhesive stress (Sec. 3.10.1.12)
Q
\= Gravitational acceleration
m
\= Modulus of sub-grade reaction
kp
\= Stiffness of soil
m
\= Coefficient of horizontal soil stress
\= Total mass of machine foundation system
5
\= Mass of foundation block
5
\= Mass of soil
\*
\= Number of pile in a group
X'ÐÐ>
\= Allowable bearing capacity of shallow foundation
X
\= Ultimate end bearing capacity pile
X)
\= Net safe ultimate bearing capacity of shallow foundation
X)D
\= Net ultimate bearing capacity of shallow foundation
X'\<
\= Safe ultimate bearing capacity
qsp
\= Safe settlement pressure of shallow foundation
XD
\= Unconfined compressive strength
XDÐ"
\= Ultimate bearing capacity of shallow foundation
Y
\= Stress reduction coefficient to allow for the deformability of the
soil column
ZD
\= undrained shear strength; same as OD
\[B
\= Liquid limit; same as LL
\= Depth
∆4
\= Thickness of any ("3) layer
a
\= Adhesion factor
c
\= Ratio of footing length to width (Sec 3.9.6.8)
c
\= Friction factor due to overburden (3.10.1)
ð, ð"
\= Unit weight of the soil
ð>
\= Unit weight of water
\= Total settlement
+
\= Consolidation settlement
\<
\= Elastic settlement
\= Immediate settlement
\= Secondary consolidation settlement
C
\= চড়রংংড়হ্থং ৎধঃরড় ড়ভ ংড়রষ
σ′
\= Initial effective stress at mid-point of a soil layer
σ!′
\= Increase in effective stress at mid-point of a soil layer due to
increase in stress
σ7′
\= Reference stress (100 kPa) for computation of pile settlement
D¼
\= The total vertical stress
D¼′
\= Effective vertical stress
D/′
\= Effective vertical stress; same as D¼′
E'
\= Maximum shear stress
k
\= Apparent angle of internal fiction
k′
\= Effective/drained angle of internal fiction
k
\= Soil shaft interface friction angle
F)
\= natural circular frequency
Division A: Site Investigations, Soil Classifications, Materials and
Foundation Types (Sections 3.4 to 3.7)
3.4
Site Investigations
3.4.1
Sub-Surface Survey
Depending on the type of project thorough investigations has to be carried out for
identification, location, alignment and depth of various utilities, e.g., pipelines,
cables, sewerage lines, water mains etc. below the surface of existing ground level.
Detailed survey may also be conducted to ascertain the topography of existing
ground.
3.4.2
Sub-Soil Investigations
Sub soil investigation shall be done describing the character, nature, load bearing
capacity and settlement capacity of the soil before constructing a new building and
structure or for alteration of the foundation of an existing structure. The aims of a
geotechnical investigation are to establish the soil, rock and groundwater conditions,
to determine the properties of the soil and rock, and to gather additional relevant
knowledge about the site. Careful collection, recording and interpretation of
geotechnical information shall be made. This information shall include ground
conditions, geology, geomorphology, seismicity and hydrology, as relevant.
Indications of the variability of the ground shall be taken into account.
An engineering geological study may be an important consideration to establish the
physiographic setting and stratigraphic sequences of soil strata of the area.
Geological and agricultural soil maps of the area may give valuable information of
site conditions.
During the various phases of sub-soil investigations, e.g. drilling of boreholes, field
tests, sampling, groundwater measurements, etc. a competent graduate engineer
having experiences in supervising sub-soil exploration works shall be employed by
the drilling contractor.
3.4.3
Methods of Exploration
Sub soil exploration process may be grouped into three types of activities such as:
reconnaissance, exploration and detailed investigations. The reconnaissance method
includes geophysical measurements, sounding or probing, while exploratory methods
involve various drilling techniques. Field investigations should comprise :
(i)
Drilling and/or excavations (test pits including exploratory boreholes) for
sampling;
(ii) Groundwater measurements;
(iii) Field tests.
Examples of the various types of field investigations are:
(i)
Field testing (e.g. CPT, SPT, dynamic probing, WST, pressuremeter tests,
dilatometer tests, plate load tests, field vane tests and permeability tests);
(ii) Soil sampling for description of the soil and laboratory tests;
(iii) Groundwater measurements to determine the groundwater table or the
pore pressure profile and their fluctuations
(iv) Geophysical investigations (e.g. seismic profiling, ground penetrating
radar, resistivity measurements and down hole logging);
(v)
Large scale tests, for example to determine the bearing capacity or the
behaviour directly on prototype elements, such as anchors.
Where ground contamination or soil gas is expected, information shall be gathered
from the relevant sources. This information shall be taken into account when
planning the ground investigation. Some of the common methods of exploration,
sampling and ground water measurements in soils are described in Appendix D.
3.4.4
Number and Location of Investigation Points
The locations of investigation points, e.g., pits and boreholes shall be selected on the
basis of the preliminary investigations as a function of the geological conditions, the
dimensions of the structure and the engineering problems involved. When selecting
the locations of investigation points, the following should be observed:
(i)
The investigation points should be arranged in such a pattern that the
stratification can be assessed across the site;
(ii)
The investigation points for a building or structure should be placed at
critical points relative to the shape, structural behaviour and expected
load distribution (e.g. at the corners of the foundation area);
(iii)
For linear structures, investigation points should be arranged at adequate
offsets to the centre line, depending on the overall width of the structure,
such as an embankment footprint or a cutting;
(iv)
For structures on or near slopes and steps in the terrain (including
excavations), investigation points should also be arranged outside the
project area, these being located so that the stability of the slope or cut
can be assessed. Where anchorages are installed, due consideration
should be given to the likely stresses in their load transfer zone;
(v)
The investigation points should be arranged so that they do not present a
hazard to the structure, the construction work, or the surroundings (e.g.
as a result of the changes they may cause to the ground and groundwater
conditions);
(vi)
The area considered in the design investigations should extend into the
neighbouring area to a distance where no harmful influence on the
neighbouring area is expected. Where ground conditions are relatively
uniform or the ground is known to have sufficient strength and stiffness
properties, wider spacing or fewer investigation points may be applied.
In either case, this choice should be justified by local experience.
(vii) The locations and spacing of sounding, pits and boreholes shall be such
that the soil profiles obtained will permit a reasonably accurate estimate
of the extent and character of the intervening soil or rock masses and
will disclose important irregularities in subsurface conditions.
(viii) For building structures, the following guidelines shall be followed:
On uniform soils, at least three borings, not in one line, should be made
for small buildings and at least five borings one at each corner and one
at the middle should be made for large buildings. As far as possible the
boreholes should be drilled closed to the proposed foundations but
outside their outlines.
Spacing of exploration depends upon nature and condition of soil, nature
and size of the project. In uniform soil, spacing of exploration (boring)
may be 30 m to 100 m apart or more and in very erratic soil conditions,
spacing of 10 m or less may be required. The following chart gives
an approximate idea about spacing of boring required for small
and multistoried buildings having different horizontal stratification of
soil.
Type of
Building
Spacing of Bore Holes (m)
Type of Soil in Horizontal Stratification
Uniform
Average
Erratic
Small buildings
Multistoried
buildings
| soil. | Col2 | Col3 | Col4 |
| ------------------------------------------------ | --------------------------------------------- | --------------------------------------------- | --------------------------------------------- |
| **Type of**
**Building** | **Spacing of Bore Holes (m)** | **Spacing of Bore Holes (m)** | **Spacing of Bore Holes (m)** |
| **Type of**
**Building** | **Type of Soil in Horizontal Stratification** | **Type of Soil in Horizontal Stratification** | **Type of Soil in Horizontal Stratification** |
| **Type of**
**Building** | **Uniform** | **Average** | **Erratic** |
| Small buildings
Multistoried
buildings | 60
45 | 30
30 | 15
15 |
(ix)
For large areas covering industrial and residential colonies, the
geological nature of the terrain will help in deciding the number of
boreholes or trial pits. The whole area may be divided into grid pattern
with Cone Penetration Tests (Appendix D) performed at every 100 m
grid points. The number of boreholes or trial pits shall be decided by
examining the variation in penetration curves. At least 67% of the
required number of borings or trial pits shall be located within the area
under the building.
3.4.5
Depth of Exploration
The depth of investigations shall be extended to all strata that will affect the project
or are affected by the construction. The depth of exploration shall depend to some
extent on the site and type of the proposed structure, and on certain design
considerations such as safety against foundation failure, excessive settlement,
seepage and earth pressure. Cognizance shall be taken of the character and sequence
of the subsurface strata. The site investigation should be carried to such a depth that
the entire zone of soil or rock affected by the changes caused by the building or the
construction will be adequately explored. A rule of thumb used for this purpose is to
extend the borings to a depth where the additional load resulting from the proposed
building is less than 10% of the average load of the structure, or less than 5% of the
effective stress in the soil at that depth. Where the depth of investigation cannot be
related to background information, the following guide lines are suggested to
determine the depth of exploration:
(i)
Where substructure units will be supported on spread footings, the
minimum depth boring should extend below the anticipated bearing
level a minimum of two footing widths for isolated, individual footings
where length 2 times of width, and four footing widths for footings
where length 5 times of width. For intermediate footing lengths, the
minimum depth of boring may be estimated by linear interpolation as a
function of length between depths of two times width and five times
width below the bearing level. Greater depth may be required where
warranted by local conditions.
(ii)
For more heavily loaded structures, such as multistoried structures and
for framed structures, at least 50% of the borings should be extended to
a depth equal to 1.5 times the width of the building below the lowest
part of the foundation.
(iii)
Normally the depth of exploration shall be 1.5 times the estimated width
or the least dimension of the footing below the foundation level. If the
pressure bulbs for a number of loaded areas overlap, the whole area may
be considered as loaded and exploration shall be carried down to one
and a half times the least dimension. In weak soils, the exploration shall
be continued to a depth at which the loads can be carried by the stratum
in question without undesirable settlement or shear failure.
(iv)
Where substructure units will be supported on deep foundations, the
depth boring should extend a minimum of 6 m below the anticipated
pile of shaft tip elevation. Where pile or shaft groups will be used, the
boring should extend at least two times the maximum pile or shaft group
dimension below the anticipated tip elevation, unless the foundation will
be end bearing on or in rock.
(v)
For piles bearing on rock, a minimum of 1.5 m of rock core should be
obtained at each boring location to ensure the boring has not been
terminated in a boulder.
(vi)
For shafts supported on or extending into rock, a minimum of 1.5 m of
rock core, or a length of rock core equal to at least three times the shaft
diameter for isolated shafts or two times the maximum shaft group
dimension for a shaft group, whichever is greater, should be obtained to
ensure that the boring had not been terminated in a boulder and to
determine the physical properties of rock within the zone of foundation
influence for design.
(vii)
The depth, to which weathering process affects the deposit, shall be
regarded as the minimum depth of exploration for a site. However, in no
case shall this depth be less than 2 m, but where industrial processes
affect the soil characteristics, this depth may be more.
(viii) At least one boring should be carried out to bedrock, or to well below
the anticipated level of influence of the building. Bedrock should be
ascertained by coring into it to a minimum depth of 3 m.
3.4.6
Sounding and Penetration Tests
Subsurface soundings are used for exploring soil strata of an erratic nature. They are
useful to determine the presence of any soft pockets between drill holes and also to
determine the density index of cohesionless soils and the consistency of cohesive
soils at desired depths. A field test called Vane Shear Test may be used to determine
the shearing strength of the soil located at a depth below the ground.
Penetration tests consist of driving or pushing a standard sampling tube or a cone.
The devices are also termed as penetrometers, since they penetrate the subsoil with a
view to measuring the resistance to penetrate the soil strata. If a sampling tube is
used to penetrate the soil, the test is referred to as Standard Penetration Test (or
simply SPT). If a cone is used, the test is called a Cone Penetration Test. If the
penetrometer is pushed steadily into the soil, the procedure is known as Static
Penetration Test. If driven into the soil, it is known as Dynamic Penetration Test.
Details of sounding and penetrations tests are presented in Appendix D.
3.4.7
Geotechnical Investigation Report
The results of a geotechnical investigation shall be compiled in the Geotechnical
Investigation Report which shall form a part of the Geotechnical Design Report. The
Geotechnical Investigation Report shall consist of the following :
(i)
A presentation of all appropriate geotechnical information on field and
laboratory tests including geological features and relevant data;
(ii) A geotechnical evaluation of the information, stating the assumptions
made in the interpretation of the test results.
The Geotechnical Investigation Report shall state known limitations of the results, if
appropriate. The Geotechnical Investigation Report should propose necessary further
field and laboratory investigations, with comments justifying the need for this further
work. Such proposals should be accompanied by a detailed programme for the further
investigations to be carried out. The presentation of geotechnical information shall
include a factual account of all field and laboratory investigations. The factual
account should include the following information :
(i)
The purpose and scope of the geotechnical investigation including a
description of the site and its topography, of the planned structure and the
stage of the planning the account is referring to;
(ii) The names of all consultants and contractors;
(iii) The dates between which field and laboratory investigations were
performed;
(iv) The field reconnaissance of the site of the project and the surrounding area
noting particularly :
evidence of groundwater;
behaviour of neighbouring structures;
exposures in quarries and borrow areas;
areas of instability;
difficulties during excavation;
history of the site;
geology of the site,
survey data with plans showing the structure and the location of all
investigation points;
local experience in the area;
information on the seismicity of the area.
The presentation of geotechnical information shall also include documentation of the
methods, procedures and results including all relevant reports of :
(i)
desk studies;
(ii) field investigations, such as sampling, field tests, groundwater
measurements and technical specifications of field equipment used
(iii) laboratory tests and test standard followed
The results of the field and laboratory investigations shall be presented and reported
according to the requirements defined in the ASTM or equivalent standards applied in
the investigations.
3.5
Identification, Classification and Description af Soils
3.5.1
Identification of Soils
Samples and trial pits should be inspected visually and compared with field logs of
the drillings so that the preliminary ground profile can be established. For soil
samples, the visual inspection should be supported by simple manual tests to identify
the soil and to give a first impression of its consistency and mechanical behaviour. A
standard visual-manual procedure of describing and identifying soils may be
followed.
Soil classification tests should be performed to determine the composition and index
properties of each stratum. The samples for the classification tests should be selected
in such a way that the tests are approximately equally distributed over the complete
area and the full depth of the strata relevant for design.
3.5.2
Particle Size Classification of Soils
Depending on particle sizes, main soil types are gravel, sand, silt and clay. However,
the larger gravels can be further classified as cobble and boulder. The soil particle
size shall be classified in accordance with Table 6.3.1.
Table 6.3.1: Particle Size Ranges of Soils
Soil Type
Particle Size
Range (mm)
Retained on Mesh
Size/ Sieve No.
Boulder
>
12″
Cobble
300 –
3″
Gravel:
Coarse Gravel
75 –
3/4″
Medium Gravel
19 –
9.5
3/8″
Fine Gravel
9.5 –
4.75
No. 4
Sand:
Coarse Sand
4.75 –
2.00
No. 10
Medium Sand
2.00 –
0.425
No. 40
Fine Sand
0.425 –
0.075
No. 200
Silt
0.075 –
0.002
-----
Clay
\<
0.002
-----
3.5.3
Engineering Classification of Soils
Soils are divided into three major groups, coarse grained, fine grained and organic.
The classification is based on classification test results namely grain size analysis and
consistency test. The coarse grained soils shall be classified using Table 6.3.2.
Outlines of organic and inorganic soil separations are also provided in Table 6.3.2.
The fine grained soils shall be classified using the plasticity chart shown in Figure
6.3.1. In this context, this Code adopts the provisions of ASTM D2487. In addition to
these classifications, a soil shall be described by its colour, particle angularity (for
coarse grained soils) and consistency. Further to the above classification soils
exhibiting swelling or collapsing characteristic shall be recorded. For undisturbed
soils information on stratification, compactness, cementation, moisture conditions
and drainage characteristics shall be included.
Table 6.3.2: Engineering Classification of Soils (Criteria for Assigning Group
Symbols and Names using Laboratory Tests A)
Classification (For particles smaller
than 75 mm and based on estimated
weights)
Group
Symbol
Group Name B
Laboratory Classification
Percent
finer than
### 0.075 mm
Other Criteria
Coarse
grained soils
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm)
Gravels
(More
than
50%of
coarse
fraction
retained
on No. 4
sieve
(4.75 mm)
Clean
gravels
GW
Well graded gravels,
sandy gravels, sand
gravel mixture, little or
no fines.D
\< 5 E
Cu 4 and
1 ≤ Cz ≤ 3 C
GP
Poorly graded gravels,
sandy gravels, Sand
gravel mixture, little or
no fines. D
Cu \< 4 and/or
1> Cz> 3 C
Gravel with
fines
GM
Silty gravels, silty sandy
gravels. D, F, G
> 12 E
> IP\< 4 or the
> limit values
> below 'A'
> line of
> plasticity
> chart
> For 4> IP >
> 7 and limit
> values
> above
> 'A' line,
> dual symbol
> required\*
> GC
> Clayey gravels, silty
> clayey gravels. . D, F, G
> IP >7 and the
> limit values
> above 'A' line
> of Plasticity
> Chart
> Sands
> (over
> 50% of
> coarse
> fraction
> smaller
> than
> 4.75
> mm)
> Clean Sands
> SW
> Well graded sand,
> gravelly sand, little or no
> fines. H
> \< 5 E
> Cu ≥ 6 and
> 1≤ Cz ≤ 3 C
> SP
> Poorly graded sands,
> gravelly sand, little or no
> fines. H
> Cu \< 6 and/or
> 1 > Cz > 3 C
> Sands with
> fines
> SM
> Silty sand, poorly graded
> sand silt mixtures. F, G, H
> 12 E
> IP \< 4 or the
> limit values
> below 'A' line
> of Plasticity
> chart
> For 4 > IP
> 7 and limit
> values
> above A-
> line, dual
> symbols
> required.
> SC
> Clayey sand, sand clay
> mixtures. F, G, H
> IP >7 and
> the limit
> values above
> 'A' line of
> plasticity
> chart
> Fine grained
> soils (Over
> 50% of the
> material
> smaller than
### 0.075 mm)
Silts &
Clays
wL \< 50
Inorganic
ML
Silt of low to medium
compressibility, very fine
sands, rock flour, silt with
sand. K, L, M
Limit values on or below 'A' line of
plasticity chart & IP \<4
CL
Clays of low to medium
plasticity, gravelly clay,
sandy clay, silty clay,
lean clay. K, L, M
Limit values above 'A' line of
plasticity chart and/or IP > 4
Organic
OL
Organic clay K, L, M, N and
Organic silt K, L, M, O
of low to medium
plasticity
Liquid limit (oven dried)
Liquid limit (undried) \< 0.75
| Classification (For particles smaller
than 75 mm and based on estimated
weights) | Col2 | Col3 | Group
Symbol | Group Name B | Laboratory Classification | Col7 | Col8 |
| ------------------------------------------------------------------------------------------------------------------------------------ | --------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------- | ------------------------- | ------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------- |
| **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Group**
**Symbol** | **Group Name\*\*\*\*\_B \_** | **Percent**
**finer than**
**0.075 mm** | **Other Criteria** | **Other Criteria** |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Gravels**
(More
than
50%of
coarse
fraction
retained
on No. 4
sieve
(4.75 mm) | Clean
gravels | GW | Well graded gravels,
sandy gravels, sand
gravel mixture, little or
no fines.*D* | \< 5\_E\_ | Cu 4 and
1 ≤ Cz ≤ 3\_C\_ | Cu 4 and
1 ≤ Cz ≤ 3\_C\_ |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Gravels**
(More
than
50%of
coarse
fraction
retained
on No. 4
sieve
(4.75 mm) | Clean
gravels | GP | Poorly graded gravels,
sandy gravels, Sand
gravel mixture, little or
no fines.*D* | Poorly graded gravels,
sandy gravels, Sand
gravel mixture, little or
no fines.*D* | Cu \< 4 and/or
1> Cz> 3\_C\_ | Cu \< 4 and/or
1> Cz> 3\_C\_ |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Gravels**
(More
than
50%of
coarse
fraction
retained
on No. 4
sieve
(4.75 mm) |
Gravel with
fines | GM | Silty gravels, silty sandy
gravels.*D, F, G* | > 12\_E\_ | IP\< 4 or the
limit values
below 'A'
line of
plasticity
chart | For 4> IP >
7 and limit
values
above
'A' line,
dual symbol
required\* |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Gravels**
(More
than
50%of
coarse
fraction
retained
on No. 4
sieve
(4.75 mm) |
Gravel with
fines | GC | Clayey gravels, silty
clayey gravels. .*D, F, G* | Clayey gravels, silty
clayey gravels. .*D, F, G* | IP >7 and the
limit values
above 'A' line
of Plasticity
Chart | IP >7 and the
limit values
above 'A' line
of Plasticity
Chart |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Sands**
(over
50% of
coarse
fraction
smaller
than
4.75
mm) | Clean Sands | SW | Well graded sand,
gravelly sand, little or no
fines.*H* | \< 5\_E\_ | Cu ≥ 6 and
1≤ Cz ≤ 3\_C\_ | Cu ≥ 6 and
1≤ Cz ≤ 3\_C\_ |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Sands**
(over
50% of
coarse
fraction
smaller
than
4.75
mm) | Clean Sands | SP | Poorly graded sands,
gravelly sand, little or no
fines.*H* | Poorly graded sands,
gravelly sand, little or no
fines.*H* | Cu \< 6 and/or
1 > Cz > 3\_C\_ | Cu \< 6 and/or
1 > Cz > 3\_C\_ |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Sands**
(over
50% of
coarse
fraction
smaller
than
4.75
mm) | Sands with
fines | SM | Silty sand, poorly graded
sand silt mixtures.*F, G, H* | > 12\_E\_ | IP \< 4 or the
limit values
below 'A' line
of Plasticity
chart | For 4 > IP
>7 and limit
values
above A-
line, dual
symbols
required. |
| **Coarse**
**grained soils**
(More than
50% of the
material
retained on
No. 200 sieve
(0.075 mm) | **Sands**
(over
50% of
coarse
fraction
smaller
than
4.75
mm) | Sands with
fines | SC | Clayey sand, sand clay
mixtures.*F, G, H* | Clayey sand, sand clay
mixtures.*F, G, H* | IP >7 and
the limit
values above
'A' line of
plasticity
chart | IP >7 and
the limit
values above
'A' line of
plasticity
chart |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL \< 50 | Inorganic | ML | Silt of low to medium
compressibility, very fine
sands, rock flour, silt with
sand.*K, L, M* | Limit values on or below 'A' line of
plasticity chart & IP \<4 | Limit values on or below 'A' line of
plasticity chart & IP \<4 | Limit values on or below 'A' line of
plasticity chart & IP \<4 |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL \< 50 | Inorganic | CL | Clays of low to medium
plasticity, gravelly clay,
sandy clay, silty clay,
lean clay.*K, L, M* | Limit values above 'A' line of
plasticity chart and/or IP > 4 | Limit values above 'A' line of
plasticity chart and/or IP > 4 | Limit values above 'A' line of
plasticity chart and/or IP > 4 |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL \< 50 | Organic | OL | Organic clay\_K, L, M, N\_ and
Organic silt\_K, L, M, O\_
of low to medium
plasticity | Liquid limit (oven dried)
Liquid limit (undried) \< 0.75 | Liquid limit (oven dried)
Liquid limit (undried) \< 0.75 | Liquid limit (oven dried)
Liquid limit (undried) \< 0.75 |
Classification (For particles smaller
than 75 mm and based on estimated
weights)
Group
Symbol
Group Name B
Laboratory Classification
Percent
finer than
### 0.075 mm
Other Criteria
Fine grained
soils (Over
50% of the
material
smaller than
### 0.075 mm)
Silts &
Clays
wL ≥ 50
Inorganic
MH
Silt of high plasticity,
micaceous fine sandy or
silty soil, elastic silt. K, L, M
Limit values on or below 'A' line of
plasticity chart
CH
High plastic clay, fat
clay. K, L, M
Limit values above 'A' line of
plasticity chart
Organic
OH
Organic clay of high
plasticity. K, L, M, P
Liquid limit (oven dried)
Liquid limit (undried) \< 0.75
Soils of high organic origin
PT
Peat and highly organic
soils. K, L, M, Q
Identified by colour, odour, fibrous
texture and spongy characteristics.
Notes:
A
Based on the material passing the 3-in. (75-mm) sieve
B
ওভ ভরবষফ ংধসঢ়ষব পড়হঃধরহবফ পড়ননষবং ড়ৎ নড়ঁষফবৎং, ড়ৎ নড়ঃয, ধফফ ুরিঃয পড়ননষবং ড়ৎ নড়ঁষফবৎং, ড়ৎ নড়ঃযচ্:ড় মৎড়ঁঢ় হধসব.
C
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
D
ওভ ংড়রষ পড়হঃধরহং ≥ ১৫ % ংধহফ, ধফফ ুরিঃয ংধহফচ্:ড় মৎড়ঁঢ় হধসব.
E
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
F
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
G
ওভ ভরহবং ধৎব ড়ৎমধহরপ, ধফফ ুরিঃয ড়ৎমধহরপ ভরহবংচ্:ড় মৎড়ঁঢ় হধসব.
H
ওভ ংড়রষ পড়হঃধরহং ≥ ১৫ % মৎধাবষ, ধফফ ুরিঃয মৎধাবষচ্:ড় মৎড়ঁঢ় হধসব.
I
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
J
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
K
ওভ ংড়রষ পড়হঃধরহং ১৫:ড় ২৯ % ঢ়ষঁং ঘড়. ২০০, ধফফ ুরিঃয ংধহফচ্ ড়ৎ ুরিঃয মৎধাবষ,চ্ যিরপযবাবৎ রং ঢ়ৎবফড়সরহধহঃ.
L
ওভ ংড়রষ পড়হঃধরহং ≥৩০ % ঢ়ষঁং ঘড়. ২০০, ঢ়ৎবফড়সরহধহঃষু ংধহফ, ধফফ ুংধহফ চ্:ড় মৎড়ঁঢ় হধসব.
M
ওভ ংড়রষ পড়হঃধরহং ≥ ৩০ % ঢ়ষঁং ঘড়. ২০০, ঢ়ৎবফড়সরহধহঃষু মৎধাবষ, ধফফ ুমৎধাবষষুচ্:ড় মৎড়ঁঢ় হধসব.
N
চও ≥ ৪ ধহফ ঢ়ষড়ঃং ড়হ ড়ৎ ধনড়াব ুঅচ্ ষরহব.
O
চও \< ৪ ড়ৎ ঢ়ষড়ঃং নবষড়ুি অচ্ ষরহব.
P
চও ঢ়ষড়ঃং ড়হ ড়ৎ ধনড়াব ুঅচ্ ষরহব.
Q
চও ঢ়ষড়ঃং নবষড়ি ুঅচ্ ষরহব.
| Classification (For particles smaller
than 75 mm and based on estimated
weights) | Col2 | Col3 | Group
Symbol | Group Name B | Laboratory Classification | Col7 |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Classification (For particles smaller**
**than 75 mm and based on estimated**
**weights)** | **Group**
**Symbol** | **Group Name\*\*\*\*\_B \_** | **Percent**
**finer than**
**0.075 mm** | **Other Criteria** |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL ≥ 50 | Inorganic | MH | Silt of high plasticity,
micaceous fine sandy or
silty soil, elastic silt.*K, L, M* | Limit values on or below 'A' line of
plasticity chart | Limit values on or below 'A' line of
plasticity chart |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL ≥ 50 | Inorganic | CH | High plastic clay, fat
clay.*K, L, M* | Limit values above 'A' line of
plasticity chart | Limit values above 'A' line of
plasticity chart |
| **Fine grained**
**soils** (Over
50% of the
material
smaller than
0.075 mm) | **Silts &**
**Clays**
wL ≥ 50 | Organic | OH | Organic clay of high
plasticity.\_K, L, M,\_P | Liquid limit (oven dried)
Liquid limit (undried) \< 0.75 | Liquid limit (oven dried)
Liquid limit (undried) \< 0.75 |
| **Soils of high organic origin** | **Soils of high organic origin** | **Soils of high organic origin** | PT | Peat and highly organic
soils.\_K, L, M,\_Q | Identified by colour, odour, fibrous
texture and spongy characteristics. | Identified by colour, odour, fibrous
texture and spongy characteristics. |
| Notes: | Notes: | Notes: | Notes: | Notes: | Notes: | Notes: |
| \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. | \_A \_
Based on the material passing the 3-in. (75-mm) sieve
\_B \_
If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
\_C \_
Cu = D60/D10, CZ = (D30)2 / (D10 ×D60)
\_D \_
If soil contains ≥ 15 % sand, add “with sand” to group name.
\_E \_
Gravels with 5 to 12 % fines require dual symbols:
GW-GM well-graded gravel with silt
GW-GC well-graded gravel with clay
GP-GM poorly graded gravel with silt
GP-GC poorly graded gravel with clay
\_F \_
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
\_G \_
If fines are organic, add “with organic fines” to group name.
\_H \_
If soil contains ≥ 15 % gravel, add “with gravel” to group name.
\_I \_
Sands with 5 to 12 % fines require dual symbols:
SW-SM well-graded sand with silt
SW-SC well-graded sand with clay
SP-SM poorly graded sand with silt
SP-SC poorly graded sand with clay.
\_J \_
If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay.
\_K \_
If soil contains 15 to 29 % plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
\_L \_
If soil contains ≥30 % plus No. 200, predominantly sand, add “sand ” to group name.
\_M \_
If soil contains ≥ 30 % plus No. 200, predominantly gravel, add “gravelly” to group name.
\_N \_
PI ≥ 4 and plots on or above “A” line.
\_O \_
PI \< 4 or plots below“ A” line.
\_P \_
PI plots on or above “A” line.
\_Q \_
PIplots below “A” line. |
If desired, the percentages of gravel, sand, and fines may be stated in terms indicating a range of percentages,
as follows:
Trace −
Particles are present but estimated to be less than 5 %
Few −
5 to 10 %
Little −
15 to 25 %
Some −
30 to 45 %
Mostly − 50 to 100 %
Figure 6.3.1 Plasticity chart (based on materials passing 425 m sieve)
3.5.4
Identification and Classification of Organic Soils
The presence of organic matter can have undesirable effects on the engineering
behaviour of soil. For example, the bearing capacity is reduced, the compressibility is
increased and, swelling and shrinkage potential is increased due to organic content.
Organic content tests are used to classify the soil. In soil with little or no clay
particles and carbonate content, the organic content is often determined from the loss
on ignition at a controlled temperature. Other suitable tests can also be used. For
example, organic content can be determined from the mass loss on treatment with
hydrogen peroxide (H2O2), which provides a more specific measure of organics.
Organic deposits are due to decomposition of organic matters and found usually in
topsoil and marshy place. A soil deposit in organic origin is said to peat if it is at the
higher end of the organic content scale (75% or more), organic soil at the low end,
and muck in between. Peat soil is usually formed of fossilized plant minerals and
characterized by fiber content and lower decomposition. The peats have certain
characteristics that set them apart from moist mineral soils and required special
considerations for construction over them. This special characteristic includes,
extremely high natural moisture content, high compressibility including significant
secondary and even tertiary compression and very low undrained shear strength at
natural moisture content.
However, there are many other criteria existed to classify the organic deposits and it
remains still as controversial issue with numerous approaches available for varying
purpose of classification. A possible approach is being considered by the American
society for Testing and Materials for classifying organic soils having varying amount
of organic matter contents. The classification is given in Table 6.3.3.
Table 6.3.3: Classification and Description of Organic Soils (after Edil, 1997)
Organic Content
(ASTM D2974-07a)
Description
\< 5 %
Little effect on behavior; considered inorganic soil.
6 \~ 20 %
Effects properties but behavior is still like mineral soils;
organic silts and clays.
21 \~ 74 %
Organic
matter
governs
properties;
traditional
soil
mechanics may be applicable; silty or clayey organic soils.
> 75 %
> Displays behavior distinct from traditional soil mechanics
> especially at low stress.
> 3.5.5
> Identification and Classification of Expansive Soils
> Expansive soils are those which swell considerably on absorption of water and shrink
> on the removal of water. In monsoon seasons, expansive soils imbibe water, become
> soft and swell. In drier seasons, these soils shrink or reduce in volume due to
> evaporation of water and become harder. As such, the seasonal moisture variation in
> such soil deposits around and beneath the structure results into subsequent upward
> and downward movements of structures leading to structural damage, in the form of
> wide cracks in the wall and distortion of floors. For identification and classification
> of expansive soils parameters like liquid limit, plasticity index, shrinkage limit, free
> swell, free swell index, linear shrinkage, swelling potential, swelling pressure and
> volume change from air dry to saturate condition should be evaluated experimentally
> or from available geotechnical correlation. Various recommended criteria for
> identification and classification of expansive soils are presented in Appendix E.
3.5.6
Identification and Classification of Collapsible Soils
Soil deposits most likely to collapse are; (i) loose fills, (ii) altered wind-blown sands,
(iii) hill wash of loose consistency and (iv) decomposed granite or other acid igneous
rocks.
A very simple test for recognizing collapsible soil is the ″sauges test″. Two
undisturbed cylindrical samples (sausages) of the same diameter and length (volume)
are carved from the soil. One sample is then wetted and kneaded to form a cylinder
of the original diameter. A decrease in length as compared to the original,
undisturbed cylinder will confirm a collapsible grain structure. Collapse is probable
when the natural void ratio, collapsible grain structure. Collapse is probable when the
natural void ratio, P is higher than a critical void ratio, P+ that depends on void ratios
PB and PA at liquid limit and plastic limits respectively. The following formula should
be used to estimate the critical void ratio.
P
L
c
e
e
e
.
.
(6.3.1)
Collapsible soils (with a degree of saturation, C7 0.6) should satisfy the following
condition:
\Erode slowly under 50 mm or 180 mm head
No colloidal erosion under 380 mm or 1020 mm head | Dispersive soils
Intermediate soils
Non-dispersive soils | D1 and D2
ND4 and ND3
ND2 and ND1 |
In below surface clays, overconsolidation may have taken place when the clay was
previously at, or close to the ground surface and above the water table, but due to
subsequent deposition the strata may now be below the surface, saturated and
overconsolidated. Partial saturation does not in itself cause engineering problems,
but may lead to laboratory testing difficulties. Soft clays have undrained shear
strengths between about 10kPa and 40kPa, in other words, from exuding between the
fingers when squeezed to being easily moulded in the fingers.
Soft clays present very special problems of engineering design and construction.
Foundation failures in soft clays are comparatively common. The construction of
buildings in soft clays has always been associated with stability problems and
settlement. Shallow foundations inevitably results in large settlements which must be
accommodated for in the design, and which invariably necessitate long-term
maintenance of engineered facilities. The following relationship among N-values
obtained from SPT, consistency and undrained shear strength of soft clays may be
used as guides.
N-value
Consistency
Undrained Shear Strength (kN/m2)
Below 2
Very soft
Less than 20
2 – 4
Soft
20 – 40
Undrained shear strength is half of unconfined compressive strength as determined
from unconfined compression test or half of the peak deviator stress as obtained from
unconsolidated undrained (UU) triaxial compression test.
3.6
Materials
All materials for the construction of foundations shall conform to the requirements of
Part 5 of this Code.
3.6.1
Concrete
All concrete materials and steel reinforcement used in foundations shall conform to
the requirements specified in Chapter 5 unless otherwise specified in this Section.
For different types of foundation the recommended concrete properties are shown in
Table 6.3.5. However, special considerations should be given for hostile environment
(salinity, acidic environment).
Table 6.3.5: Properties of Concrete for Different Types of Foundations
Foundation Type Minimum cement
content (kg/m3)
Specified Min.
28 days Cylinder
Strength (MPa)
Slump
(mm)
Remarks
Footing/raft
25 to 125
Retarder and
plasticizer
recommended.
Slump test shall be
performed as per
ASTM C143.
Drilled shaft/Cast-
in-situ pile
(tremie concrete)
125 to 200
Driven pile
25 to 125
3.6.2
Steel
All steel reinforcement and steel materials used in foundations shall conform to the
requirements specified in Chapter 5 unless otherwise specified in this Section.
However, this Section considers the corrosivity of soil that is described as under.
Corrosion in soil, water or moist out-door environment is caused by electro-chemical
processes. The process takes place in corrosion cells on the steel surface, which
consists of an anodic surface, a cathodic surface (where oxygen is reduced) and the
electrolyte, which reacts with these surfaces. In the case of general corrosion, the
surface erosion is relatively even across the entire surface. Local corrosion however
is concentrated to a limited surface area. Pronounced cavity erosion is rather unusual
on unprotected carbon steel in soil or water.
In many circumstances, steel corrosion rates are low and steel piles may be used for
permanent works in an unprotected condition. The degree of corrosion and whether
protection is required depend upon the working environment which can be variable,
even within a single installation. Underground corrosion of steel piles driven into
undisturbed soils is negligible irrespective of the soi1 type and characteristics. The
insignificant corrosion attack is attributed to the low oxygen levels present in
undisturbed soil. For the purpose of calculations, a maximum corrosion rate of 0.015
mm per side per year may be used. In recent-fill soils or industrial waste soils, where
corrosion rates may be higher, protection systems should be considered.
(a)
Atmospheric Corrosion
Atmospheric corrosion of steel of 0.035 mm/side per year may be used
for most atmospheric environments.
(b)
Corrosion in Fresh Water
Corrosion losses in fresh water immersion zones are generally lower than
for sea water so the effective life of steel piles is normally proportionately
longer. However, fresh waters are variable and no general advice can be
given to quantify the increase in the length of life.
(c)
Corrosion in Marine Environment
Marine environments may include several exposure zones with different
aggressivity and different corrosion performance.
(i)
Below the bed level: Where piles are below the bed level little
corrosion occurs and the corrosion rate given for underground
corrosion is applicable, that is, 0.015 mm/side per year.
(ii) Seawater immersion zone: Corrosion of steel pilling in immersion
conditions is normally low, with a mean corrosion rate of 0.035
mm/side per year.
(iii) Tidal zones: Marine growths in this zone give significant protection
to the piling, by sheltering the steel from wave action between tides
and by limiting the oxygen supply to the steel surface. The
corrosion rate of steels in the tidal zone is similar to that of
immersion zone corrosion, i.e. 0.035 mm/side per year. Protection
should be provided where necessary, to the steel surfaces to prevent
the removal or damage of the marine growth.
(iv) Low water zone: In tidal waters, the low water level and the splash
zone are reasons of highest thickness losses, where a mean
corrosion rate of 0.075 mm/side per year occurs. Occasionally
higher corrosion rates are encountered at the lower water level
because of specific local conditions.
(v)
Splash and atmospheric zones: In the splash zone, which is a more
aggressive environment than the atmospheric zone, corrosion rates
are similar to the low water level, i.e. 0.075 mm/side per year. In
this zone thick stratified rust layers may develop and at thicknesses
greater than 10 mm this tend to spall from steel especially on curved
parts of the piles such as the shoulders and the clutches. Rust has a
much greater volume than the steel from which it is derived so that
the steel corrosion losses are represented by some 10 % to 20 % of
the rust thickness. The boundary between splash and atmospheric
zones is not well defined, however, corrosion rates diminish
rapidly with distance above peak wave height and mean
atmospheric corrosion rate of 0.035 mm/side per year can be
used.
(d)
Method of Assessing Soil Corrosivity
The following variables attributes to accelerated corrosion: (i) acidity and
alkalinity; (ii) soluable salts; (iii) bacteria (sulphates usually promote
bacteria; (iv) resistivity; (v) moisture content; (vi) pH; and so on. The
following charts, Tables 6.3.6a and 6.3.6b provide guides in assessing the
corrosivity of soils. The parameters should be measured following
relevant Standards of ASTM.
Table 6.3.6a: Soil Corrosivity Scores for Various Parameters
Item/Parameter
Measured value
Score/Mark
Soil composition
Calcareous, marly limestone, sandy marl, non-
stratified sand
+2
Sandy silt, sandy clay, clayey silt
Clay, silty clay
-2
Peat, marshy soil
-4
Ground water
None
Exist
-1
Vary
-2
Item/Parameter
Measured value
Score/Mark
Resistivity
10,000 ohm-cm or more
10,000-5,000
-1
5,000-2,300
-2
2,300-1,000
-3
1,000 or less
-4
Moisture content
20% or less
More than 20%
-1
pH
6 or more
Less than 6
-2
Sulphide and hydrogen
sulphide
None
Trace
-2
Exist
-4
Carbonate
5% or more
+2
5% - 1%
+1
Less than 1%
Chloride
100 mg/kg or less
More than 100 mg/kg
+1
Sulphate
200 mg/kg or less
200 – 500 mg/kg
-1
500 – 1000 mg/kg
-2
More than 1000 mg/kg
-3
Cinder and coke
None
Exist
-4
Table 6.3.6b: Soil Corrosivity Rating
Score/Mark
Corrosivity Rating
0 and above
Non-corrosive
0 to -4
Slightly corrosive
-5 to -10
Corrosive
-10 or less
Highly corrosive
(e)
Methods of Increasing Effective Life
The effective life of unpainted or otherwise unprotected steel piling
depends upon the combined effects of imposed stresses and corrosion.
Where measures for increasing the effective life of a structure are
necessary, the following should be considered; introduction of a corrosion
allowance (i.e. oversized cross-sections of piles, high yield steel etc), anti-
corrosion painting, application of a polyethylene (PE) coating (on steel
tube piles), zinc coating, electro-chemical (cathodic) protection, casting in
cement mortar or concrete, and use of atmospheric corrosion resistant
steel products instead of ordinary carbon steel in any foundation work
involving steel.
(i)
Use of a heavier section: Effective life may be increased by the use
of additional steel thickness as a corrosion allowance. Maximum
corrosion seldom occurs at the same position as the maximum
bending moment. Accordingly, the use of a corrosion allowance
is a cost effective method of increasing effective life. It is preferable
to use atmospheric corrosion resistant high strength low alloy
steel.
(ii) Use of a high yield steel: An alternative to using mild steel in a
heavier section is to use a higher yield steel and retain the same
section.
(iii) Zinc coatings: Steel piles should normally be coated under shop
conditions. Paints should be applied to the cleaned surface by airless
spraying and then cured rapidly to produce the required coating
thickness in as few coats as possible. Hot zinc-coating of steel piles
in soil can achieve normally long-lasting protection, provided that
the zinc layer has sufficient thickness. In some soils, especially
those with low pH-values, the corrosion of zinc can be high, thereby
shortening the protection duration. Low pH-values occur normally
in the aerated zone above the lowest ground water level. In such a
case, it is recommended to apply protection paint on top of the zinc
layer.
(iv) Concrete encasement: Concrete encasement may be used to protect
steel piles in marine environment. The use of concrete may be
restricted to the splash zone by extending the concrete cope to
below the mean high water level, both splash and tidal zones may
be protected by extending the cope to below the lowest water level.
The concrete itself should be a quantity sufficient to resist seawater
attack.
(v)
Cathodic protection: The design and application of cathodic
protection systems to marine piles structures is a complex operation
requiring the experience of specialist firms. Cathodic protection
with electric current applied to steel sheet pile wall. Rod-type
anodes are connected directly with steel sheet pile. Cathodic
protection is considered to be fully effective only up to the half-tide
mark. For zones above this level, including the splash zone,
alternative methods of protection may be required, in addition to
cathodic protection. Where cathodic protection is used on marine
structures, provision should be made for earthing ships and buried
services to the quay.
(vi) Polyetheline coating: Steel tube piles can be protected effectively by
application of a PE-cover of a few millimeter of thickness. This
cover can be applied in the factory and is usually placed on a
coating of epoxy. Steel tube piles in water, where the mechanical
wear is low, can in this way be protected for long time periods.
When the steel tube piles with the PE-cover are driven into coarse-
grained soil, the effect of damaging the protection layer must be
taken into consideration.
(vii) Properly executed anti-corrosion measures, using high-quality
methods can protect steel piles in soil or water over periods of 15 to
20 years. PE-cover in combination with epoxy coating can achieve
even longer protection times.
3.6.3
Timber
Timber may be used only for foundation of temporary structure and shall conform to
the standards specified in Sec 2.9 of Part 5 of this Code. Where timber is exposed to
soil or used as load bearing pile above ground water level, it shall be treated in
accordance with BDS 819:1975.
3.7
Types of Foundation
3.7.1
Shallow Foundations
Shallow foundations spread the load to the ground at shallow depth. Generally, the
capacity of this foundation is derived from bearing.
3.7.2
Footing
Footings are foundations that spread the load to the ground at shallow depths. These
include individual column footings, continuous wall footings, and combined
footings. Footings shall be provided under walls, pilasters, columns, piers, chimneys
etc. bearing on soil or rock, except that footings may be omitted under pier or
monolithic concrete walls if safe bearing capacity of the soil or rock is not exceeded.
3.7.3
Raft/Mat
A foundation consisting of continuous slab that covers the entire area beneath the
structure and supports all walls and columns is considered as a raft or mat
foundation. A raft foundation may be one of the following types:
(i)
Flat plate or concrete slab of uniform thickness usually supporting
columns spaced uniformly and resting on soils of low compressibility.
(ii) Flat plates as in (a) but thickened under columns to provide adequate
shear and moment resistance.
(iii) Two way slab and beam system supporting largely spaced columns on
compressible soil.
(iv) Cellular raft or rigid frames consisting of slabs and basement walls,
usually used for heavy structures.
3.7.4
Deep Foundations
A cylindrical/box foundation having a ratio of depth to base width greater than 5 is
considered a Deep Foundation. Generally, its capacity is derived from friction and
end bearing.
3.7.5
Driven Piles
A slender deep foundation unit made of materials such as steel, concrete, wood, or
combination thereof, which is pre-manufactured and placed by driving, jacking,
jetting or screwing and displacing the soil.
(i)
Driven Precast Concrete Piles: Pile structure capable of being driven into
the ground and able to resist handling stresses shall be used for this
category of piles.
(ii) Driven Cast-in-situ Concrete Piles : A pile formed by driving a steel
casing or concrete shell in one or more pieces, which may remain in
place after driving or withdrawn, with the inside filled with concrete, falls
in this category of piles. Sometimes an enlarged base may be formed by
driving out a concrete plug.
(iii) Driven Prestressed Concrete Pile: A pile constructed in prestressed
concrete in a casting yard and subsequently driven in the ground when it
has attained sufficient strength.
(iv) Timber Piles: Structural timber (Sec 2.9 Part 5) shall be used as piles for
temporary structures for directly transmitting the imposed load to soil.
Driven timber poles are used to compact and improve the deposit.
3.7.6
Bored Piles/Cast-in-Situ Piles
A deep foundation of generally small diameter, usually less than 600 mm,
constructed using percussion or rotary drilling into the soil. These are constructed by
concreting bore holes formed by auguring, rotary drilling or percussion drilling with
or without using bentonite mud circulation. Excavation or drilling shall be carried out
in a manner that will not impair the carrying capacity of the foundations already in
place or will not damage adjacent foundations. These foundations may be tested for
capacity by load test or for integrity by sonic response or other suitable method.
Under-reaming drilled piers can be constructed in cohesive soils to increase the end
bearing.
3.7.7
Drilled Pier/Drilled Shafts
Drilled pier is a bored pile with larger diameter (more than 600 mm) constructed by
excavating the soil or sinking the foundation.
3.7.8
Caisson/Well
A caisson or well foundation is a deep foundation of large diameter relative to its
length that is generally a hollow shaft or box which is sunk to position. It differs
from other types of deep foundation in the sense that it undergoes rigid body
movement under lateral load, whereas the others are flexible like a beam under such
loads. This type of foundation is usually used for bridges and massive structures.
Division B: Design of Foundations (Sections 3.8 to 3.11)
3.8
Shallow Foundation
This Section shall be applicable to isolated Footings, Combined Footings and
Raft/Mats.
3.8.1
Distribution of Bearing Pressure
Footing shall be designed to keep the maximum imposed load within the safe bearing
values of soil and rock. To prevent unequal settlement footing shall be designed to
keep the bearing pressure as nearly uniform as practical. For raft design, distribution
of soil pressures should be consistent with the properties of the foundation materials
(subsoil) and the structure (raft thickness) and with the principles of geotechnical
engineering.
Mat or raft and floating foundations shall only be used when the applied load of
building or structure is so arranged as to result in practically uniformly balanced
loading, and the soil immediately below the mat is of uniform bearing capacity.
3.8.2
Dimension of Footings
Footings shall generally be proportioned from the allowable bearing pressure and
stress limitations imposed by limiting settlement.
The angle of spread of the load from the wall base to outer edge of the ground
bearing shall not exceed the following:
Brick or stone masonry
horizontal to 1 vertical
Lime concrete
2 horizontal to 1 vertical
Cement concrete
1 horizontal to 1 vertical
A footing shall be placed to depth so that:
(a)
adequate bearing capacity is achieved,
(b)
in case of clayey soil , shrinkage and swelling due to seasonal weather
change is not significant,
(c)
it is below possible excavation close by, and
(d)
it is at least 500 mm below natural ground level unless rock or other
weather resistant material is at the surface.
Where footings are to be founded on a slope, the distance of the sloping surface at
the base level of the footing measured from the centre of the footing shall not be less
than twice the width of the footing.
When adjacent footings are to be placed at different levels, the distance between the
edges of footings shall be such as to prevent undesirable overlapping of structures in
soil and disturbance of the soil under the higher footing due to excavation of the
lower footing.
On a sloping site, footing shall be on a horizontal bearing and stepped. At all changes
of levels, footings shall be lapped for a distance of at least equal to the thickness of
foundation or three times the height of step, whichever is greater. Adequate
precautions shall be taken to prevent tendency for the upper layers of soil to move
downhill.
3.8.3
Thickness of Footing
The minimum thickness for different types of footing for light structures (two stories
or less in occupancy category A, B, C and D), shall be as follows:
Type of Footing
Minimum Thickness
Remark
Masonry
250 mm; twice the maximum
projection from the face of the
wall
Greater of the two
values
shall
be
selected
Plain concrete
200 mm, or twice the maximum
offset in a stepped footing
---------------------------
Reinforced concrete
(depth above bottom
reinforcement)
150 mm
300 mm
Resting on soil
Resting on pile
3.8.4
Footings in Fill Soil
Footings located in fill are subject to the same bearing capacity, settlement, and
dynamic ground stability considerations as footings in natural soil. The behavior of
both fill and underlying natural soil should be considered.
3.8.5
Soil and Rock Property Selection
Soil and rock properties defining the strength and compressibility characteristics of
foundation materials are required for footing design. Foundation stability and
settlement analysis for design shall be conducted using soil and rock properties based
on the results of field and laboratory testing.
3.8.6
Minimum Depth of Foundation
The minimum depth of foundation shall be 1.5 m for exterior footing of permanent
structures in cohesive soils and 2 m in cohesionless soils. For temporary structures
the minimum depth of exterior footing shall be 400 mm. In case of expansive and
soils susceptible to weathering effects, the above mentioned minimum depths will be
not applicable and may have to be increased.
3.8.7
Scour
Footings supported on soil shall be embedded sufficiently below the maximum
computed scour depth or protected with a scour countermeasure.
3.8.8
Mass Movement of Ground in Unstable Areas
In certain areas mass movement of ground may occur from causes independent of the
loads applied to the foundation. These include mining subsidence, landslides on
unstable slopes and creep on clay slopes. In areas of ground subsidence, foundations
and structures should be made sufficiently rigid and strong to withstand the probable
worst loading conditions. The construction of structures on slopes which are
suspected of being unstable and subject to landslip shall be avoided. Spread
foundations on such slopes shall be on a horizontal bearing and stepped. For
foundations on clay slopes, the stability of the foundation should be investigated.
3.8.9
Foundation Excavation
Foundation excavation below ground water table particularly in sand shall be made
such that the hydraulic gradient at the bottom of the excavation is not increased to a
magnitude that would case the foundation soils to loosen due to upward flow of
water. Further, footing excavations shall be made such that hydraulic gradients and
material removal do not adversely affect adjacent structures. Seepage forces and
gradients may be evaluated by standard flow net procedures. Dewatering or cutoff
methods to control seepage shall be used when necessary. In case of soil excavation
for raft foundations, the following issues should be additionally taken into
consideration:
(i)
Protection for the excavation using shore or sheet piles and/or retaining
system with or without bracing, anchors etc.
(ii) Consideration of the additional bearing capacity of the raft for the depth
of the soil excavated.
(iii) Consideration of the reduction of bearing capacity for any upward
buoyancy pressure of water.
(iv) Other considerations as mentioned in Sec 3.12.
#### 3.8.10 Design Considerations for Raft foundation
Design provisions given in Sec 3.9.2 shall generally apply. In case the raft supports
structure consisting of several parts with varying loads and height, it is advisable to
provide separate joints between these parts. Joints shall also be provided wherever
there is a change in the direction of the raft. The minimum depth of foundation shall
generally be not less than 1.5 m in cohesive soil and 2 m in cohesionless soils.
Foundations subject to heavy vibratory loads shall preferably be isolated.
##### 3.8.10.1 Dimensioning
The size and shape of the foundation shall be decided taking into consideration the
magnitude of subgrade modulus, the long term deformation of the supporting soil
and the distribution of contact pressure.
Distribution of contact pressure underneath a raft is affected by the physical
characteristics of the supporting soil. Consideration shall be given to the increased
contact pressure developed along the edges of foundation on cohesive soils and the
decrease in pressure on granular soils. Both long term and short term deformation
and settlement effects shall be considered in the design.
##### 3.8.10.2 Eccentricity
Since raft foundation usually occupies the entire area of a building, it may not be
feasible to proportion the raft so that the centroid of the raft coincides with the line of
action of the resultant force due to building. In such cases, the effect of eccentricity
on the contact pressure distribution shall be considered in the design.
##### 3.8.10.3 Rigidity of Foundation
The rigidity of foundation affects soil pressure distribution which in turn produces
additional stresses in the raft due to moments etc. A rigid foundation also generates
high secondary stresses. The effects of such rigidity shall be taken into consideration
in designing rafts.
##### 3.8.10.4 Methods of Analysis
The essential part of analysis of a raft foundation is the determination of distribution
of contact pressure below the mat which is a complex function of the rigidity of raft,
and the rigidity of the superstructure and the supporting soil. Any analytical method
shall therefore use simplifying assumptions which are reasonably valid for the
condition analysed. Choice of a particular method shall therefore be governed by the
validity of the assumptions in the particular case.
3.9
Geotechnical Design of Shallow Foundations
3.9.1
General
Shallow foundations on soil shall be designed to support the design loads with
adequate bearing and structural capacity and with tolerable settlements. In addition,
the capacity of footings subjected to seismic and dynamic loads shall be
appropriately evaluated. The location of the resultant pressure on the base of the
footings should be maintained preferably within B/6 of the centre of the footing.
3.9.2
Design Load
(a)
Shallow foundation design considering bearing capacity due to shear strength
shall consider the most unfavourable effect of the following combinations of
loading:
(i)
Full Dead Load + Normal Live Load
(ii)
Full Dead Load + Normal Live Load + Wind Load or Seismic Load
(iii) 0.9 ×(Full Dead Load) + Buoyancy Pressure
(b)
Shallow foundation design considering settlement shall consider the most
unfavourable effect of the following combinations of loading:
SAND
(i)
Full Dead Load + Normal Live Load
(ii)
Full Dead Load + Normal Live Load + Wind Load or Seismic Load
CLAY
Full Dead Load + 0.5× Normal Live Load
Normal Live Load is a live load considering floor area reduction factor as used in
column design (Sec 2.3.13).
3.9.3
Bearing Capacity of Shallow Foundations
When physical characteristics such as cohesion, angle of internal friction, density etc.
are available, the bearing capacity shall be calculated from stability considerations.
Established bearing capacity equations shall be used for calculating bearing capacity.
A factor of safety of between 2.0 to 3.0 (depending on engineering judgement on the
extent of soil exploration, quality control and monitoring of construction) shall be
adopted to obtain allowable bearing pressure when dead load and normal live load is
used. Thirty three percent (33%) overstressing above allowable pressure shall be
allowed in case of design considering wind or seismic loading. Allowable load shall
also limit settlement between supporting elements to a tolerable limit.
3.9.3.1
Presumptive bearing capacity for preliminary design
For lightly loaded and small sized structures (two storied or less in occupancy
category A, B, C & D) and for preliminary design of any structure, the presumptive
bearing values (allowable) as given in Table 6.3.7 may be assumed for uniform soil
in the absence of test results.
3.9.3.2
Allowable increase of bearing pressure due to wind and earthquake forces
The allowable bearing pressure of the soil determined in accordance with this Section
may be increased by 33 percent when lateral forces due to wind or earthquake
act simultaneously with gravity loads. No increase in allowable bearing pressure
shall be permitted for gravity loads acting alone. In a zone where seismic forces
exist, possibility of liquefaction in loose sand, silt and sandy soils shall be
investigated.
Table 6.3.7: Presumptive Values of Bearing Capacity for Lightly Loaded Structures\*
Soil
Type
Soil Description
Safe Bearing
Capacity, kPa
Soft Rock or Shale
Gravel, sandy gravel, silty sandy gravel; very dense and
offer high resistance to penetration during excavation (soil
shall include the groups GW, GP, GM, GC)
400\*\*
Sand (other than fine sand), gravelly sand, silty sand; dry
(soil shall include the groups SW, SP, SM, SC)
200\*\*
Fine sand; loose & dry (soil shall include the groups SW, SP)
100\*\*
Silt, clayey silt, clayey sand; dry lumps which can be
easily crushed by finger (soil shall include the groups
ML, SC & MH)
Clay, sandy clay; can be indented with strong thumb
pressure (soil shall include the groups CL & CH)
Soft clay; can be indented with modest thumb pressure
(soil shall include the groups CL & CH)
Very soft clay; can be penetrated several centimeters with
thumb pressure (soil shall include the groups CL & CH)
Organic clay & Peat (soil shall include the groups
OH, OL, Pt)
To be determined
after investigation.
Fills
To be determined
after investigation.
\*
Two stories or less (Occupancy category A, B, C and D)
\*\* 50% of these values shall be used where water table is above the base, or below it
within a distance equal to the least dimension of foundation
3.9.4
Settlement of Shallow Foundation
Foundation shall be so designed that the allowable bearing capacity is not exceeded,
and the total and differential settlement are within permissible values. Foundations
can settle in various ways and each affects the performance of the structure. The
simplest mode consists of the entire structure settling uniformly. This mode does not
distort the structure. Any damage done is related to the interface between the
structure and adjacent ground or adjacent structures. Shearing of utility lines could be
a problem. Another possibility is that one side of the structure settles much more than
the opposite side and the portions in between settle proportionately. This causes the
structure to tilt, but it still does not distort. A nominal tilt will not affect the
performance of the structure, although it may create aesthetic and public confidence
problems. However, as a result of difference in foundation settlement the structure
may settle and distort causing cracks in walls and floors, jamming of doors and
windows and overloading of structural members.
3.9.4.1
Total settlement
Total settlement () is the absolute vertical movement of the foundation from its as-
constructed position to its loaded position. Total settlement of foundation due to net
imposed load shall be estimated in accordance with established engineering
principle. An estimate of settlement with respect to the following shall be made.
(i)
Elastic compression of the underlying soil below the foundation and of
the foundation.
(ii) Consolidation settlement.
(iii) Secondary consolidation/compression of the underlying soil.
(iv) Compression and volume change due to change in effective stress or soil
migration associated with lowering or movement of ground water.
(v)
Seasonal swelling and shrinkage of expansive clays.
(vi) Ground movement on earth slopes, such as surface erosion, creep or
landslide.
(vii) Settlement due to adjacent excavation, mining subsidence and
underground erosion.
In normal circumstances of inorganic and organic soil deposits the total settlement is
attributed due to the first three factors as mentioned above. The other factors are
regarded as special cases. Because soil settlement can have both time-depended and
notime-dependent components, it is often categorized in terms short-term settlement
(or immediate settlement) which occurs as quickly as the load is applied, and long-
term settlement (or delayed settlement), which occurs over some longer period.
Many engineers associate consolidation settlement solely with the long term
settlement of clay. However, this is not strictly true. Consolidation is related to
volume change due to change in effective stress regardless of the type of soil or the
time required for the volume change.
3.9.4.2
Elastic/distortion settlement
Elastic Settlement \< of foundation soils results from lateral movements of the soil
without volume change in response to changes in effective vertical stress. This is
হড়হ-ঃরসব ফবঢ়বহফবহঃ ঢ়যবহড়সবহড়হ ধহফ ংরসরষধৎ:ড়:যব চড়রংংড়হ্থং বভভবপঃ যিবৎব ধহ ড়নলবপঃ
is loaded in the vertical direction expands laterally. Elastic or distortion settlements
primarily occur when the load is confined to a small area, such as a structural
foundation, or near the edges of large loaded area such as embankments.
3.9.4.3
Immediate settlement/short term settlement
This vertical compression occurs immediately after the application of loading either
on account of elastic behaviour that produces distortion at constant volume and on
account of compression of air void. This is sometimes designated as for sandy soil,
even the consolidation component is immediate.
3.9.4.4
Primary consolidation settlement
Primary consolidation settlement or simply the consolidation settlement + of
foundation is due to consolidation of the underlying saturated or nearly saturated soil
especially cohesive silt or clay. The full deal load and 50% of total live load shall be
considered when computing the consolidation settlement of foundations on clay
soils.
3.9.4.5
Secondary consolidation settlement
Secondary consolidation settlement of the foundation is due to secondary
compression or consolidation of the underlying saturated or nearly saturated cohesive
silt or clay. This is primarily due to particle re-orientation, creep, and decomposition
of organic materials. Secondary compression is always time-dependent and can be
significant in highly plastic clays, organic soils, and sanitary landfills, but it is
negligible in sands and gravels.
3.9.4.6
Differential settlement
Differential settlement is the difference in total settlement between two foundations
or two points in the same foundation. It occurs as a result of relative movement
between two parts of a building. The related terms describing the effects of
differential settlement on the structural as a whole or on parts of it are tilt, rotation
and angular distortion/relative rotation which are defined below. Due consideration
shall be given to estimate the differential settlement that may occur under the
building structure under the following circumstances:
(i)
Non-uniformity in subsoil formation within the area covered by the
building due to geologic or man-made causes, or anomalies in type,
structure, thickness and density of the formation.
(ii) Non-uniform pressure distribution due to non-uniform and incomplete
loading.
(iii) Ground water condition during and after construction.
(iv) Loading influence of adjacent structures.
(v)
Uneven expansion and contraction due to moisture migration, uneven
drying, wetting or softening.
3.9.4.7
Rotation and tilt of shallow foundation
(a)
Rotation
Rotation is the angle between the horizontal line and an imaginary
straight line connecting any two foundations or two points in a single
foundation.
(b)
Tilt
Tilt is rotation of the entire superstructure or a well-defined part of it as a
result of non-uniform or differential settlement of foundation as a result
of which one side of the building settles more than the other thus
affecting the verticality of the building.
(c)
Angular Distortion/Relative Rotation
Angular distortion or relative rotation is the angle between imaginary
straight line indicating the overall tilt of a structure and the imaginary
connecting line indicating the inclination of a specific part of it. It is
measured as the ratio of differential settlement to the distance between the
two points.
(d)
Tolerable Settlement, Tilt and Rotation
Allowable or limiting settlement of a building structure will depend on
the nature of the structure, the foundation and the soil. Different types of
structures have varying degrees of tolerance to settlements and
distortions. These variations depend on the type of construction, use of
the structure, rigidity of the structure and the presence of sensitive
finishes. As a general rule, a total settlement of 25 mm and a differential
settlement of 20 mm between columns in most buildings shall be
considered safe for buildings on isolated pad footings on sand for
working load (un-factored). A total settlement of 40 mm and a differential
settlement of 20 mm between columns shall be considered safe for
buildings on isolated pad footings on clay soil for working load.
Buildings on raft can usually tolerate greater total settlements. Limiting
tolerance for distortion and deflections introduced in a structure is
necessarily a subjective process, depending on the status of the building
and any specific requirements for serviceability. The limiting values,
given in Table 6.3.8 may be followed as guidelines.
Table 6.3.8: Permissible Total Settlement, Differential Settlement and Angular Distortion
(Tilt) for Shallow Foundations in Soils (in mm) (Adapted from NBCI, 2005)
Type of
Structure
Isolated Foundations
Raft Foundation
Sand and Hard Clay
Plastic Clay
Sand and Hard Clay
Plastic Clay
Maximum
Settlement
Differential
Settlement
Angular
Distortion
Maximum
Settlement
Differential
Settlement
Angular
Distortion
Maximum
Settlement
Differential
Settlement
Angular
Distortion
Maximum
Settlement
Differential
Settlement
Angular
Distortion
Steel Structure
### 0.0033 L
1/300
### 0.0033 L
1/300
### 0.0033 L
1/300
### 0.0033 L
1/300
RCC Structures
### 0.0015 L
1/666
### 0.0015 L
1/666
### 0.0021 L
1/500
### 0.002 L
1/500
Multistoried Building
(a) RCC or steel
framed
building with
panel walls
### 0.002 L
1/500
### 0.002 L
1/500
### 0.0025 L
1/400
### 0.0033 L
1/300
(b) Load bearing walls
(i) L/H = 2 \*
### 0.0002 L 1/5000
### 0.0002 L
1/5000
Not likely to be encountered
(ii) L/H = 7 \*
### 0.0004 L 1/2500
### 0.0004 L
1/2500
Not likely to be encountered
Silos
### 0.0015 L
1/666
### 0.0015 L
1/666
### 0.0025 L
1/400
### 0.0025 L
1/400
Water Tank
### 0.0015 L
1/666
### 0.0015 L
1/666
### 0.0025 L
1/400
### 0.0025 L
1/400
Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
* For intermediate ratios of L/H, the values can be interpolated.
#### 3.9.5 Dynamic Ground Stability or Liquefaction Potential for Foundation Soils
Soil liquefaction is a phenomenon in which a saturated soil deposit loses most, if not
all, of its strength and stiffness due to the generation of excess pore water pressure
during earthquake-induced ground shaking. It has been a major cause for damage of
structures during past earthquakes (e.g., 1964 Niigata Earthquake). Current
knowledge of liquefaction is significantly advanced and several evaluation methods
are available. Hazards due to liquefaction are routinely evaluated and mitigated in
seismically active developed parts of the world.
| Type of
Structure | Isolated Foundations | Col3 | Col4 | Col5 | Col6 | Col7 | Raft Foundation | Col9 | Col10 | Col11 | Col12 | Col13 |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Type of**
**Structure** | **Sand and Hard Clay** | **Sand and Hard Clay** | **Sand and Hard Clay** | **Plastic Clay** | **Plastic Clay** | **Plastic Clay** | **Sand and Hard Clay** | **Sand and Hard Clay** | **Sand and Hard Clay** | **Plastic Clay** | **Plastic Clay** | **Plastic Clay** |
| **Type of**
**Structure** | **Maximum**
\*\*Settlement \*\* | **Differential**
**Settlement** | **Angular**
**Distortion** | **Maximum**
**Settlement** | **Differential**
**Settlement** | **Angular**
**Distortion** | **Maximum**
**Settlement** | **Differential**
**Settlement** | **Angular**
**Distortion** | **Maximum**
**Settlement** | **Differential**
**Settlement** | **Angular**
**Distortion** |
| **Steel Structure** | 50 | 0.0033 L | 1/300 | 50 | 0.0033 L | 1/300 | 75 | 0.0033 L | 1/300 | 100 | 0.0033 L | 1/300 |
| **RCC Structures** | 50 | 0.0015 L | 1/666 | 75 | 0.0015 L | 1/666 | 75 | 0.0021 L | 1/500 | 100 | 0.002 L | 1/500 |
| **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** | **Multistoried Building** |
| (a) RCC or steel
framed
building with
panel walls | 60 | 0.002 L | 1/500 | 75 | 0.002 L | 1/500 | 75 | 0.0025 L | 1/400 | 125 | 0.0033 L | 1/300 |
| (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls | (b) Load bearing walls |
| (i) L/H = 2 \* | 60 | 0.0002 L | 1/5000 | 60 | 0.0002 L | 1/5000 | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered |
| (ii) L/H = 7 \* | 60 | 0.0004 L | 1/2500 | 60 | 0.0004 L | 1/2500 | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered | Not likely to be encountered |
| **Silos** | 50 | 0.0015 L | 1/666 | 75 | 0.0015 L | 1/666 | 100 | 0.0025 L | 1/400 | 125 | 0.0025 L | 1/400 |
| **Water Tank** | 50 | 0.0015 L | 1/666 | 75 | 0.0015 L | 1/666 | 100 | 0.0025 L | 1/400 | 125 | 0.0025 L | 1/400 |
| Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. | Notes: The values given in the Table may be taken only as a guide and the permissible total settlement,
differential settlement and tilt (angular distortion) in each case should be decided as per requirements
of the designer.
L denotes the length of deflected part of wall/ raft or centre to centre distance between columns.
H denotes the height of wall from foundation footing.
\* For intermediate ratios of L/H, the values can be interpolated. |
Liquefaction can be analyzed by a simple comparison of the seismically induced
shear stress with the similarly expressed shear stress required to cause initial
liquefaction or whatever level of shear strain amplitude is deemed intolerable in
design. Usually, the occurrence of 5% double amplitude (DA) axial strain is adopted
to define the cyclic strength consistent with 100% porewater pressure build-up. The
corresponding strength (CRR) can be obtained by several procedures. Thus, the
liquefaction potential of a sand deposit is evaluated in terms of factor of safety FL ,
defined as in Eq. 6.3.6. The externally applied cyclic stress ratio (CSR) can be
evaluated using Equations 6.3.7a, 6.3.7b and 6.3.8.
\&B 5
+SS
+S
(6.3.6)
If the factor of safety \&B is \< 1, liquefaction is said to take place. Otherwise,
liquefaction does not occur. The factor of safety obtained in this way is generally
used to identify the depth to which liquefaction is expected to occur in a future
earthquake. This information is necessary if countermeasure is to be taken in an in
situ deposit of sands.
The cyclic shear stress induced at any point in level ground during an earthquake due
to the upward propagation of shear waves can be assessed by means of a simple
procedure proposed. If a soil column to a depth z is assumed to move horizontally
and if the peak horizontal acceleration on the ground surface is L' , the maximum
shear stress E' acting at the bottom of the soil column is given by
E' 5 L' Y (ð")(4/Q)
(6.3.7a)
Y 5 1 −0.0154
(6.3.7b)
Where, ð" is unit weight of the soil, Q is the gravitational acceleration, 4 is the depth
and Y is a stress reduction coefficient to allow for the deformability of the soil
column ( Y \< 1). It is recommended to use the empirical formula given in Eq.
6.3.7b to compute stress reduction coefficient Y , where 4 is in meters. Division of
both sides of Eq. 6.3.7a by the effective vertical stress D¼′ gives
C@ 5
I
J·′
'
Y
J·
J·′
(6.3.8)
Where, D¼ 5 ð"4 is the total vertical stress. Eq. 6.3.8 has been used widely to assess
the magnitude of shear stress induced in a soil element during an earthquake. The
peak ground acceleration, L' should be taken from seismic zoning map. One of
the advantages of Eq. 6.3.8 is that all the vast amount of information on the
horizontal accelerations that has ever been recorded on the ground surface can be
used directly to assess the shear stress induced by seismic shaking in the horizontal
plane within the ground.
The second step is to determine the cyclic resistance ratio (CRR) of the in situ soil.
The cyclic resistance ratio represents the liquefaction resistance of the in situ soil.
The most commonly used method for determining the liquefaction resistance is to
use the data obtained from the standard penetration test. A cyclic triaxial test may
also be used to estimate CRR more accurately. Site response analysis of a site may be
carried out to estimate the site amplification factor. For this purpose, dynamic
parameters such as shear modulus and damping factors need to be estimated. The
site amplification factor is required to estimate L5L for a given site properly. The
following points are to be noted as regards to soil liquefaction:
Sandy and silty soils tend to liquefy; clay soils do not undergo liquefaction
except the sensitive clays.
Resistance to liquefaction of sandy soil depends on fine content. Higher the
fine content lower is the liquefaction potential.
As a rule of thumb, any soil that has a SPT value higher than 30 will not
liquefy.
Fine grained soils (silty clays/ clayey silt) are susceptible to liquefaction if (Finn et.
al., 1994):
Fraction finer than 0.005 mm
≤ 10%
Liquid limit (LL)
≤ 36%
Natural water content
≤ 0.9 × LL
Liquidity index
≤ 0.75
3.9.6
Structural Design of Shallow Foundations
The foundation members should have enough strength to withstand the stresses
induced from soil-foundation interaction. The following important factors should be
considered in the structural design of foundations.
3.9.6.1
Loads and reactions
Footings shall be considered as under the action of downward forces, due to the
superimposed loads, resisted by an upward pressure exerted by the foundation
materials and distributed over the area of the footings as determined by the
eccentricity of the resultant of the downward forces. Where piles are used under
footings, the upward reaction of the foundation shall be considered as a series of
concentrated loads applied at the pile centers, each pile being assumed to carry the
computed portion of the total footing load.
3.9.6.2
Isolated and multiple footing reactions
When a single isolated footing supports a column, pier or wall, the footing shall be
assumed to act as a cantilever element. When footings support more than one
column, pier, or wall, the footing slab shall be designed for the actual conditions of
continuity and restraint.
3.9.6.3
Raft foundation reactions
For determining the distribution of contact pressure below a raft it is analyzed either
as a rigid or flexible foundation considering the rigidity of the raft, and the rigidity of
the superstructure and the supporting soil. Consideration shall be given to the
increased contact pressure developed along the edges of raft on cohesive soils and
the decrease in contact pressure along the edges on granular soils. Any appropriate
analytical method reasonably valid for the condition may be used. Choice of a
particular method shall be governed by the validity assumptions used. Numerical
analysis of rafts using appropriate software may also be used for determination of
reactions, shears and moments.
Both analytical (based on beams on elastic foundation, Eq. 6.3.9) and numerical
methods require values of the modulus of subgrade reaction of the soil. For use in
preliminary design, indicative values of the modulus of subgrade reaction (k) for
cohesionless soils and cohesive soils are shown in Tables 6.3.9a and 6.3.9b,
respectively.
m 5 0.65. ¸
H
AK
Hµ ¹
1 1
»
.
H
(1ÒêÈ) .
A
(6.3.9)
Where, L= Modulus of elasticity of soil; LM = Flexural rigidity of foundation;
ঘ = ডরফঃয ড়ভ ভড়ঁহফধঃরড়হ; ঙ = চড়রংংড়হ্থং ৎধঃরড় ড়ভ ংড়রষ.
Table 6.3.9a: Modulus of Subgrade Reaction (k) for Cohesionless Soils
Soil Characteristic
\*Modulus of Sub-grade Reaction (k) of Soil
(kN/m3)
Relative
Density
Standard Penetration Test
Value (N) (Blows per 300
mm)
For Dry or Moist
State
For Submerged
State
Loose
Medium
Dense
\<10
10 to 30
30 and over
15000 to 47000
47000 to 180000
9000 to 29000
29000 to 108000
\*The above values apply to a square plate 300 mm x 300 mm or beams 300 mm wide.
Table 6.3.9b: Modulus of Subgrade Reaction (k) for Cohesive Soils
Soil Characteristic
Modulus of Subgrade
Reaction, k (kN/m3)
Consistency
Unconfined Compressive Strength (kN/m2)
Stiff
Very Stiff
Hard
100 to 200
200 to 400
400 and over
27000 to 54000
54000 to 108000
* The values apply to a square plate 300 mm x 300 mm. The above values are based
on the assumption that the average loading intensity does not exceed half the ultimate
bearing capacity.
3.9.6.4
Critical section for moment
External moment on any section of a footing shall be determined by passing a
vertical plane through the footing and computing the moment of the forces acting
over the entire area of the footing on one side of that vertical plane. The critical
section for bending shall be taken at the face of the column, pier, or wall. In the case
of columns that are not square or rectangular, the section shall be taken at the side of
the concentric square of equivalent area. For footings under masonry walls, the
critical section shall be taken halfway between the middle and edge of the wall. For
footings under metallic column bases, the critical section shall be taken halfway
between the column face and the edge of the metallic base. For mat foundations and
combined footings critical section should be determined on the basis of maximum
positive and negative moments obtained from soil-foundation interaction.
3.9.6.5
Critical section for shear
Computation of shear in footings, and location of critical section shall be in
accordance with relevant sections of the structural design part of the Code. Location
of critical section shall be measured from the face of column, pier or wall, for
footings supporting a column, pier, or wall. For footings supporting a column or pier
with metallic base plates, the critical section shall be measured from the location
defined in the critical section for moments for footings.
3.9.6.6
Critical section for footings on driven piles/bored piles/drilled piers
Shear on the critical section shall be in accordance with the following. Entire reaction
from any driven pile or bored piles, and drilled pier whose center is located !/2
( ! = diameter of the pile) or more outside the critical section shall be considered as
producing shear on that section. Reaction from any driven pile or drilled shaft whose
center is located !/2 or more inside the critical section shall be considered as
producing no shear on that section. For the intermediate position of driven pile or
drilled shaft centers, the portion of the driven pile or shaft reaction to be considered
as producing shear on the critical section shall be based on linear interpolation
between full value at !/2 outside the section and zero value at !/2 inside the
section.
3.9.6.7
Transfer of Forces at the Base of Column.
All forces and moments applied at base of column or pier shall be transferred to top
of footing. If the strength of concrete of footing is less than that of column, then
bearing stress of footing concrete and reinforcement should be checked against
imposed loading.
Lateral forces shall be transferred to supporting footing in accordance with shear
transfer provisions of the relevant sections of the structural design part of the Code.
Bearing on concrete at contact surface between supporting and supported member
shall not exceed concrete bearing strength for either surface.
3.9.6.8
Reinforcement
Reinforcement shall be provided across interface between supporting and supported
member either by extending main longitudinal reinforcement into footings or by
dowels. Reinforcement across interface shall be sufficient to satisfy all of the
following:
(i)
Reinforcement shall be provided to transfer all force that exceeds
concrete bearing strength in supporting and supported member.
(ii) If it is required that loading conditions include uplift, total tensile force
shall be resisted by reinforcement only.
(iii) Area of reinforcement shall not be less than 0.005 times gross area of
supported member (column) with a minimum of 4 bars.
(iv) Minimum reinforcement of footing and raft shall be governed by
temperature and shrinkage reinforcement as per Sec 8.1.11 Chapter 8 of
this Part.
Reinforcement of square footings shall be distributed uniformly across the entire
width of footing. Reinforcement of rectangular footings shall be distributed
uniformly across the entire width of footing in the long direction. In the short
direction, the portion of the total reinforcement given by the following equation shall
be distributed uniformly over a band width (centered on center line of column or
pier) equal to the length of the short side of the footing.
S\< )7+\<\<)" ) 4') > "ℎ
e"'Ð 7\< )7+\<\<)" ) ℎ7" 7\<+" ) 5
(γ1)
(6.3.10)
Here, c is the ratio of the footing length to width. The remainder of reinforcement
required in the short direction shall be distributed uniformly outside the center band
width of footing.
3.9.6.9
Development length and splicing
Computation of development length of reinforcement in footings shall be in
accordance with the relevant sections of the structural design part of the Code.
For transfer of force by reinforcement, development length of reinforcement in
supporting and supported member required splicing shall be in accordance with the
relevant sections (Part. 6, Chapters 6 and 8) of the structural design part of the Code.
Critical sections for development length of reinforcement shall be assumed at the
same locations as defined above as the critical section for moments and at all other
vertical planes where changes in section or reinforcement occur.
##### 3.9.6.10 Dowel size
Diameter of dowels, if used, shall not exceed the diameter of longitudinal
reinforcements.
3.10
Geotechnical Design of Deep Foundations
#### 3.10.1 Driven Precast Piles
The provisions of this article shall apply to the design of axially and laterally loaded
driven piles in soil. Driven pile foundation shall be designed and installed on the
basis of a site investigation report that will include subsurface exploration at
locations and depths sufficient to determine the position and adequacy of the bearing
soil unless adequate data is available upon which the design and installation of the
piles can be based. The report shall include:
(i)
Recommended pile type and capacities
(ii) Driving and installation procedure
(iii) Field inspection procedure
(iv) Requirement of pile load test
(v)
Durability and quality of pile material
(vi) Designation of bearing stratum or strata
A plan showing clearly the designation of all piles by an identifying system shall be
filed prior to installation of such piles. All detailed records for individual piles
shall bear an identification corresponding to that shown on the plan. A copy of
such plan shall be available at the site for inspection at all times during the
construction.
The design and installation of driven pile foundations shall be under the direct
supervision of a competent geotechnical/foundation engineer who shall certify that
the piles as installed satisfy the design criteria.
##### 3.10.1.1 Application
Pile driving may be considered when footings cannot be founded on granular or stiff
cohesive soils within a reasonable depth. At locations where soil conditions would
normally permit the use of spread footings but the potential for scour exists, piles
may be driven as a protection against scour. Piles may also be driven where an
unacceptable amount of settlement of spread footings may occur.
##### 3.10.1.2 Materials
Driven piles may be cast-in-place concrete, pre-cast concrete, pre-stressed concrete,
timber, structural steel sections, steel pipe, or a combination of materials.
##### 3.10.1.3 Penetration
Pile penetration shall be determined based on vertical and lateral load capacities of
both the pile and subsurface materials. In general, the design penetration for any pile
shall be not less than 3D into a hard cohesive or a dense granular material, and not
less than 6D into a soft cohesive or loose a granular material.
##### 3.10.1.4 Estimated pile length
Estimated pile lengths of driven piles shall be shown on the drawing and shall be
based upon careful evaluation of available subsurface information, axial and lateral
capacity calculations, and/or past experience. The maximum length/diameter ratio
should not exceed 50 for a single segmental pile.
##### 3.10.1.5 Types of driven piles
Driven piles shall be classified as "friction" or "end bearing" or a combination of
both according to the manner in which load transfer is developed. The ultimate load
capacity of a pile consists of two parts. One part is due to friction called skin friction
or shaft friction or side shear, and the other is due to end bearing at the base or tip of
the pile. If the skin friction is greater than about 80% of the end bearing load
capacity, the pile is deemed a friction pile and, if the reverse, an end bearing pile. If
ঃযব বহফ নবধৎরহম রং হবমষবপঃবফ,:যব ঢ়রষব রং পধষষবফ ধ ুভষড়ধঃরহম ঢ়রষবচ্.
##### 3.10.1.6 Batter piles
When the lateral resistance of the soil surrounding the piles is inadequate to
counteract the horizontal forces transmitted to the foundation, or when increased
rigidity of the entire structure is required, batter piles should be used in the
foundation. Where negative skin friction loads are expected, batter piles should be
avoided, and an alternate method of providing lateral restraint should be used.
Free standing batter piles are subject to bending moments due to their own weight, or
external forces from other sources. Batter piles in loose fill or consolidating deposits
may become laterally loaded due to settlement of the surrounding soil. In
consolidating clay, special precautions, like provision of permanent casing, shall be
taken.
##### 3.10.1.7 Selection of soil and rock properties
Soil and rock properties defining the strength and compressibility characteristics of
the foundation materials, are required for driven pile design.
##### 3.10.1.8 Pile driving equipment
The pile driving process needs to fulfil assumptions and goals of the design engineer
just as much as the design process has to foresee the conception and installation of
the pile at the site. This is only possible through the selection of the right driving
equipment especially hammer with proper assembly mounted on the most suitable
leader, operated according to the specified practices of installation that consists of a
series of principle and subsidiary procedures.
There are three principal methods of installing precast displacement piles: jacking,
vibratory driving and driving. Jacking is comparatively new method and vibratory
driving is suitable to limited soil and pile types (e.g. loose saturated sand, sheet
piles). The most common method of installing displacement piles is by driving the
piles into the ground by blows of an impact hammer. Because of this, piles installed
in this manner are referred to as driven piles. An efficient method of installation
requires proper use of the equipment for driving.
The pile driving equipment mainly consists of the components like pile hammer, pile
driving leader and driving system components like anvil, cap block, driving head,
follower, pile cushion etc. The key to efficient pile driving is a good match of the pile
with the hammer and the other system components. Mismatches, often result either
inability to drive the pile as specified or in pile damage. A brief account of pile
driving equipment especially related to driving by impact hammers is provided in
Appendix-F.
##### 3.10.1.9 Design capacity of driven precast pile
The design pile capacity is the maximum load that the driven pile shall support with
tolerable movement. In determining the design pile capacity the following items shall
be considered:
(i)
Ultimate geotechnical capacity (axial and lateral).
(ii) Structural capacity of pile section (axial and lateral).
(iii) The allowable axial load on a pile shall be the least value of the above
two capacities.
In determining the design axial capacity, consideration shall be given to the
following:
(i)
The influence of fluctuations in the elevation of ground water table on
capacity.
(ii) The effects of driving piles on adjacent structure and slopes.
(iii) The effects of negative skin friction or down loads from consolidating
soil and the effects of lift loads from expansive or swelling soils.
(iv) The influence of construction techniques such as augering or jetting on
pile capacity.
(v)
The difference between the supporting capacity single pile and that of a
group of piles.
(vi) The capacity of an underlying strata to support load of the pile group.
(vii) The possibility of scour and its effect on axial lateral capacity.
##### 3.10.1.10 Ultimate Geotechnical Capacity of Driven Precast Pile for Axial Load
The ultimate load capacity, ?DÐ", of a pile consists of two parts. One part is due to
friction called skin friction or shaft friction or side shear, ? and the other is due to
end bearing at the base or tip of the pile, ?4.The ultimate axial capacity (?DÐ") of
driven piles shall be determined in accordance with the following for compression
loading.
?DÐ" 5 ? + ?4 −I
(6.3.11)
For uplift loading;
?DÐ" ≤0.7? + I
(6.3.12)
The allowable or working axial load shall be determined as:
?'ÐÐ> 5 ?DÐ"/\&C
(6.3.13)
Where, I is the weight of the pile and \&C is a gross factor of safety as suggested in
Tables 6.3.10a and 6.3.10b. Often, for compression loading, the weight term is
neglected if the weight, I, is considered in estimating imposed loading. The ultimate
bearing capacity (skin friction and/or end bearing) of a single vertical pile may be
determined by any of the following methods.
(i)
By the use of static bearing capacity equations
(ii) By the use of SPT and CPT
(iii) By load tests
(iv) By dynamic methods
##### 3.10.1.11 Static bearing capacity equations for driven precast pile capacity
The skin friction, ? and end bearing ?4 can be calculated as:
? 5 B
(6.3.14a)
?4 5 4B4
(6.3.14b)
Where, = skin friction area (perimeter area) of the pile=Perimeter × Length
B = skin frictional resistance on unit surface area of pile that depends on
soil properties and loading conditions (drained or undrained)
4 = end bearing area of the pile = Cross-sectional area of pile tip (bottom)
B4 = end bearing resistance on unit tip area of pile, that depends on soil
properties to a depth of 2B (B is the diameter for a circular pile section
or length of sides for a square pile section) from the pile tip and loading
conditions (drained or undrained)
For a layered soil system containing n number of layers, end bearing resistance can
be calculated considering soil properties of the layer at which the pile rests, and the
skin friction resistance considers all the penetrating layers calculated as:
? 5 ∑
∆K
)
× (;PY5P±PY) × (B)
(6.3.15)
Where, ∆K represents the thickness of any "ℎ layer and (;PY5P±PY) is the
perimeter of the pile in that layer. The manner in which skin friction is transferred to
the adjacent soil depends on the soil type. In fine-grained soils, the load transfer is
nonlinear and decreases with depth. As a result, elastic compression of the pile is not
uniform; more compression occurs on the top part than on the bottom part of the pile.
For coarse-grained soils, the load transfer is approximately linear with depth (higher
loads at the top and lower at the bottom).
In order to mobilize skin friction and end bearing, some movement of the pile is
necessary. Field tests revealed that to mobilize the full skin friction a vertical
displacement of 5 to 10 mm is required. The actual vertical displacement depends on
the strength of soil and is independent of the pile length and diameter. The full end
bearing resistance is mobilized in driven piles when the vertical displacement is
about 10% of the pile tip diameter. For bored piles or drilled shafts, a vertical
displacement of about 30% of the pile tip diameter is required. The full end bearing
resistance is mobilized when slip or failure zones similar to shallow foundations are
formed. The end bearing resistance can then be calculated by analogy with shallow
foundations. The important bearing capacity factor is :>.
The full skin friction and full end bearing are not mobilized at the same
displacement. The skin friction is mobilized at about one-tenth of the displacement
required to mobilize the end bearing resistance. This is important in deciding on the
factor of safety to be applied to the ultimate load. Depending on the tolerable
settlement, different factors of safety can be applied to skin friction and to end
bearing.
Generally, piles driven into loose, coarse-grained soils tend to density the adjacent
soil. When piles are driven into dense, coarse-grained soils, the soil adjacent to the
pile becomes loose. Pile driving usually remolds fine-grained soils near the pile shaft.
The implication of pile installation is that the intact shear strength of the soil is
changed and one must account for this change in estimations of the load capacity.
3.10.1.12
Axial capacity of driven precast pile in cohesive soil using static bearing
capacity equations
The ultimate axial capacity of driven piles in cohesive may be calculated from static
formula, given by Equations 6.3.14a, 6.3.14b and 6.3.15, using a total stress method
for undrained loading conditions, or an effective stress method for drained loading
conditions. Appropriate values of adhesion factor (α) and coefficient of horizontal
soil stress (m) for cohesive soils that are consistent with soil condition and pile
installation procedure may be used. There are basically two approaches for
calculating skin friction:
(i)
The α-method that is based on total stress analysis and is normally used to
estimate the short term load capacity of piles embedded in fine grained
soils. In this method, a coefficient α is used to relate the undrained shear
strength OD or ZD to the adhesive stress (B) along the pile shaft. As such,
? 5 aOD
(6.3.16)
a = 1.0
for clays with OD ≤ 25 kN/m2
a = 0.5
for clays with OD ≥ 70 kN/m2
a 5 1 −¸
+Òr
s= ¹
for clays with 25 kN/m2 \< OD \< 70 kN/m2
The end bearing in such a case is found by analogy with shallow foundations and
is expressed as:
?4 5 (OD)4(:+)44
(6.3.17)
:+ is a bearing capacity factor and for deep foundation the value is usually 9. OD
রং:যব ঁহফৎধরহবফ ংযবধৎ ংঃৎবহমঃয ড়ভ ংড়রষ ধঃ:যব নধংব ড়ভ:যব ঢ়রষব. ঞযব ংঁভভরী ্তুন্থং ধৎব
indicatives of base of pile. The general equation for :+ is, however, as follows.
:+ 5 6 P1 + 0.2 ¸
B
38¹Q ≤9
(6.3.18)
\#4 represents the diameter of the pile at base and L is the total length of pile. The
skin friction value, B4 5 (OD)4(:+)4 should not exceed 4.0 MPa.
(ii) The c -method is based on an effective stress analysis and is used to
determine both the short term and long term pile load capacities. The
ভৎরপঃরড়হ ধষড়হম:যব ঢ়রষব ংযধভঃ রং ভড়ঁহফ ঁংরহম ঈড়ঁষড়সন্থং ভৎরপঃরড়হ ষধ,ি যিবৎব
the friction stress is given by B 5 CD′ 5 D′ ±L*k′. The lateral effective
stress, D′ is proportional to vertical effective stress, D/′ by a co-efficient,
K. As such,
B 5 0D/′ ±L*k′ 5 cD/′
(6.3.19a)
Where,
c 5 0±L*k′ 5 0±L*k′ 5 (1 −Z*k′)√@ @
(6.3.19b)
k′ is the effective angle of internal friction of soil and OCR is the over-
consolidation ratio. For normally consolidated clay, c varies from 0.25 to 0.29.
The value of c decreases for a very long pile, as such a correction factor is used.
SYYPO±S* BLO±SY BSY c 5 VSQ ¸
1(=
B ¹ ≥0.5
(6.3.19c)
The end bearing capacity is calculated by analogy with the bearing capacity of
shallow footings and is determined from:
B4 5 (D¼′ )4Ô:>Õ4
(6.3.20)
Where, :> is a bearing capacity factor that depends on angle of internal friction
শ′ ড়ভ:যব ংড়রষ ধঃ:যব নধংব ড়ভ:যব ঢ়রষব, ধং ঢ়ৎবংবহঃবফ রহ ঋরমঁৎব ৬.৩.২. ঝঁনংপৎরঢ়ঃ ুনচ্
designates the parameters at the base soil.
Figure 6.3.2 Bearing capacity factor õ for deep foundation (After Berezantzev et. al. 1961)
Bearing Capacity Factor, Nq
Angle of Internal Friction, φ (Degree)
| Col1 | 1000
Nq
Factor,
100
Capacity
Bearing
10
20 25 30 35 40 45 50
Angle of Internal Friction, φ (Degree) | Col3 |
| ---- | ------------------------------------------------------------------------------------------------------------------------------------------- | ---- |
##### 3.10.1.13 Axial Capacity of driven precast pile in cohesive soil using SPT values
Standard Penetration Test N-value is a measure of consistency of clay soil and
indirectly the measure of cohesion. The skin friction of pile can thus be estimated
from N-value. The following relation may be used for preliminary design of ultimate
capacity of concrete piles in clay soil.
For skin friction the relationship is as under.
B 5 1.8:\_º= (\* mPL) ≤70 mPL
(6.3.21)
For end bearing, the relationship is as under.
B4 5 45:º= (in kPa) ≤4000 mPL
(6.3.22)
Where, :\_º= is the average N-value over the pile shaft length and :º= is the N-value
in the vicinity of pile tip. A factor of safety of 3.5 shall be used to estimate allowable
capacity.
##### 3.10.1.14 Axial capacity of driven precast pile in cohesionless soil using static
bearing capacity equations
Piles in cohesionless soils shall be designed by effective stress methods of analysis
for drained loading conditions. The ultimate axial capacity of piles in cohesionless
soils may also be calculated using empirical effective stress method or from in-situ
methods and analysis such as the cone penetration or pressure meter tests. Dynamic
formula may be used for driven piles in cohesionless soils such as gravels, coarse
sand and deposits where pore pressure developed due to driving is quickly dissipated.
For piles in cohesionless soil, the ultimate side resistance may be estimated using the
following formula:
B 5 cD/′
(6.3.23)
Where, D/′ is the effective vertical stress at the level under consideration. The values
for β are as under.
c = 0.10
for k = 33
c = 0.20
for k = 35
c = 0.35
for k = 37
For uncemented calcareous sand the value of c varies from 0.05 to 0.10.
The following equation, as used for cohesive soil, may be used to compute the
ultimate end bearing capacity of piles in sandy soil in which, the maximum effective
stress, D/′ allowed for the computation is 240 kPa. Figure 6.3.2 may also be used to
estimate the value of :>.
B4 5 (D¼′ )4Ô:>Õ4
(6.3.24)
:> = 8 to 12
for loose sand
:> = 12 to 40
for medium sand
:> = 40
for dense sand
##### 3.10.1.15 Critical depth for end bearing and skin friction
The vertical effective stress (D¼′ or D/′ ) increases with depth. Hence the skin friction
should increase with depth indefinitely. In reality skin friction does not increase
indefinitely. It is believed that skin friction would become a constant at a certain
depth. This depth is named critical depth. Pile end bearing in sandy soils is also related
to effective stress. Experimental data indicates that end bearing capacity does not also
increase with depth indefinitely. Due to lack of a valid theory, Engineers use the same
critical depth concept adopted for skin friction for end bearing capacity as well. Both
the skin friction and the end bearing capacity are assumed to increase till the critical
depth, #+ and then maintain a constant value. Following approximations may be used
for the critical depth in relation to diameter of pile, D.
\#+ 5 10#
for loose sand
\#+ 5 15#
for medium dense sand
\#+ 5 20#
for dense sand
##### 3.10.1.16 Axial Capacity of Driven Precast Pile in Cohesionless Soil using SPT
Values
Standard Penetration Test N-value is a measure of relative density hence angle of
internal friction of cohesionless soil. The skin friction of pile can thus be estimated
from N-value. The following relation may be used for ultimate capacity of concrete
piles in cohesionless soil and non-plastic silt.
For skin friction the relationship is as under.
For sand:
B 5 2:\_º= (in kPa) ≤60 kPa
(6.3.25)
For non-plastic silt:
B 5 1.7:\_º= (in kPa) ≤60 kPa
(6.3.26)
For end bearing, the relationship is as under.
For sand:
B4 5 40:º= ¸
B
3¹ (in kPa) ≤400:º= and ≤11000 kPa
(6.3.27)
For non-plastic silt:
B4 5 30:º= ¸
B
3¹ (in kPa) ≤300:º= and ≤11000 kPa
(6.3.28)
Where, :\_º= is the average N-value over the pile shaft length and :º= is the N-value
in the vicinity of pile tip. A higher factor of safety of 3.5 should be used to estimate
allowable capacity.
##### 3.10.1.17 Axial capacity of driven precast pile using pile load Test
Generally, the load on test pile to determine ultimate capacity is twice the design
load. The test load on service/working pile is 1.5 times the design load. The
following criteria should be met in deciding the allowable/safe pile capacity.
Safe Load for Single Pile
(a)
Two thirds of the final load at which the load displacement attains a value
of 12 mm unless otherwise required in a given case on the basis of nature
and type of structure in which case, the safe load should be corresponding
to the stated total displacement permissible.
(b)
Fifty (50) percent of the final load at which the total displacement equals
to 10 percent of pile diameter case of uniform diameter piles and 7.5
percent of bulb diameter in case of under-reamed piles.
Safe Load for Pile Group
(a)
Final load at which the load displacement attains a value of 25 mm unless
otherwise required in a given case on the basis of nature and type of
structure, and
(b)
Two thirds of the final load at which the total displacement attains a value
of 40 mm.
3.10.1.18
Selection of factor of safety for driven precast pile
Driven pile in soil shall be designed for a minimum overall factor of safety of 2.0
against bearing capacity failure (end bearing, side resistance or combined) when the
design is based on the results of a load test conducted at the site, with good quality
control. Otherwise, it shall be designed for a minimum factor of safety 3.0. The
minimum recommended overall factor of safety is based on an assumed normal level
of field quality control during construction. If a normal level of field quality control
cannot be assured, higher minimum factors of safety shall be used. The
recommended values of overall factor of safety on ultimate axial load capacity based
on specified construction control is given in Tables 6.3.10a and 6.3.10b.
Partial factor of safety may be used independently for skin friction and end bearing.
The values of partial factor of safety may be taken as 1.5 and 3.0 respectively for
skin friction and end bearing. The design/allowable load may be taken as the
minimum of the values considering overall and partial factor of safety.
Table 6.3.10a: Factor of Safety for Deep Foundation for Downward and Upward
Load
Structure
Design
Life (yrs.)
Probability
of Failure
Design Factor of Safety
Good
Control
Normal
Control
Poor
Control
V. Poor
Control
Monument
> 100
> 10-5
> 2.30
> 3.00
> 3.50
> 4.00
> Permanent
> 25 -100
> 10-4
> 2.00
> 2.50
> 2.80
> 3.00
> Temporary
> \< 25
> 10-3
> 1.40
> 2.00
> 2.30
> 2.80
> Table 6.3.10b: Guidelines for Investigation, Analysis and Construction Control
> Item
> Good
> Control
> Normal
> Control
> Poor
> Control
> V. Poor
> Control
> Proper Subsoil Investigation
> Yes
> Yes
> Yes
> Yes
> Proper Review of Subsoil
> Report
> Yes
> Yes
> Yes
> Yes
> Supervision by Competent
> Geotechnical/Foundation
> Engineer
> Yes
> Yes
> Yes
> No
Item
Good
Control
Normal
Control
Poor
Control
V. Poor
Control
Load Test Data
Yes
Yes
Yes
No
Qualification of Contractor
Yes
Yes
No
No
Proper Construction
ঊয়ঁরঢ়সবহঃ্থং
Yes
No
No
No
Maintaining Proper
Construction Log
Yes
No
No
No
##### 3.10.1.19 Group piles and group capacity of driven precast piles
All piles shall be braced to provide lateral stability in all directions. Three or more
piles connected by a rigid cap shall be considered as being braced (stable), provided
that the piles are located in a radial direction from the centroid of the group, not less
than 60o apart circumferentially. A two pile group in a rigid cap shall be considered
to be braced along the axis connecting the two piles. Piles supporting walls shall be
driven alternately in lines at least 300 mm apart and located symmetrically under the
centre of gravity of the wall load, unless effective measures are taken to cater for
eccentricity and lateral forces, or the wall piles are adequately braced to provide
lateral stability. Individual piles are considered stable if the pile tops are laterally
braced in two directions by construction, such as a structural floor slab, grade beams,
struts, or walls.
Group pile capacity of driven piles should be determined as the product of the group
efficiency, number of piles in the group and the capacity of a single pile. In general, a
group efficiency value of 1.0 should be used except for friction piles driven in
cohesive soils. The minimum center-to-center pile spacing of 2.5B is recommended.
The nominal dimensions and length of all the piles in a group should be similar.
##### 3.10.1.20 Pile caps
Pile caps shall be of reinforced concrete. The soil immediately below the pile cap
shall not be considered as carrying any vertical load. The tops of all piles shall be
embedded not less than 75 mm into pile caps and the cap shall extend at least 100
mm beyond the edge of all piles. The tops of all piles shall be cut back to sound
material before capping. The pile cap shall be rigid enough, so that the imposed load
can be distributed on the piles in a group equitably. The cap shall generally be cast
over a 75 mm thick levelling course of concrete. The clear cover for the main
reinforcement in the cap slab under such condition shall not be less than 50 mm.
##### 3.10.1.21 Lateral load capacity on driven precast piles
Lateral capacity of vertical single piles shall be the least of the values calculated on
the basis of soil failure, structural capacity of the pile and deflection of the pile head.
In the analysis, pile head conditions (fixed-head or free-head) should be considered.
For estimating the depth of fixity, established method of analysis shall be used. The
main reinforcement of pile foundation is usually governed by the lateral load
capacity and vice versa. Deflection calculations require horizontal subgrade modulus
of the surrounding soil. When considering lateral load on piles, the effect of other
coexistent loads, including axial load on the pile, shall be taken into consideration for
checking structural capacity of the shaft.
To determine lateral load capacity, lateral load tests shall be performed with at least
two times the proposed design working load. Allowable lateral load capacity will be
the least from the following criteria.
(i)
Half of the lateral load at which lateral movement of the pile head is 12
mm or lateral load corresponding to any other specified displacement as
per performance requirements.
(ii) Final load at which the total displacement corresponds to 5 mm or lateral
load corresponding to any other specified displacement as per
performance requirements.
All piles standing unbraced in air, water or soils not capable of providing lateral
support shall be designed as columns in accordance with the provisions of this Code.
##### 3.10.1.22 Vertical ground movement and negative skin friction in driven precast
piles
The potential for external loading on a pile by vertical ground movements shall be
considered as part of the design. Vertical ground movements may result in negative
skin friction or downdrag loads due to settlement of compressible soils or may result
in uplift loads due to heave of expansive soils. For design purposes, the full
magnitude of maximum vertical ground movement shall be assumed.
Driven piles installed in compressible fill or soft soil subject to compression shall be
designed against downward load due to downdrag. The potential for external loading
on a pile by negative skin friction/downdrag due to settlement of compressible soil
shall be considered as a part of the design load. Evaluation of negative skin friction
shall include a load-transfer method of analysis to determine the neutral point (i.e.,
point of zero relative displacement) and load distribution along shaft. Due to the
possible time dependence associated with vertical ground movement, the analysis
shall consider the effect of time on load transfer between the ground and shaft and
the analysis shall be performed for the time period relating to the maximum axial
load transfer to the pile. Negative skin friction loads may be reduced by application
of bitumen or other viscous coatings to the pile surfaces. In estimating negative skin
friction the following factors shall be considered :
(i)
Relative movement between soil and pile shaft.
(ii)
Relative movement between any underlying compressible soil and pile
shaft.
(iii)
Elastic compression of the pile under the working load.
(iv)
The rate of consolidation of the compressible layer.
(v)
Negative skin friction is mobilized only when tendency for relative
movement between pile shaft and surrounding soil exists.
##### 3.10.1.23 Driven precast pile in expansive soils (upward movement)
Piles driven in swelling soils may be subjected to uplift forces in the zone of seasonal
moisture change. Piles shall extend a sufficient distance into moisture-stable soils to
provide adequate resistance to swelling uplift forces. In addition, sufficient clearance
shall be provided between the ground surface and the underside of pile caps or grade
beams to preclude the application of uplift loads at the pile cap. Uplift loads may be
reduced by application of bitumen or other viscous coatings to the pile surface in the
swelling zone.
##### 3.10.1.24 Dynamic/Seismic Design of Driven Precast Pile
In case of submerged loose sands, vibration caused by earthquake may cause
liquefaction or excessive total and differential settlements. This aspect of the problem
shall be investigated and appropriate methods of improvements should be adopted to
achieve suitable values of N. Alternatively, large diameter drilled pier foundation
shall be provided and taken to depths well into the layers which are not likely to
liquefy.
3.10.1.25
Protection against corrosion and abrasion in driven precast pile
Where conditions of exposure warrant a concrete encasement or other corrosion
protections shall be used on steel piles and steel shells. Exposed steel piles or steel
shells shall not he used in salt or brackish water, and only with caution in fresh water.
Details are given in Sec 3.6.2.
3.10.1.26
Dynamic monitoring of driven precast pile
Dynamic monitoring may be specified for piles installed in difficult subsurface
conditions such as soils with obstructions and boulders to evaluate compliance with
structural pile capacity. Dynamic monitoring may also be considered for
geotechnical capacity verification, where the size of the project or other limitations
deters static load testing.
##### 3.10.1.27 Maximum allowable driving stresses in driven precast pile
Maximum allowable driving stresses in pile material for top driven piles shall not
exceed 0.9BH (compression), 0.9BH (tension) for steel piles, 0.85B+′ concrete
(compression) and 0.7BH steel reinforcement (tension) for concrete piles and
0.85B+′ −B!+ (compression) for prestressed concrete piles.
3.10.1.28
Effect of buoyancy in driven precast pile
The effects of hydrostatic pressure shall be considered in the design of driven piles,
where used with foundation subjected to buoyancy forces.
3.10.1.29
Protection against Deterioration of Driven Precast Piles
(a)
Steel Pile
A steel pile design shall consider that steel piles may be subject to
corrosion, particularly in fill soils (low pH soils, acidic, pH value \<5.5)
and marine environments. In fact, extremely acid soils (below pH 4.5)
and very strongly alkaline soils (above pH 9.1) have significantly high
corrosion loss rates when compared to other soils. For structural
elements, the Code considers a site to be corrosive if one or more of the
following conditions exist for the representative soil and/or water samples
taken at the site: Chloride concentration is 500 ppm or greater, sulfate
concentration is 2000 ppm or greater, or the pH is less than 6. A field
electric resistivity survey or resistivity testing and pH testing of soil and
ground water samples should be used to evaluate the corrosion potential.
Methods of protecting steel piling in corrosive environments include use
of protective coatings, cathodic protection, and increased steel area. The
corrosion guidelines are provided in Tables 6.3.6a and 6.3.6b.
(b)
Concrete Pile
A concrete pile foundation design shall consider that deterioration of
concrete piles can occur due to sulfates in soil, ground water, or sea
water; chlorides in soils and chemical wastes; acidic ground water an
organic acids. Laboratory testing of soil and ground water samples for
sulfates and pH is usually sufficient to assess pile deterioration potential.
A full chemical analysis of soil and water samples is recommended when
chemical wastes are suspected. Methods of protecting concrete piling
include dense impermeable concrete, sulfate resisting Portland cement,
minimum cover requirements for reinforcement and use of epoxies,
resins, or other protective coatings.
(c)
Timber Pile
A timber pile foundation (used for temporary structures) design shall
consider that deterioration of timber piles can occur due to decay from
wetting and drying cycles or from insects or marine borers Methods of
protecting timber piling include pressure treating with creosote or other
wood preservers.
3.10.1.30
Pile spacing, clearance and embedment in driven precast pile
End bearing driven piles shall be proportioned such that the minimum center-to-
center pile spacing shall exceed the greater of 750 mm or 2.5 pile diameters/widths.
The distance from the side of any pile to the nearest edge of the pile cap shall not be
less than 100 mm. The spacing of piles shall be that the average load on the
supporting strata will not exceed the safe bearing value of those strata as determined
by test boring or other established methods.
Piles deriving their capacity from frictional resistance shall be sufficiently apart to
ensure that the zones of soil from which the piles derive their support do not overlap
to such an extent that their bearing values are reduced. Generally, in such cases, the
spacing shall not be less than 3.0 times the diameter of the shaft. The tops of piles
shall project not less than 75 mm into concrete after all damaged pile material has
been removed.
3.10.1.31
Structural capacity of driven precast pile section
The cross-section of driven piles shall be of sufficient size and pile material shall
have the necessary structural strength to resist all handling stresses during driving or
installation and the necessary strength to transmit the load imposed on them to the
underlying and surrounding soil. Pile diameter/cross-section of a pile shaft at any
level shall not be less than the designated nominal diameter/cross-section. The
structural design of piles must consider each of the following loading conditions.
(i)
Handling loads are those imposed on the pile between the time it is
fabricated and the time it is in the pile driver leads and ready to be driven.
They are generated by cranes, forklifts, and other construction equipment.
(ii) Driving loads are produced by the pile hammer during driving.
(iii) Service loads are the design loads from the completed structures.
The maximum allowable stress on a pile shall not exceed 0.33B+′ for precast concrete
piles and 33B+′ −B!+ for prestressed concrete piles and 0.25BH for steel H-piles. The
axial carrying capacity of a pile fully embedded in soil with undrained shear strength
greater than 10 kN/m2 shall not be limited by its strength as long column. For driven
piles in weaker soils (undrained shear strength less than 10 kN/m2), due
consideration shall be given to determine whether the shaft behaves as a long column
or not. If necessary, suitable reductions shall be made in its structural strength
considering buckling. The effective length of a pile not secured against buckling by
adequate bracing shall be governed by fixity conditions imposed on it by the
structure it supports and by the nature of the soil in which it is installed.
Minimum Reinforcement in Driven Concrete Pile
The longitudinal and transverse steel provided in piles should enable the pile to :
Withstand handling stresses
Endure driving stresses
Provide the necessary structural capacity
The maximum bending stress is produced while handling if the pile is pitched at the
head. To prevent whipping during handling, length/diameter ratio of the pile should
never exceed 50. Otherwise, segmental pile should be used. Considering all of these,
the recommended area of main reinforcement for precast concrete piles, designed
mainly for vertical load with small lateral capacity, should not be less than the
following percentages of the cross sectional area of the piles. In all cases, its
adequacy for handling stresses shall be checked. The following reinforcement
provisions may not be valid for laterally loaded piles or piles for uplift resistance.
(i)
Pile length \< 30 times the least width : 1.00%
(ii) Pile length 30 to 40 times the least width : 1.5%
(iii) Pile length > 40 times the least width : 2%
The lateral reinforcement resists the driving stresses induced in the piles and should
be in the form hoops or links of diameter not less than 6 mm. The volume of lateral
reinforcement shall not be less than the following :
(i)
At each end of the pile for a distance of about three times the least
width/diameter – not less than 0.4% of the gross volume of the pile.
(ii) In the body of the pile – not less than 0.2% of the gross volume of the
pile.
(iii) The transition between closer spacing and the maximum should be
gradual over a length of 3 times the least width/diameter.
Minimum Grades of Concrete
The minimum 28 days cylinder strength of concrete for driven piles is 21 MPa.
Depending on driving stresses, the following grades of concrete should be used.
(i)
For hard driving (driving stress > 1000 kN/m2) – 28 MPa
(ii) For easy driving (driving stress ≤ 1000 kN/m2) – 21 MPa
#### 3.10.2 Driven Cast-in-Place Concrete Piles
Driven cast-in-place concrete piles shall be in general cast in metal shells driven into
the soil that will remain permanently in place. However, other types of cast-in-place
piles, plain or reinforced, cased or uncased, may be used if the soil conditions permit
their use and if their design and method of placing are satisfactory.
##### 3.10.2.1 Shape
Cast-in-place concrete piles may have a uniform cross-section or may be tapered
over any portion.
##### 3.10.2.2 Minimum area
The minimum area at the butt of the pile shall be 650 cm2 and the minimum diameter
at the tip of the pile shall be 200 mm.
##### 3.10.2.3 General reinforcement requirements
Depending on the driving and installation conditions and the loading condition, the
amount of reinforcement and its arrangement shall vary. Cast-in-place piles, carrying
axial loads only, where the possibility of lateral forces being applied to the piles is
insignificant, need not be reinforced where the soil provides adequate lateral support.
Those portions of cast-in-place concrete piles that are not supported laterally shall be
designed as reinforced concrete columns and the reinforcing steel shall extend 3000
mm below the plane where the soil provides adequate lateral restraint. Where the
shell is smooth pipe and more than 3 mm in thickness, it may be considered as load
carrying in the absence of corrosion. Where the shell is corrugated and is at least 2
mm in thickness, it may be considered as providing confinement in the absence of
corrosion.
##### 3.10.2.4 Reinforcement in superstructure
Sufficient reinforcement shall be provided at the junction of the pile with the
superstructure to make a suitable connection. The embedment of the reinforcement
into the cap shall be as specified for precast piles.
##### 3.10.2.5 Shell requirements
The shell shall be of sufficient thickness and strength, so as to hold its original form
and show no harmful distortion after it and adjacent shells had driven and the driving
core, if any, has been withdrawn. The plans shall stipulate that alternative designs of
the shell must be approved by the Engineer before driving is done.
##### 3.10.2.6 Splices
Piles may be spliced provided the splice develops the full strength of the pile. Splices
should be detailed on the contract plans. Any alternative method of splicing
providing equal results may be considered for approval.
##### 3.10.2.7 Reinforcement cover
The reinforcement shall be placed a clear distance of not less than 50 mm from the
cased or uncased sides. When piles are in corrosive or marine environments, or when
concrete is placed by the water or slurry displacement methods, the clear distance
shall not be less than 75 mm for uncased piles and piles with shells not sufficiently
corrosion resistant. Reinforcements shall extend to within 100 mm of the edge of the
pile cap.
##### 3.10.2.8 Installation
Steel cased piles shall have the steel shell mandrel driven their full length in contact
with surrounding soil, left permanently in place and filled with concrete. No pile
shall be driven within 4.5 times the average pile diameter of a pile filled with
concrete less than 24 hours old. Concrete shall not be placed in steel shells within the
heave range of driving.
##### 3.10.2.9 Concreting
For bored or driven cast-in-situ piles, concrete shall be deposited in such a way as to
preclude segregation. Concrete shall be deposited continuously until it is brought to
the required level. The top surface shall be maintained as level as possible and the
formation of seams shall be avoided.
For under-reamed piles, the slump of concrete shall range between 100 mm and 150
mm for concreting in water free holes. For large diameter holes concrete may be
placed by tremie or by drop bottom bucket; for small diameter boreholes a tremie
shall be utilized.
A slump of 125 mm to 200 mm shall be maintained for concreting by tremie. In case
of tremie concreting for piles of smaller diameter and length up to 10 m, the
minimum cement content shall be 350 kg/m3 of concrete. For larger diameter and/or
deeper piles, the minimum cement content shall be 400 kg/m3 of concrete.
For concreting under water, the concrete shall contain at least 10 percent more
cement than that required for the same mix placed in the dry. The amount of coarse
aggregate shall be not less than one and a half times, nor more than two times, that of
the fine aggregate. The materials shall be so proportioned as to produce a concrete
having a slump of not less than 125 mm, nor more than 200 mm.
##### 3.10.2.10 Structural integrity
Bored piles shall be installed in such a manner and sequence as to prevent distortion
or damage to piles being installed or already in place, to the extent that such
distortion or damage affects the structural integrity of pile.
#### 3.10.3 Prestressed Concrete Piles
##### 3.10.3.1 Shape and size
Prestressed concrete piles that are generally octagonal, square or circular shall be of
approved size and shape. Concrete in prestressed piles shall have a minimum
compressive strength (cylinder), B+′ of 35 MPa at 28 days. Prestressed concrete piles
may be solid or hollow. For hollow piles, precautionary measures should be taken to
prevent breakage due to internal water pressure during driving.
##### 3.10.3.2 Reinforcement
Within the context of this Code, longitudinal prestressing is not considered as load-
bearing reinforcement. Sufficient prestressing steel in the form of high-tensile wire,
strand, or bar should be used so that the effective prestress after losses is sufficient to
resist the handling, driving, and service-load stresses. Post-tensioned piles are cast
with sufficient mild steel reinforcement to resist handling stresses before stressing.
For pretensioned piles, the longitudinal prestressing steel should be enclosed in a
steel spiral with the minimum wire size ranging from ACI 318 W3.5 (nominal area
### 0.035 in2, nominal dia=0.211 inch) to W5 (nominal area 0.05 in2, nominal dia=0.252
inch) depending on the pile size. The wire spiral should have a maximum 6 in. (150
mm) pitch with closer spacing at each end of the pile and several close turns at the tip
and pile head. The close spacing should extend over at least twice the diameter or
thickness of the pile, and the few turns near the ends are often at 1 in. (25 mm)
spacing. Occasionally, prestressed piles are designed and constructed with
conventional reinforcement in addition to the prestressing steel to increase the
structural capacity and ductility of the pile. This reinforcement reduces the stresses in
the concrete and should be taken into account.
For prestressed concrete piles, the effective prestress after all losses should not be
less than 700 lb/in2 (4.8 MPa). Significantly higher effective prestress values are
commonly used and may be necessary to control driving stresses in some situations.
Bending stresses shall be investigated for all conditions of handling, taking into
account the weight of the pile plus 50 percent allowance for impact, with tensile
stresses limited to 5tB+′.
##### 3.10.3.3 Vertical and spiral reinforcement
The full length of vertical reinforcement shall be enclosed within spiral
reinforcement. For piles up to 600 mm in diameter, spiral wire shall be No.5 (U.S.
Steel Wire Gage). Spiral reinforcement at the ends of these piles shall have a pitch of
75 mm for approximately 16 turns.
In addition, the top 150 mm of pile shall have five turns of spiral winding at 25 mm
pitch. For the remainder of the pile, the vertical steel shall be enclosed with spiral
reinforcement with not more than 150 mm pitch. For piles having diameters greater
than 600 mm. spiral wire shall be No.4 (U.S. Steel Wire Gauge). Spiral
reinforcement at the end of these piles shall have a pitch of 50 mm for approximately
16 turns. In addition, the top 150 mm of pile shall have four turns of spiral winding at
38 mm pitch. For the remainder of the pile, the vertical steel shall be enclosed
with spiral reinforcement with not more than 100 mm pitch. The reinforcement shall
be placed at a clear distance from the face of the prestressed pile of not less than
50 mm.
##### 3.10.3.4 Driving and handling stresses
A prestressed pile shall not be driven before the concrete has attained a compressive
strength of at least 28 MPa, but not less than such strength sufficient to withstand
handling and driving forces.
#### 3.10.4 Bored Piles
In bored cast in place piles, the holes are first bored with a permanent or temporary
casing or by using bentonite slurry to stabilize the sides of the bore. A prefabricated
steel cage is then lowered into the hole and concreting is carried by tremie method.
##### 3.10.4.1 Shape and size
Bored cast-in-situ concrete piles that are generally circular in section shall be of
approved size and shape. Concrete in bored cast-in-situ concrete piles shall have a
minimum compressive strength (cylinder), B+′ of 21 MPa at 28 days.
##### 3.10.4.2 Dimension
All shafts should be sized in 50 mm increments with a minimum shaft diameter of
400 mm.
##### 3.10.4.3 Ultimate geotechnical capacity of bored pile for axial load
The basic concept of ultimate bearing capacity and useful equations for axial load
capacity are identical to that of driven pile as described in Art. 3.10.1.10.
##### 3.10.4.4 Axial capacity of bored piles in cohesive soil using static bearing capacity
equations
The ultimate axial capacity of bored piles in cohesive may be calculated from the
same static formula as used for driven piles, given by Equations 6.3.14a, 6.3.14b and
6.3.15, using a total stress method for undrained loading conditions, or an effective
stress method for drained loading conditions. The skin friction B may be taken as
2/3rd the value of driven piles and the end bearing B4 may be taken as 1/3rd of that of
driven pile.
##### 3.10.4.5 Axial capacity of bored piles in cohesive soil using SPT values
The following relations may be used for preliminary design of ultimate capacity of
concrete bored piles in clay soils.
For skin friction the relationship is as under.
B 5 1.2:\_º= (in kPa) ≤70 kPa
(6.3.29)
For end bearing, the relationship is as under.
B4 5 25:º= (in kPa) ≤4000 kPa
(6.3.30)
Where, :\_º= is the average N-value over the pile shaft length and :º= is the
N-value in the vicinity of pile tip. A higher factor of safety of 3.5 should be used to
estimate allowable capacity.
##### 3.10.4.6 Axial capacity of bored piles in cohesionless soil using static bearing
The ultimate axial capacity of bored piles in cohesive soil may be calculated from the
same static formula as used for driven piles described in Sec 3.10.1.10. The skin
friction B may be taken as 2/3rd the value of driven pile and the end bearing B4 may
be taken as 1/3rd of driven pile.
Critical Depth for End Bearing and Skin Friction
Similar to driven piles, following approximations may be used for the critical depth
in relation to pile diameter, D.
\#+ 5 10#
for loose sand
\#+ 5 15#
for medium dense sand
\#+ 5 20#
for dense sand
##### 3.10.4.7 Axial capacity of bored piles in cohesionless soil using SPT values
The following relations may be used for preliminary design of ultimate capacity of
concrete bored piles in sand and non-plastic silty soils.
For skin friction the relationship is as under :
For sand
B 5 1.0:\_º= (in kPa) ≤60 kPa
(6.3.31)
For non-plastic silt:
B 5 0.9:\_º= (in kPa) ≤60 kPa
(6.3.32)
For end bearing, the relationship is as under.
For sand
B4 5 15:º= ¸
B
3¹ (in kPa) ≤150:º= and ≤4000 kPa
(6.3.33)
For non-plastic silt:
B4 5 10:º= ¸
B
3¹ (in kPa) ≤100:º= and ≤4000 kPa
(6.3.34)
Where,
N
is the average N-value over the pile shaft length and N60 is the N-value
in the vicinity of pile tip (down to a depth of 3D). A higher factor of safety of 3.5
should be used to estimate allowable capacity.
##### 3.10.4.8 Axial capacity of bored pile using pile load test
The procedures and principles of pile load test for ultimate capacity are similar to
that of driven piles.
##### 3.10.4.9 Structural capacity of bored concrete pile/drilled shaft
Minimum Reinforcement in Bored Concrete Pile
For piles loaded in compression alone, it is generally only necessary to reinforce the
shaft to a depth of 2 m greater than the depth of temporary casing to prevent any
tendency for concrete lifting when pulling the casing. Piles subject to tension or
lateral forces and eccentric loading (possibly being out of position or out of plumb)
do however require reinforcement suitable to cope with these forces. The following
criteria for typical nominal reinforcement for piles in compression shall be
considered. Table 6.3.11 may be used as guidelines. The restrictions that apply to the
use of this Table have to be carefully considered in any particular application.
Table 6.3.11: Guidance on the Minimum Reinforcing Steel for Bored Cast-in-place Piles
Pile Diameter
(mm)
Main Reinforcement
Lateral (Hoop)
Reinforcement
Bar Size
(mm)
No. of Bars
Bar Size (mm)
Pitch (mm)
Notes:
(a)
Yield strength of steel = 420 MN/m2
(b)
ঞযব ধনড়াব মঁরফবষরহবং ধৎব ভড়ৎ ুনঁরষফ-ধনরষরঃুচ্ ড়হষু: ঞযবু ধৎব হড়ঃ
appropriate Where:
(i)
Piles are required to resist any applied tensile or bending forces- the
reinforcement has to be designed for the specific loading conditions.
(ii)
Piles are required to accommodate positional and verticality
tolerances, or where they are constructed through very soft alluvial
deposits (cu \< 10 kN/m2). Specific reinforcement design is then
necessary.
(c)
Minimum depth of reinforcement is taken as 3 m below cutoff for simple
bearing only. Any lateral loads or moments taken by the pile will require
reinforcement to extend to some depth below the zone subjected to
bending forces. This zone may be determined from a plot of the bending
moment with depth. Furthermore the reinforcement would normally
extend at least 1 m below the depth of any temporary casing.
(d)
Even with the appropriate reinforcement care will still be required to
prevent damage to piles by construction activities especially during
cutting-down or in the presence of site traffic.
The longitudinal reinforcement shall be of high yield steel bars (min BH = 420 Mpa)
and shall not be less than:
0.5% of +
for + ≤ 0.5 m2;
0.375% of +
for 0.5 m2 \< + ≤ 1 m2;
0.25% of +
for + > 1.0 m2;
Where, + is the gross cross-sectional area of the pile. The minimum diameter for the
longitudinal bars should not be less than 16 mm for large diameter (diameter ≥ 600
mm) piles. Piles should have at least 6 longitudinal bars.
The assembled reinforcement cage should be sufficiently strong to sustain lifting and
lowering into the pile bore without permanent distortion or displacement of bars or in
addition bars should not be so densely packed that concrete aggregate cannot pass
freely between them. Hoop reinforcement (for shear) is not recommended closer than
100 mm centres. Minimum Concrete cover to the reinforcement periphery shall be 75
mm. This guidance is only applicable for piles with vertical load.
Minimum Grades of Concrete
The integrity of pile shaft is of paramount importance, and the concreting mixes and
methods that have been evolved for bored piles are directed towards this as opposed
to the high strength concrete necessary for precast piles or structural work above
ground. This prerequisite has led to the adoption of highly workable mixes, and the
ুঃড়ঃধষ পড়ষষধঢ়ংবচ্ সরী ভড়ৎ:ৎবসরব ঢ়রষবং যধং নববহ সবহঃরড়হবফ. ওহ ড়ৎফবৎ:ড় বহংঁৎব:যধঃ:যব
concrete flows between the reinforcing bars with ease, and into the interstices of the
soil, a high slump, self-compacting mix is called for. A minimum cement content of
350 kg/m3 is generally employed under dry placement condition, increasing to 400
kg/m3 under submerged condition at slumps greater than 125 mm, with a
corresponding increase in fine aggregate content to maintain the cohesion of the mix.
The water cement ratio in all cases is recommended as 0.45. Three mixes as
recommended are given in Table 6.3.12.
Table 6.3.12: Recommended Concrete Slumps for Cast-in-place Bored Piles
Mix
Slump (mm)
Conditions of use
A
Poured into water-free unlined bore. Widely spaced
reinforcement leaving ample room for free movement of the
concrete between bars
Mix
Slump (mm)
Conditions of use
B
Where reinforcement is not placed widely enough to give
free movement of concrete between bars. Where cutoff level
of concrete is within casing. Where pile diameter is \< 600
mm.
C
Where concrete is to be placed by tremie under water or
bentonite in slurry.
3.10.4.10
Selection of factor of safety for bored pile
Selection of factor of safety for axial capacity of bored pile is similar to that used for
driven piles.
3.10.4.11
Group capacity of bored pile
The behavior of group bored piles is almost similar to that of driven piles. For the
pile cap, lateral load capacity, vertical ground movement, negative skin friction, piles
in expansive soil, dynamic and seismic design, corrosion protection, dynamic
monitoring and buoyancy. Sec 3.10.1.18 should be consulted as they are similar for
both driven and bored piles. However, Individual bored piles are considered stable if
the pile tops are laterally braced in two directions by construction, such as a
structural floor slab, grade beams, struts, or walls. Generally, the use of a single pile
as foundation is not recommended unless the diameter is 600 mm or more.
#### 3.10.5 Settlement of Driven and Bored Piles
The settlement of axially loaded piles and pile groups at the allowable loads shall be
estimated. Elastic analysis, load transfer and/or finite element techniques may be
used. The settlement of the pile or pile group shall not exceed the tolerable
movement limits as recommended for shallow foundations (Table 6.3.7). When a pile
is loaded, two things would happen involving settlement.
The pile would settle into the soil
The pile material would compress due to load
The settlement of a single pile can be broken down into three distinct parts.
Settlement due to axial deformation, C'
Settlement at the pile tip, C!"
Settlement due to skin friction, C
C"( )Ð\<) 5 C' + C!" + C
(6.3.35a)
Moreover, piles acting in a group could undergo long term consolidation settlement.
Settlement due to axial deformation of a single pile can be estimated as :
C' 5
ÔRT³'R
ÕB
wHU
(6.3.35b)
Where,
?! = Load transferred to the soil at tip level
? = Total skin friction load
L = Length of the pile
A = Cross section area of the pile
%অ = ণড়ঁহম্থং সড়ফঁষঁং ড়ভ ঢ়রষব সধঃবৎরধষ
L = 0.5 for clay and silt soils
\= 0.67 for sandy soil
Pile tip settlement, C!" can be estimated as :
C!" 5
+TRT
3>Ý
(6.3.35c)
Where,
?! = Load transferred to the soil at tip level
# = Diameter of the pile
X = Ultimate end bearing capacity
! = Empirical coefficient as given in Table 6.3.13
Table 6.3.13: Typical Values of "V for Settlement Calculation of Single Pile
Soil Type
Values of "V
Driven Pile
Bored Pile
Dense Sand
0.02
0.09
Loose Sand
0.04
0.18
Stiff Clay
0.02
0.03
Soft Clay
0.03
0.06
Dense Silt
0.03
0.09
Loose Silt
0.05
0.12
Skin friction acting along the shaft would stress the surrounding soil. Skin friction
acts upward direction along the pile. The force due to pile on surrounding soil would
be in downward direction. When the pile is loaded, the pile would slightly move
down. The pile would drag the surrounding soil with it. Hence, the pile settlement
would occur due to skin friction as given by :
C 5
\+
R
3>Ý
(6.3.36)
Where,
= Empirical coefficient 5 ¸0.93 + 0.16
B
3¹ !
! = Empirical coefficient as given in Table 6.3.9
? = Total skin friction load
# = Diameter of the pile
X = Ultimate end bearing capacity
Short Term Pile Group Settlement
Short term or elastic pile group settlement can be estimated using the following
relation.
C 5 C"( )Ð\<) ¸
A
3¹
\=.r
(6.3.37)
Where,
C = Settlement of the pile group
C"( )Ð\<) = Total settlement of a single pile
\= Smallest dimension of the pile group
# = Diameter of the pile
Interestingly, geometry of the group does not have much of an influence on the
settlement. As such, Group Settlement Ratio, @ of a pile group consisting of n
number of piles can be approximated as follows :
@ 5
Þ
\*(
Ú ÞWX) 5 (\*)=.r
(6.3.38)
The settlement of the group can be estimated as the highest value as obtained from
Equations 6.3.37 and 6.3.38.
Long Term Settlement for Pile Group
For pile groups, settlement due to consolidation is more important than for single
piles. Consolidation settlement of pile group in clay soil is computed using the
following simplified assumptions.
The pile group is assumed to be a solid foundation with a depth 2/3rd the
length of the piles
Effective stress at mid-point of the clay layer is used to compute
settlement
If soil properties are available, the consolidation settlement (S) may be obtained from
the following equation. The depth of significant stress increase (10%) or the depth of
bed rock whichever is less should be taken for computation of settlement. Stress
distribution may be considered as 2 vertical to 1 horizontal.
C 5
+G
1³\<Ý VSQ
σÝ′ ³σT′
σÝ′
(6.3.39)
Where,
* \= Compression index of soil
P = initial void ratio
- \= Thickness of the clay layer
σ
′ = Initial effective stress at mid-point of the clay layer
σ!
′ = Increase in effective stress at mid-point of the clay layer due to pile
load.
In absence of soil properties the following empirical equations may be used to
estimate the long term consolidation settlement of clay soils.
For clay:
C 5
G
2 Ln ¸
σÍ′
σÝ′ ¹
(6.3.40)
For sand:
C 5
G
2 Y¸
σÍ′
σ)′ ¹
−¸
σÝ′
σ)′ ¹
Z
(6.3.41)
Where,
* \= Thickness of the clay layer
σ′ = Initial effective stress at mid-point of the clay layer
σ1
′ = New effective stress at mid-point of the clay layer after pile load.
σ7′ = Reference stress (100 kPa)
9 = Dimensionless modulus number as obtained from Table 6.3.14
d = Stress exponent as obtained from Table 6.3.14.
Table 6.3.14: Settlement Parameters
Soil
Density
Modulus
Number, M
Stress
Exponent, j
Till
V. Dense to Dense
1000 - 300
1.0
Gravel
*
400 - 40
0.5
Sand
Dense
400 - 250
0.5
Sand
Medium Dense
250 - 150
0.5
Sand
Loose
150 - 100
0.5
Silt
Dense
200 - 80
0.5
Silt
Medium Dense
80 - 60
0.5
Silt
Loose
60 - 40
0.5
Silty Clay
Stiff
60 - 40
0.5
Silty Clay
Medium Stiff
20 - 10
0.5
Silty Clay
Soft
10 - 5
0.5
Marine Clay
Soft
20 - 5
0.0
Organic Clay
Soft
20 - 5
0.0
Peat
----
5 - 1
0.0
#### 3.10.6 Drilled Shafts/ Drilled Piers
Large diameter (more than 600 mm) bored piles are sometimes classified as drilled
shaft or drilled piers. They are usually provided with enlarged base called bell. The
provisions of this article shall apply to the design of axially and laterally loaded
drilled shafts/ drilled piers in soil or extending through soil to or into rock.
##### 3.10.6.1 Application of drilled shaft
Drilled shafts may be considered when spread footings cannot be founded on suitable
soil within a reasonable depth and when piles are not economically viable due to high
loads or obstructions to driving. Drilled shafts may be used in lieu of spread footings
as a protection against scour. Drilled shafts may also be considered to resist high
lateral or uplift loads when deformation tolerances are small.
##### 3.10.6.2 Materials for drilled shaft
Shafts shall be cast-in-place concrete and may include deformed bar steel
reinforcement, structural steel sections, and/or permanent steel casing as required by
design.
##### 3.10.6.3 Embedment for Drilled Shaft
Shaft embedment shall be determined based on vertical and lateral load capacities of
both the shaft and sub-surface materials.
##### 3.10.6.4 Batter drilled shaft
The use of battered shafts to increase the lateral capacity of foundations is not
recommended due to their difficulty of construction and high cost. Instead,
consideration should first be given to increasing the shaft diameter to obtain the
required lateral capacity.
##### 3.10.6.5 Selection of soil properties for drilled shaft
Soil and rock properties defining the strength and compressibility characteristics of
the foundation materials are required for drilled shaft design.
##### 3.10.6.6 Geotechnical design of drilled shafts
Drilled shafts shall be designed to support the design loads with adequate bearing
and structural capacity, and with tolerable settlements. The response of drilled shafts
subjected to seismic and dynamic loads shall also be evaluated. Shaft design shall be
based on working stress principles using maximum un-factored loads derived from
calculations of dead and live loads from superstructures, substructures, earth (i.e.,
sloping ground), wind and traffic. Allowable axial and lateral loads may be
determined by separate methods of analysis.
The design methods presented herein for determining axial load capacity assume
drilled shafts of uniform cross section, with vertical alignment, concentric axial
loading, and a relatively horizontal ground surface. The effects of an enlarged base,
group action, and sloping ground are treated separately.
##### 3.10.6.7 Bearing capacity equations for drilled shaft
The ultimate axial capacity ?DÐ" of drilled shafts shall be determined in accordance
with the principles laid for bored piles.
Cohesive Soil
Skin friction resistance in cohesive soil may be determined using either the α-method
or the β-method as described in the relevant section of driven piles. However, for
clay soil, α-method has wide been used by the engineers. This method gives:
B 5 aZD
(6.3.42)
Where,
B = Skin friction
ZD = undrained shear strength of soil along the shaft
a = adhesion factor =0.55 for undrained shear strength ≤ 190 kPa (4000 psf)
For higher values of ZD the value of a may be taken from Figure 6.3.3 as obtained
from test data of previous investigators.
Figure 6.3.3 Adhesion factor α for drilled shaft (after Kulhawy and Jackson, 1989)
The skin friction resistance should be ignored in the upper 1.5 m of the shaft and
along the bottom one diameter of straight shafts because of interaction with the end
bearing. If end bearing is ignored for some reasons, the skin friction along the bottom
one diameter may be considered. For belled shaft, skin friction along the surface of
the bell and along the shaft for a distance of one shaft diameter above the top of bell
should be ignored. For end bearing of cohesive soil, the following relations given by
Equations 6.3.43 and 6.3.44 are recommended.
B4 5 :+CD ≤4000 kPa
(6.3.43)
Where,
:+ 5 6 P1 + 0.2 ¸
B
38¹Q ≤9
Where,
B4 = End bearing stress
CD = undrained shear strength of soil along the shaft
:+ = Bearing capacity factor
6 = Length of the pile (Depth to the bottom of the shaft)
\#4 = Diameter of the shaft base
If the base diameter is more than 1900 mm, the value of B4 from Eq. 6.3.43 could
produce settlements greater than 25 mm, which would be unacceptable for most
buildings. To keep settlement within tolerable limits, the value of B4 should be
reduced to B4
′ by multiplying a factor &7 such that:
B4
′ 5 &7B4
(6.3.44a)
&7 5
.r
1= \[Í 38/A)³\[È ≤ 1.0
(6.3.44b)
F1 5 0.0071 + 0.0021 ¸
B
38¹ ≤ 0.0015
(6.3.44c)
F 5 1.59Ç
J) 0.5 ≤ ω2 ≤1.5
(6.3.44d)
Where,
7 = Reference width=1 ft = 0.3 m = 12 inch = 300 mm
D7 = Reference stress = 100 kPa = 2000 psf
Cohesionless Soil
Skin friction resistance in cohesionless soil is usually determined using the β-method.
The relevant equation is reproduced again:
B 5 cD/′
(6.3.45)
c 5 0±L\*k
(6.3.46)
Where,
B = Skin friction
D/′ = Effective vertical stress at mid-point of soil layer
0 = Coefficient of lateral earth pressure
k = Soil shaft interface friction angle
The values of K and k can be obtained from the chart of Tables 6.3.15, from the soil
friction angle, k and preconstruction coefficient of lateral earth pressure 0.
However, 0 is very difficult to determine. An alternative is to compute β directly
using the following empirical relation.
c 5 1.5 −0.135Ç
/
A)
(6.3.47)
Where,
Br = Reference width=1 ft = 0.3 m = 12 inch = 300 mm
z = Depth from the ground surface to the mid-point of the strata
Table 6.3.15: Typical \/\ and }/}] Values for the Design of Drilled Shaft
Construction Method
\/\
Construction Method
}/}]
Open hole or temporary casing
1.0
Dry construction with minimal side
wall disturbance and prompt
concreting
Slurry method – minimal slurry
cake
1.0
Slurry construction – good
workmanship
Slurry method – heavy slurry
cake
0.8
Slurry construction – poor
workmanship
2/3
Permanent casing
0.7
Casing under water
5/6
The unit end bearing capacity for drilled shaft in cohesionless soils will be less than
that for driven piles because of various reasons like soil disturbance during augering,
temporary stress relief while the hole is open, larger diameter and depth of influence
etc. The reasons are not well defined, as such the following empirical formula
ফবাবষড়ঢ়বফ নু জববংব ধহফ ঙ্থ ঘবষষ (১৯৮৯) সধু নব ংঁমমবংঃবফ:ড় ঁংব:ড় বংঃরসধঃব বহফ
bearing stress.
B4 5 0.60D7: ≤ 4500 kPa
(6.3.48)
Where,
B4 = Unit bearing resistance
D7′ = Reference stress = 100 kPa = 2000 psf
N = Mean SPT value for the soil between the base of the shaft and a depth
equal to two times the base diameter below the base. No overburden
correction is required (N= N60)
If the base diameter is more than 1200 mm, the value of B4 from Eq. 6.3.48 could
produce settlements greater than 25 mm, which would be unacceptable for most
buildings. To keep settlement within tolerable limits, the value of B4 should be
reduced to B4
′ by multiplying a factor &7 such that:
B4
′ 5 &7B4
(6.3.49a)
&7 5 4.17
A)
38 ≤ 1.0
(6.3.49b)
Where,
7 = Reference width=1 ft = 0.3 m = 12 inch = 300 mm
\#4 =Base diameter of drilled shaft
##### 3.10.6.8 Other methods of evaluating axial load capacity of drilled shaft
A number of other methods are available to estimate the ultimate axial load capacity
of drilled shafts. These methods are based on N-values obtained from Standard
Penetration Test (SPT) and on angle of internal friction of sand. These methods may
also be used to estimate the ultimate load carrying capacity of drilled shafts. Three of
these methods are as follows and they are summarized in Appendix G.
Method based on the Standard Penetration Test (CGS, 1985)
Method based on Theory of Plasticity (CGS, 1985)
Tomlinson (1995) Method
##### 3.10.6.9 Factor of safety for drilled shaft
Similar to bored and driven piles, drilled shafts shall be designed for a minimum
overall factor of safety of 2.0 against bearing capacity failure (end bearing, side
resistance or combined) when the design is based on the results of a load test
conducted at the site. Otherwise, it shall be designed for a minimum overall factor of
safety 3.0. The minimum recommended overall factor of safety is based on an
assumed normal level of field quality control during construction. If a normal level of
field quality control cannot be assured, higher minimum factors of safety shall be
used. The recommended values of overall factor of safety on ultimate axial load
capacity based on specified construction control is presented in Tables 6.3.10a and
6.3.10b.
##### 3.10.6.10 Deformation and settlement of axially loaded drilled shaft
Similar to driven and bored piles, settlement of axially loaded shafts at working or
allowable loads shall be estimated using elastic or load transfer analysis methods. For
most cases, elastic analysis will be applicable for design provided the stress levels in
the shaft are moderate relative to ?DÐ". Analytical methods are similar to that
provided in Sec 3.10.1.10 for driven and bored piles. The charts provided in
Appendix G may also be used to estimate the settlement of drilled shaft.
##### 3.10.6.11 Drilled shaft in layered soil profile
The short-term settlement of shafts in a layered soil profile may be estimated by
summing the proportional settlement components from layers of cohesive and
cohesionless soil comprising the subsurface profile.
##### 3.10.6.12 Tolerable movement of drilled shaft
Tolerable axial displacement criteria for drilled shaft foundations shall be developed
by the structural designer consistent with the function and type of structure, fixity of
bearings, anticipated service life, and consequences of unacceptable displacements
on the structure performance. Drilled shaft displacement analyses shall be based on
the results of in-situ/laboratory testing to characterize the load-deformation behavior
of the foundation materials.
##### 3.10.6.13 Group loading of drilled shaft
Cohesive Soil
Evaluation of group capacity of shafts in cohesive soil shall consider the presence
and contact of a cap with the ground surface and the spacing between adjacent shafts.
For a shaft group with a cap in firm contact with the ground, ?DÐ" may be computed
as the lesser of (1) the sum of the individual capacities of each shaft in the group or
(2) the capacity of an equivalent pier defined in the perimeter area of the group. For
the equivalent pier, the shear strength of soil shall not be reduced by any factor (e.g.,
α1) to determine the ? component of ?DÐ", the total base area of the equivalent pier
shall be used to determine the QT component of ?DÐ" and the additional capacity of
the cap shall be ignored. If the cap is not in firm contact with the ground, or if the
soil at the surface is loose or soft, the individual capacity of each shaft should be
reduced to ζ times QT for an isolated shaft, where ζ = 0.67 for a center-to-center
(CTC) spacing of 3B (where B is the shaft diameter) and ζ = 1.0 for a CTC spacing
of 6B. For intermediate spacings, the value of ζ may be determined by linear
interpolation. The group capacity may then be computed as the lesser of (1) the sum
of the modified individual capacities of each shaft in group, or (2) the capacity of an
equivalent pier as stated above.
Cohesionless Soil
Evaluation of group capacity of shafts in cohesion soil shall consider the spacing
between adjacent shafts. Regardless of cap contact with the ground, the individual
capacity of each shaft should be reduced to times QT for an isolated shaft, where ζ =
### 0.67 for a center-lo-center (CTC) spacing of 3B and ζ = 1.0 for a CTC spacing of 8B.
For intermediate spacings, the value of ζ may be determined by linear interpolation.
The group capacity may be computed as the lesser of (I) sum of the modified
individual capacities of each shaft in the group or (2) capacity of an equivalent pier
circumscribing the group including resistance over the entire perimeter and base
areas.
##### 3.10.6.14 Drilled shaft in strong soil overlying weak soil
If a group of shafts is embedded in a strong soil deposit which overlies a weaker
deposit (cohesionless and cohesive soil), consideration shall be given to the potential
for a punching failure of the lip into the weaker soil strata. For this case, the unit tip
capacity XH of the equivalent shaft may be determined using the following:
XH 5
GA)
1= (X^A −XB) ≤X^A
(6.3.50)
In the above equation X^A is the ultimate unit capacity of an equivalent shaft bearing
in the stronger upper layer and XB is the ultimate unit capacity of an equivalent shaft
bearing in the weaker underlying soil layer. If the underlying soil unit is a weaker
cohesive soil strata, careful consideration shall be given to the potential for large
settlements in the weaker layer.
##### 3.10.6.15 Lateral loads on drilled shaft
Soil Layering
The design of laterally loaded drilled shafts in layered soils shall be based on
evaluation of the soil parameters characteristic of the respective layers
Ground Water
The highest anticipated water level shall be used for design
Scour
The potential for loss of lateral capacity due to scour shall be considered in the
design. If heavy scour is expected, consideration shall be given to designing the
portion of the shaft that would be exposed as a column. In all cases, the shaft length
shall be determined such that the design structural load can be safely supported
entirely below the probable scour depth.
Group action
There is no reliable rational method for evaluating the group action for closely
spaced, laterally loaded shafts. Therefore, as a general guide, drilled shaft with
diameter B in a group may be considered to act individually when the center-to-
center (CTC) spacing is greater than 2.5B in the direction normal to loading, and
CTC > 8B in the direction parallel to loading. For shaft layout not conforming to
these criteria, the effects of shaft interaction shall be considered in the design. As a
general guide, the effects of group action for in-line CTC \<8B may be considered
using the ratios (CGS, 1985) appearing as below, Table 6.3.16:
Table 6.3.16: Ratio of Group and Single Plie Shaft Resistance
Centre to Centre Shaft
Spacing for In-line
Loading
Ratio of Lateral Resistance of
Shaft in Group to Single Shaft
8B
1.00
6B
0.70
4B
0.40
3B
0.25
Cyclic Loading
The effects of traffic, wind, and other non-seismic cyclic loading on the load-
deformation behavior of laterally loaded drilled shafts shall be considered during
design. Analysis of drilled shafts subjected to cyclic loading may he considered in
the COM624 analysis (Reese et. al., 1984).
Combined Axial and Lateral Loading
The effects of lateral loading in combination with axial loading shall be considered in
the design. Analysis of drilled shafts subjected to combined loading may be
considered in the COM624 analysis (Reese et. al., 1984).
Sloping Ground
For drilled shafts which extend through or below sloping ground. The potential for
additional lateral loading shall be considered in the design. The general method of
analysis developed by Borden and Gabr (1987) may be used for the analysis of shafts
instable slopes. For shafts in marginally stable slopes. Additional consideration
should be given for smaller factors of safety against slope failure or slopes showing
ground creep, or when shafts extend through fills overlying soft foundation soils and
bear into more competent underlying soil or rock formations. For unstable ground,
detailed explorations, testing and analysis are required to evaluate potential
additional lateral loads due to slope movements
Tolerable Lateral Movements
Tolerable lateral displacement criteria for drilled shaft foundations shall be
developed by the structural designer consistent with the function and type of
structure, fixity, anticipated service life, and consequences of unacceptable
displacements on the structure performance. Drilled shaft lateral displacement
analysis shall be based on the results of in-situ and/or laboratory testing to
characterize the load-deformation behavior of the foundation materials.
##### 3.10.6.16 Uplift loads on drilled shaft
Uplift capacity shall rely only on side resistance in conformance with related articles
for driven piles. If the shaft has an enlarged base, ? shall be determined in
conformance with related articles for driven piles.
##### 3.10.6.17 Consideration of vertical ground movement
The potential for external loading on a shaft by vertical ground movement (i.e.,
negative skin friction down-drag due to settlement of compressible soil or uplift due
to heave of expansive soil) shall be considered as a part of design. For design
purposes, it shall be assumed that the full magnitude of maximum potential vertical
ground movement occurs.
##### 3.10.6.18 Negative skin friction
Evaluation of negative skin friction shall include a load-transfer method of analysis
to determine the neutral point (i.e., point of zero relative displacement) and load
distribution along shaft (e.g., Reese and O'Neill, 1988). Due to the possible time
dependence associated with vertical ground movement, the analysis shall consider
the effect of time on load transfer between the ground and shaft and the analysis shall
be performed for the time period relating to the maximum axial load transfer to the
shaft. Evaluation of negative skin friction shall include a load-transfer method of
analysis to determine the neutral point (i.e., point of zero relative displacement) and
load distribution along shaft (e.g., Reese and O'Neill, 1988). Due to the possible time
dependence associated with vertical ground movement, the analysis shall consider
the effect of time on load transfer between the ground and shaft and the analysis shall
be performed for the time period relating to maximum axial load transfer to the shaft.
##### 3.10.6.19 Expansive soils
Shafts designed for and constructed in expansive soil shall extend to a sufficient
depth into moisture-stable soils to provide adequate anchorage to resist uplift
movement in addition; sufficient clearance shall be provided between the ground
surface and underside of caps or beams connecting shafts to preclude the application
of uplift loads at the shaft/cap connection from swelling ground conditions.
##### 3.10.6.20 Dynamic/seismic design of drilled shaft
Refer to Seismic Design section of this Code and Lam and Martin (1986a; 1986b) for
guidance regarding the design of drilled shafts subjected to dynamic and seismic
loads.
##### 3.10.6.21 Structural shaft design, shaft dimensions and shaft spacing
Drilled shafts shall be designed to resist failure loads to insure that the shaft will not
collapse or suffer loss of serviceability due to excessive stress and/or deformation.
Dimensions
All shafts should be sized in 50 mm increments with a minimum shaft diameter of
600 mm. The diameter of columns supported by shafts shall be less than or equal to
the shaft diameter B.
Center to Center Spacing
The center-to-center spacing of drilled shafts of diameter B should be 3B or greater
to avoid interference between adjacent shafts during construction. If closer spacing is
required, the sequence of construction shall be specified and the interaction effects
between adjacent shafts shall be evaluated by the designer.
Reinforcement
Where the potential for lateral loading is insignificant, drilled shafts need to be
reinforced for axial loads only. Those portions of drilled shafts that are not supported
laterally shall be designed as reinforced concrete columns in accordance with
relevant sections in structural design part of the Code and the reinforcing steel shall
extend a minimum of 5 m below the plane where the soil provides adequate lateral
restraint. Where permanent steel casing is used and the shell is smooth pipe and more
than 3 mm in thickness, it may be considered as load carrying in the absence of
corrosion.
The design of longitudinal and spiral reinforcement shall be in conformance with the
requirements of the relevant sections of the structural design part of the Code.
Development of length of deformed reinforcement shall be in conformance with the
relevant sections of the structural design part of the Code.
Longitudinal Bar Spacing
The minimum clear distance between longitudinal reinforcement shall not be less
than 3 times the bar diameter nor 3 times the maximum aggregate size. If bars are
bundled in forming the reinforcing cage, the minimum clear distance between
longitudinal reinforcement shall not be less than 3 times the diameter of the bundled
bars. Where heavy reinforcement is required, consideration may be given to an inner
and outer reinforcing cage.
Splices
Splices shall develop the full capacity of the bar in tension and compression. The
location of splices shall be staggered around the perimeter of the reinforcing cage so
as not to occur at the same horizontal plane. Splices may be developed by lapping,
welding, and special approved connectors. Splices shall be in conformance with the
relevant sections of the structural design part of the Code.
Transverse Reinforcement
Transverse reinforcement shall be designed to resist stresses caused by fresh concrete
flowing from inside the cage to the side of the excavated hole. Transverse
reinforcement may be constructed of hoops or spiral steel.
Handling Stresses
Reinforcement cages shall be designed to resist handling and placement stresses.
Reinforcement Cover
The reinforcement shall be placed a clear distance of not less than 50 mm from the
permanently cased or 75 mm from the uncased sides. When shafts are constructed in
corrosive or marine environments, or when concrete is placed by the water or slurry
displacement methods, the clear distance shall not be less than 100 mm for uncased
shafts and shafts with permanent casings not sufficiently corrosion resistant.
The reinforcement cage shall be centered in the hole using centering devices. All
steel centering devices shall be epoxy coated.
Reinforcement into Superstructure
Sufficient reinforcement shall be provided at tit junction of the shaft with the
superstructure to make a suitable connection. The embedment of the reinforcement
into the cap shall be in conformance with relevant articles of the structural design
part of the Code.
3.10.6.22
Enlarged base of drilled shaft
Enlarged bases shall be designed to insure that plain concrete is not overstressed. The
enlarged base shall slope at a side angle not less than 30 degrees from the vertical
and have a bottom diameter not greater than 3 times diameter of the shaft. The
thickness of the bottom edge of enlarged base shall not be less than 150 mm.
3.10.6.23
Construction of drilled shaft
Drilled shafts may be constructed using the dry, casing, or wet method of
construction, or a combination of methods. In every case, excavation of hole,
placement of concrete, and all other aspects of shaft construction shall be performed
in conformance with the provisions of this Code.
The load capacity and deformation behavior of drilled shafts can be greatly affected
by the quality and methods of construction. The effects of construction methods are
incorporated in design by application of factor of safety consistent with the expected
construction methods and level of field quality control measures undertaken as
described in the relevant sections for driven piles.
Where the spacing between shafts in a group is restricted, consideration shall be
given to the sequence of construction to minimize the effect of adjacent shaft
construction operations on recently constructed shafts. The following construction
procedure shall be followed:
(i)
Place permanent/temporary steel casing in position and embed casing toe
into firm strata.
(ii)
Bore and excavate inside the steel casing down to casing toe level, or to a
level approved, and continue excavation to final pile tip level using
drilling mud. The fluid level inside casings shall at all times be at least 2
metres higher than outside the casings.
(iii) Carefully clean up all mud or sedimentation from the bottom of borehole.
(iv) Place reinforcement cage, inspection pipes etc.
(v)
Concrete continuously under water, or drilling fluid, by use of the tremie
method.
(vi) After hardening, break out the top section of the concrete pile to reach
sound concrete.
In drilling of holes for all piles, bentonite and any other material shall be mixed
thoroughly with clean water to make a suspension which shall maintain the stability
of the pile excavation for the period necessary to place concrete and complete
construction. The control tests shall cover the determination of' density, viscosity, gel
strength and pH values. Bentonite slurry shall meet the Specifications as shown in
Table 6.3.17.
Table 6.3.17: Specifications of Bentonite Slurry
Item to be Measured
Range of Results
at 20 C
Test Method
Density during drilling to
support excavation
greater than 1.05
g/ml
Mud density Balance (ASTM D4380)
Density prior to concreting
less than 1.25 g/ml
Mud density Balance (ASTM D4380)
Viscosity
30 - 90 seconds
Marsh Cone Method (ASTM D6910)
pH
### 9.5 to 12
pH indicator paper strips or electrical pH
meter (ASTM D4972)
Liquid limit
> 450%
> Casagrande apparatus (ASTM D4318)
Temporary casing of approved quality or an approved alternative method shall be
used to maintain the stability of pile excavations, which might otherwise collapse.
Temporary casings shall be free from significant distortion.
Where a borehole is formed using drilling fluid for maintaining the stability of a
boring, the level of the water or fluid in the excavation shall be maintained so that the
water or fluid pressure always exceeds the pressure exerted by the soils and external
ground water. The water or fluid level shall be maintained at a level not less than 2 m
above the level of ground water.
The reinforcement shall be placed as indicated on the Drawings. Reinforcement in
the form of a cage shall be assembled with additional support, such as Spreader forks
and lacings, necessary to form a rigid cage. Hoops, links or helical reinforcement
shall fit closely around the main longitudinal bars and be bound to them by approved
wire, the ends of which shall be turned into the interior of the pile or pour.
Reinforcement shall be placed and maintained in position. The cover to all
reinforcement for pile cap and bored cast in place pile shall be not less than 75 mm.
Joints in longitudinal steel bars shall be permitted unless otherwise specified. Joints
in reinforcement shall be such that the full strength of the bar is effective across the
joint and shall be made so that there is no relative displacement of the reinforcement
during the construction of the pile. Joints in longitudinal bars in piles with tension
(for instance for test loading) shall be carried out by welding or other approved
method.
Concrete to be placed under water or drilling fluid shall be placed by tremie
equipment and shall not be discharged freely into the water or drilling fluid. The
tremie equipment shall be designed to minimize the occurrence of entrapped air and
other voids, so that it causes minimal surface disturbance, which is particularly
important when a concrete-water interface exists. It shall be so designed that external
projections are minimised, allowing the tremie to pass through reinforcing cages
without causing damage. The internal face of the pipe of the tremie shall be free from
projections. The tremie pipes shall meet the following requirements:
(i)
The tremie pipes shall be fabricated of heavy gage steel pipe to withstand
all anticipated handling stress. Aluminium pipe shall not be used for
placing concrete.
(ii) Tremie pipes should have a diameter large enough to ensure that
aggregates-caused blockage will not occur. The diameter of the tremie
pipe shall be 200 mm to 300 mm.
(iii) The tremie pipes shall be smooth internally.
(iv) Since deep placement of concrete will be carried out, the tremie shall be
made in sections/lengths with detachable joints that allow the upper
sections/lengths to be removed as the placement progresses.
(v)
Sections may be joined by flanged, bolted connections (with gaskets) or
may be screwed together. Whatever joint technique is selected, joints
between tremie sections must be watertight. The joint system selected
shall be tested for water tightness before beginning of concrete
placement.
(vi) The joint system to be used shall need approval of the Engineer.
(vii) The tremie pipe should be marked to allow quick determination of the
distance from the surface of the water to the mouth of the tremie.
(viii) The tremie should be provided with adequately sized funnel or hopper to
facilitate transfer of sufficient concrete from the delivery device to the
tremie.
Before placing concrete, it shall be ensured that there is no accumulation of silt, other
material, or heavily contaminated bentonite suspension at the base of the boring,
which could impair the free flow of concrete from the pipe of the tremie. Flushing of
boreholes before concreting with fresh drilling fluid/mud is preferred. A sample of
the bentonite suspension shall be taken from the base of the boring using an approved
sampling device. If the specific gravity of the suspension exceeds 1.25, the placing of
concrete shall not proceed. In this event the Contractor shall modify the mud quality.
During and after concreting, care shall be taken to avoid damage to the concrete from
pumping and dewatering operations.
The hopper and pipe of the tremie shall be clean and watertight throughout. The pipe
shall extend to the base of the boring and a sliding plug or barrier shall be placed in
the pipe to prevent direct contact between the first charge of concrete in the pipe of
the tremie and the water or drilling fluid. The pipe shall at all times penetrate the
concrete, which has previously been placed and shall not be withdrawn from the
concrete until completion of concreting. The bottom of the tremie pipe shall be
embedded in the fresh concrete at least 2.0 m and maintained at that depth
throughout concreting. At all times a sufficient quantity of concrete shall be
maintained within the pipe to ensure that the pressure from it exceeds that from the
water or drilling fluid.
To ensure the quality of concrete being free from mud, clay lumps or any other
undesirable materials mixed with concrete at the top portion of the pile, fresh
concrete shall be overflowed sufficiently at the end of the each pour. The level of
concrete poured at the end of concreting operation shall be at least 600 mm higher
than the elevation of the pile at cut-off.
3.10.6.24
Concreting of drilled shaft
In drilled shafts/cast-in-situ bored piles, concrete shall be placed only after
excavation has been completed, inspected and accepted, and steel reinforcement
accurately placed and adequately supported. Concrete shall be placed in one
continuous operation in such a manner as to ensure the exclusion of any foreign
matter and to secure a full sized shaft. Concrete shall not be placed through water
except where tremie methods are approved. When depositing concrete from the top
of the pile, the concrete shall not be chuted directly into the pile but shall be poured
in a rapid and continuous operation through a funnel hopper centred at the top of the
pile.
For large diameter holes concrete may be placed by tremie or by drop bottom bucket;
for small diameter boreholes a tremie shall be utilized. In tremie concreting, toe of
the tremie shall be set at a maximum of 150 mm above the bottom of the borehole.
Maximum permissible siltation in bore hole prior to start of concrete operation shall
be 75 mm. A slump of 125 mm to 150 mm shall be maintained for concreting by
tremie. In case of tremie concreting for piles of smaller diameter and length up to 10
m, the minimum cement content shall be 350 kg/m3 of concrete. For larger diameter
and/or deeper piles, the minimum cement content shall be 400 kg/m3 of concrete. See
relevant sections of the Code for further specification.
For uncased concrete piles, if pile shafts are formed through unstable soil and
concrete is placed in an open drill hole, a steel liner shall be inserted in the hole prior
to placing concrete. If the steel liner is withdrawn during concreting, the level of
concrete shall be maintained above the bottom of the liner to a sufficient height to
offset any hydrostatic or lateral earth pressure.
If concrete is placed by pumping through a hollow stem auger, the auger shall not be
permitted to rotate during withdrawal and shall be withdrawn in a steady continuous
motion. Concrete pumping pressures shall be measured and shall be maintained high
enough at all times to offset hydrostatic and lateral earth pressure. Concrete volumes
shall be measured to ensure that the volume of concrete placed in each pile is equal
to or greater than the theoretical volume of the hole created by the auger. If the
installation process of any pile is interrupted or a loss of concreting pressure occurs,
the hole shall be redrilled to original depth and reformed.
Augured cast-in-situ pile shall not be installed within 6 pile diameters centre to
centre of a pile filled with concrete less than 24 hours old. If concrete level in any
completed pile drops, the pile shall be rejected and replaced. Bored cast-in-situ
concrete piles shall not be drilled/bored within a clear distance of 3 m from an
adjacent pile with concrete less than 48 hours old. For under-reamed piles, the slump
of concrete shall range between 100 mm and 150 mm for concreting in water free
holes.
For concreting under water, the concrete shall contain at least 10 percent more
cement than that required for the same mix placed in the dry. The amount of coarse
aggregate shall be not less than one and a half times, nor more than two times, that of
the fine aggregate. The materials shall be so proportioned as to produce a concrete
having a slump of not less than 100 mm, nor more than 150 mm, except where
plasticizing admixtures is used in which case, the slump may be 175 mm.
Successful placement of concrete under water requires preventing flow of water
across or through the placement site. Once flow is controlled, the tremie placement
consists of the following three basic steps:
(i)
The first concrete placed is physically separated from the water by using a
ুৎধননরঃচ্ ড়ৎ মড়-ফবারষ রহ:যব ঢ়রঢ়ব, ড়ৎ নু যধারহম:যব ঢ়রঢ়ব সড়ঁঃয পধঢ়ঢ়বফ ড়ৎ
sealed and the pipe dewatered.
(রর) ঙহপব ভরষষবফ রিঃয পড়হপৎবঃব,:যব ঢ়রঢ়ব রং ৎধরংবফ ংষরমযঃষু:ড় ধষষড়ি:যব ুৎধননরঃচ্
to escape or to break the end seal. Concrete will then flow out and
develop a mound around the mouth of the pipe. This is termed as
ুবংঃধনষরংযরহম ধ ংবধষচ্.
(iii) Once the seal is established, fresh concrete is injected into the mass of
existing concrete.
Two methods are normally used for the placement of concrete using tremie pipe,
হধসবষু,:যব পধঢ়ঢ়বফ:ৎবসরব ঢ়রঢ়ব ধঢ়ঢ়ৎড়ধপয ধহফ:যব ুৎধননরঃচ্ ঢ়ষঁম ধঢ়ঢ়ৎড়ধপয. ওহ:যব
capped tremie approach the tremie pipe should have a seal, consisting of a bottom
plate that seals the bottom of the pipe until the pipe reaches the bottom of excavation.
The tremie pipe should be filled with enough concrete before being raised off the
bottom. The tremie pipe should then be raised a maximum of 150 mm (6 inch) to
initiate flow. The tremie pipe should not be lifted further until a mound is established
around the mouth of the tremie pipe. Initial lifting of the tremie should be done
slowly to minimize disturbance of material surrounding the mouth of the tremie.
ওহ:যব ুৎধননরঃচ্ ঢ়ষঁম ধঢ়ঢ়ৎড়ধপয, ড়ঢ়বহ:ৎবসরব ঢ়রঢ়ব ংযড়ঁষফ নব ংবঃ ড়হ:যব নড়ঃঃড়স,:যব
ুৎধননরঃচ্ ঢ়ষঁম রহংবৎঃবফ ধঃ:যব:ড়ঢ় ধহফ:যবহ পড়হপৎবঃব ংযড়ঁষফ নব ধফফবফ:ড়:যব:ৎবসরব
ংষড়ষিু:ড় ভড়ৎপব:যব ুৎধননরঃচ্ ফড়হিধিৎফ ংবঢ়ধৎধঃরহম:যব পড়হপৎবঃব ভৎড়স:যব ধিঃবৎ. ঙহপব
ঃযব:ৎবসরব ঢ়রঢ়ব রং ভঁষষু পযধৎমবফ ধহফ:যব ুৎধননরঃচ্ ৎবধপযবং:যব সড়ঁঃয ড়ভ:যব:ৎবসরব,:যব
tremie pipe should be lifted a maximum of 150 mm (6 inch) off the bottom to allow
ঃযব ুৎধননরঃচ্:ড় বংপধঢ়ব ধহফ:ড় ংঃধৎঃ:যব পড়হপৎবঃব ভষড়রিহম. অভঃবৎ:যরং, ধ:ৎবসরব ঢ়রঢ়ব
should not be lifted again until a sufficient mound is established around the mouth of
the tremie.
Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5
ft) and maintained at that depth throughout concreting to prevent entry of water into
the pipe. Rapid raising or lowering of the tremie pipe should not be allowed. All
vertical movements of the tremie pipe must be done slowly and carefully to prevent
ুষড়ংং ড়ভ ংবধষচ্. ওভ ুষড়ংং ড়ভ ংবধষচ্ ড়পপঁৎং রহ ধ:ৎবসরব, ঢ়ষধপবসবহঃ ড়ভ পড়হপৎবঃব:যৎড়ঁময:যব
tremie must be halted immediately. The tremie pipe must be removed and the end
plate must be restarted using the capped tremie approach. In order to prevent washing
ড়ভ পড়হপৎবঃব রহ ঢ়ষধপব, ধ ুৎধননরঃচ্ ঢ়ষঁম ধঢ়ঢ়ৎড়ধপয সঁংঃ হড়ঃ নব ঁংবফ:ড় ৎবংঃধৎঃ ধ:ৎবসরব
ধভঃবৎ ুষড়ংং ড়ভ ংবধষচ্.
Means of raising or lowering tremie pipes and of removing pipes smoothly without
loss of concrete and without disturbing placed concrete or trapping air in the concrete
shall be provided. Pipes shall not be moved horizontally while they are embedded in
placed concrete or while they have concrete within them.
Underwater concrete shall be placed continuously for the whole of a pour to its full
depth approved by the Engineer, without interruption by meal breaks, change of
shift, movements of placing positions, and the like. Delays in placement may allow
the concrete to stiffen and resist flow once placement resumes. The rate of pour from
individual tremie shall be arranged so that concrete does not rise locally to a level
greater than 500 mm above the average level of the surrounding concrete.
Tremie blockages which occur during placement should be cleared extremely
carefully to prevent loss of seal. If a blockage occurs, the tremie should be quickly
raised 150 to 600 mm (6 inch to 2 ft) and then lowered in an attempt to dislodge the
blockage. The depth of pipe embedment must be closely monitored during all such
attempts. If the blockage cannot be cleared readily, the tremie shall be removed,
cleared, resealed, and restarted.
The volume of concrete in place should be monitored throughout the placement.
Underruns are indicative of loss of tremie seal since the washed and segregated
aggregates will occupy a greater volume. Overruns are indicative of loss of concrete
from the inside of the steel pile.
3.11
Field Tests for Driven Piles and Drilled Shafts
#### 3.11.1 Integrity Test
Low strain integrity testing of piles is a tool for quality control of long structural
elements that function in a manner similar to foundation piles, regardless of their
method of installation, provided that they are receptive to low strain impact testing.
The test provides velocity (and optionally force) data, which assists evaluation of pile
integrity and pile physical dimensions (i.e., cross-sectional area, length), continuity
and consistency of pile material. The test does not give any information regarding the
pile bearing capacity or about pile reinforcement. Integrity test principles have been
well documented in literature (ASTM 5882; Klingmuller, 1993). There exist two
methods of integrity testing, namely, Pulse Echo Method (PEM) and Transient
Response Method (TRM). In Pulse Echo Method, the pile head motion is measured
as a function of time. The time domain record is then evaluated for pile integrity. In
Transient Response Method, the pile head motion and force (measured with an
instrumented hammer) are measured as a function of time. The data are then
evaluated usually in the frequency domain.
In order to check the structural integrity of the piles Integrity tests shall be performed
on the piles in accordance with the procedure outlined in ASTM D5882. The test is
carried out by pressing a transducer onto a pile top while striking the pile head with a
hand hammer. The Sonic Integrity Testing (SIT)-system registers the impact of the
hammer followed by the response of the pile and shows the display. If instructed by
the operator, the signal will be stored in the memory of the SIT-system together with
other information, such as pile number, date, time, site, amplification factor, filter
length etc. The reflectograms are horizontally scaled and vertically amplified to
compensate external soil friction, which facilitate the interpretation. Consequently,
the reflection of the pile toe matches the length of the pile which will be confirmed
by the SIT-system. In case of any defects, the exact location can be determined from
the graph on the display.
For any project where pile has been installed, integrity tests shall be performed on
100% of the piles. Integrity testing may not identify all imperfections, but it can be
used in identifying major defects within effective length. In literature, there are many
examples that highlight success of low strain integrity testing (Klingmuller, 1993).
Factors Influencing Implication of Pile Integrity Test
(a)
This sonic echo pile integrity testing or dynamic response method is
based on measuring (or observing on an oscilloscope) the time it takes for
a reflected compression stress wave to return to the top of the pile.
(b)
Some waves will be reflected by a discontinuity in the pile shaft. When
the compressive strength is known for the pile material involved, the
depth to the discontinuity and the pile length can be determined.
(c)
On the other hand, area of pile shaft and hence its diameter, is determined
from impedance of wave response, while impedance in any section is a
function of elastic modulus of pile material, shaft area and wave velocity
propagating through that section. If the concrete material is uniform
throughout the pile length, elastic modulus and the wave velocity
(provided disturbance from other source of vibration nearby is
insignificant) are constant for that pile. In that case, changes in impedance
usually indicate changes of pile cross-sectional area.
(d)
While evaluating pile integrity (i.e., pile length and shaft diameter), the
wave velocity is assumed to be constant throughout pile length. Thus, the
reliability of integrity evaluation entirely depends on the pile material and
its uniformity throughout shaft length while casting was done. The length
and diameter obtained from pile integrity test is an indication of the actual
length and diameter of the tested piles.
(e)
Besides, this test can only assess shaft integrity and gives no information
for pile bearing capacity determination. However, if a large number of
piles are tested, it is generally easy to focus the piles having unusual
responses. Therefore, whenever an integrity testing is contemplated,
consideration must be given to the limitations of the various
methods/process of pile installation (i.e. pile driving or casting) and the
possible need for further investigation (such as pile load test) to check the
results of such testing.
(f)
It should be noted here that pile integrity test is an indicative test about
the length and quality of concrete in the pile. This test does not give any
idea about its actual load capacity. It is usually suggestive to substantiate
the findings of integrity test by excavation or pull out of the pile to
facilitate decisions about final acceptance or rejection of any pile.
Because of the large cost involved in a pile load test, the necessity of
integrity test in facilitating the selection of piles for load test is a rational
approach for quality and safety assurance of piled foundations.
#### 3.11.2 Axial Load Tests for Compression
Where accurate estimate of axial load carrying capacity of a pile is required tests in
accordance with "Standard Test Method for Deep Foundations Under Static Axial
Compressive Load", (ASTM D1143) or equivalent shall be performed on individual
piles. For a major project, at least 2% of piles (test piles plus service piles) shall be
tested in each area of uniform subsoil conditions. Where necessary, additional piles
may be load tested to establish the safe design capacity. The ultimate load carrying
capacity of a single pile may be determined with reasonable accuracy from load
testing. The load test on a pile shall not be carried out earlier than 4 (four) weeks
from the date of casting the pile. A minimum of one pile at each project shall be load
tested for bored cast-in-situ piles.
Two principal types of test may be used for compression loading on piles - the
constant rate of penetration (CRP) test and the maintained load (ML) test. The CRP
test was developed by Whitaker (1963). The CRP method is essentially a test to
determine the ultimate load on a pile and is therefore applied only to preliminary test
piles or research type investigations where fundamental pile behaviour is being
studied. In this test the compressive force is progressively increased to cause the pile
to penetrate the soil at constant rate until failure occurs. The rate of penetration
selected usually corresponds to that of shearing soil samples in unconfined
compression tests. However, rate does not affect results significantly. In CRP test the
recommended rates of penetration are 0.75 mm/min for friction piles in clay and
### 1.55 mm/min for piles end bearing in granular soil. The CRP test shall not be used
for checking compliance with specification requirements for maximum settlement at
given stages of loading.
Maintained load (ML) test is so far the most usual one in practice. In the ML test the
load is increased in stages to 1.5 times or twice the working load with time settlement
curve recorded at each stage of loading and unloading. The general procedure is to
apply static loads in increments of 25% of the anticipated design load. The ML test
may also be taken to failure by progressively increasing the load in stages. In the ML
test, the load test arrangements as specified in (ASTM D1143) shall be followed.
According to ASTM D1143 each load increment is maintained until the rate of
settlement is not greater than 0.25 mm/hr or 2 hours is elapsed, whichever occurs
first. After that the next load increment is applied. This procedure is followed for all
increments of load. After the completion of loading if the test pile has not failed the
total test load is removed any time after twelve hours if the butt settlement over one
hour period is not greater than 0.25 mm otherwise the total test load is kept on the
pile for 24 hours. After the required holding time, the test load is removed in
decrement of 25% of the total test load with 1 hour between decrement. If failure
occurs, jacking the pile is continued until the settlement equals 15% of the pile
diameter or diagonal dimension. Selection of an appropriate load test method shall be
based on an evaluation of the anticipated types and duration of loads during service,
and shall include consideration of the following:
(i)
The immediate goals of the load test (i.e., to proof load the foundation
and verify design capacity)
(ii) The loads expected to act on the production foundation (compressive
and/or uplift, dead and/or live), and the soil conditions predominant in the
region of concern.
(iii) The local practice or traditional method
As a minimum, the written test procedures should include the following:
(i)
Apparatus for applying loads including reaction system and loading
system.
(ii) Apparatus for measuring movements.
(iii) Apparatus for measuring loads.
(iv) Procedures for loading including rates of load application, load cycling
and maximum load.
(v)
Procedures for measuring movements.
(vi) Safety requirements.
(vii) Data presentation requirements and methods of data analysis.
(viii) Drawings showing the procedures and materials to be used to construct
the load test apparatus.
##### 3.11.2.1 Load test evaluation methods for axial compressive
A number of arbitrary or empirical methods are used to serve as criteria for
determining the allowable and ultimate load carrying capacity from pile load test.
Some are based on maximum permissible gross or net settlement as measured at the
pile butt while the others are based on the performance of the pile during the progress
of testing (Chellis, 1961; Whitaker, 1976; Poulos and Davis, 1980; Fuller, 1983). It is
recommended to evaluate the load carrying capacity of piles and drilled shaft using
any of the following methods along with the arbitrary methods:
(a)
Davission Offset Limit
(b) British Standard Institution Criterion
(c)
Indian Standard Criteria
(d) Butler-Hoy Criterion
(e)
Brinch-Hansen 90% Criterion
(f)
Other methods approved by the Geotechnical Engineer
The recommended criteria to be used for evaluating the ultimate and allowable load
carrying capacity of piles and drilled shaft are summarized below.
(a)
A very useful method of computing the ultimate failure load has been
reported by Davisson (1973). This method is based on offset method that
defines the failure load. The elastic shortening of the pile, considered as
point bearing, free standing column, is computed and plotted on the load-
settlement curve, with the elastic shortening line passing through the
origin. The slope of the elastic shortening line is 20o. An offset line is
drawn parallel to the elastic line. The offset is usually 0.15 inch plus a
quake factor, which is a function of pile tip diameter. For normal size
piles, this factor is usually taken as 0.1D inch, where D is the diameter of
pile in foot. The intersection of offset line with gross load-settlement
curve determines the arbitrary ultimate failure load. Davisson method is
too restrictive for drilled piles, unless the resistance is primarily friction.
This method is recommended for driven precast piles.
(b)
Terzaghi (1942) reported that the ultimate load capacity of a pile may be
considered as that load which causes a settlement equal to 10% of the pile
diameter. However, this criterion is limited to a case where no definite
failure point or trend is indicated by the load-settlement curves. This
পৎরঃবৎরড়হ যধং নববহ রহপড়ৎঢ়ড়ৎধঃবফ রহ ইঝ ৮০০৪ চ্ঈড়ফব ড়ভ চৎধপঃরপব ভড়ৎ
ঋড়ঁহফধঃরড়হংচ্ যিরপয ৎবপড়সসবহফং:যধঃ:যব ঁষঃরসধঃব ষড়ধফ পধঢ়ধপরঃু ড়ভ ঢ়রষব
should be that which causes the pile to settle a depth of 10% of pile width
or diameter.
(c)
The allowable load capacity of pile should be 50% of the final load,
which causes the pile to settle a depth of 10% of pile width or diameter
(BS 8004).
(d)
Ultimate load capacity of pile is smaller of the following two (IS: 2911
Part-4):
(i)
Load corresponding to a settlement equal to 10% of the pile
diameter in the case of normal uniform diameter pile or 7.5% of
base diameter in case of under-reamed or large diameter cast in-situ
pile.
(ii) Load corresponding to a settlement of 12 mm.
(e)
Allowable load capacity of pile is smaller of the following (IS: 2911
Part-4):
(i)
Two thirds of the final load at which the total settlement attains a
value of 12 mm.
(ii) Half of the final load at which total settlement equal to 10% of the
pile diameter in the case of normal uniform diameter pile or 7.5% of
base diameter in case of under-reamed pile.
(f)
Butler and Hoy (1977) states that the intersection of tangent at initial
straight portion of the load-settlement curve and the tangent at a slope
point of 1.27 mm/ton determines the arbitrary ultimate failure load.
(g)
The Brinch Hansen (1963) proposed a definition for ultimate load
capacity as that load for which the settlement is twice the settlement
under 90 percent of the full test load.
(h)
Where failure occurs, the ultimate load may be taken to calculate the
allowable load using a factor of safety of 2.0 to 2.5.
For load test on working pile/shaft, the safe load should be determined using the
criteria of Sec 3.10.1.16.
##### 3.11.2.2 Some factors influencing interpretations of load test results for axial
compression
The following factors should be taken into account while interpreting the test results
from pile load tests:
(a)
Potential residual loads (strains) in the pile which could influence the
interpreted distribution of load along the pile shaft.
(b)
Possible interaction of friction loads from test pile with downward
friction transferred to the soil from reaction piles obtaining part or all of
their support in soil at levels above the tip level of the test pile.
(c)
Changes in pore water pressure in the soil caused by pile driving,
construction fill and other construction operations which may influence
the test results for frictional support in relatively impervious soils such as
clay and silt.
(d)
Differences between conditions at time of testing and after final
construction such as changes in grade groundwater level.
(e)
Potential loss of soil resistance from events such as excavation, or scour,
or both of surrounding soil.
(f)
Possible difference in the performance of a pile in a group or of a pile
group from that of a single pile.
(g)
Effect on long term pile performance of factors such as creep,
environmental effects on pile material, friction loads from swelling soils
and strength losses.
(h)
Type of structure to be supported, including sensitivity of structure to
movement and relations between live and dead loads.
(i)
Special testing procedures which may be required for the application of
certain acceptance criteria or methods of interpretation.
(j)
Requirement of all conditions for non-tested piles be basically identical to
those for test pile including such thing as subsurface conditions, pile type,
length, size and stiffness, and pile installation methods and equipment so
that application or extrapolation of the test results to such other piles is
valid.
#### 3.11.3 Load Test for Uplift Capacity of Driven Pile, Bored Pile and Drilled
Shaft
Where required by the design, the uplift capacity of pile and drilled shaft shall be
determined by an approved method or analysis based on a minimum factor of safety
of three or by load tests conducted in accordance with ASTM D3689 (Standard Test
Method for Deep Foundations Under Static Axial Tensile Load). The maximum
allowable uplift load shall not exceed the ultimate load capacity as determined using
the results of load test conducted in accordance with ASTM D3689, divided by a
factor of safety of 2.0. Where uplift is due to wind or seismic loading, the minimum
factor of safety shall be 2.0 where capacity is determined by an analysis and 1.5
where capacity is determined by load tests.
For group pile subjected to uplift, the allowable working uplift load for the group
shall be calculated by an approved method of analysis where the piles in the group
are placed at centre-to-centre spacing of at least 2.5 times the least horizontal
dimension of the largest pile, the allowable working uplift load for the group is
permitted to be calculated as the lesser of the two:
(i)
The proposed individual working load times the number of piles in the
group.
(ii) Two-thirds of the effective weight of the group and the soil contained
within a block defined by the perimeter of the group and the embedded
length of the pile.
(iii) One-half the effective weight of the pile group and the soil contained
within a block defined by the perimeter of the group and the embedded
pile length plus one-half the total soil shear on the peripheral surface of
the group
Uplift or tension test on piles subject to tension/uplift shall be performed by a
continuous rate of uplift (CRU) or an incremental loading (i.e. ML) test. Where uplift
loads are intermittent or cyclic in character, as in wave loading on a marine structure,
it is recommended to adopt repetitive loading on the test pile. The tests shall be
performed in accordance with ASTM D3689. Safe load shall be taken as the least of
the following:
(a)
Two thirds of the load at which the total displacement (pile top) is 12 mm
or the load corresponding to a specified permissible uplift, and
(b)
Half of the load at which the load displacement curve shows a clear break
(downward trend).
The initial load test (on test pile/shaft) shall be carried out up to twice the estimated
design load or the load displacement curve shows a clear break. The routine test on
working pile shall be done up to one and a half times the design load or 12 mm total
displacement whichever occurred earlier.
#### 3.11.4 Load Tests for Lateral Load Capacity
Load test for lateral capacity shall be performed as per the procedure of ASTM
D3966. Safe load capacity shall be determined as per criteria mentioned in 3.10.1.20
for driven piles.
Division C: Additional Considerations in Planning, Design and
Construction of Building Foundations (Sections 3.12 To 3.22)
3.12
Excavation
Excavation for building foundation or for other purpose shall be done in a safe
manner so that no danger to life and property prevails at any stage of the work or
after completion. The requirements of this Section shall be satisfied for all such
works in addition to those of Sec 3.3 of Part 7.
Permanent excavations shall have retaining walls of sufficient strength made of steel,
masonry, or reinforced concrete to retain the embankment, together with any
surcharge load.
Excavations for any purpose shall not extend within 300 mm under any footing or
foundation, unless such footing or foundation is properly underpinned or protected
against settlement, beforehand.
The design and construction of deep excavation work more than 6 m depth or
excavation in soft soil or erratic soil must be checked by a competent Geotechnical
Engineer.
#### 3.12.1 Notice to Adjoining Property
Prior to any excavation close to an adjoining building in another property, a written
notice shall be given to the owner of the adjoining property at least 10 days ahead of
the date of excavation. The person undertaking the excavation shall, where
necessary, incorporate adequate provisions and precautionary measures to ensure
safety of the adjoining property and shall supply the details of such measures in the
notice to the owner of the adjoining property. He shall obtain approval of the
Authority regarding the protective provisions, and permission of the owner of the
adjoining property regarding the proposed excavation in writing. The protective
measures shall incorporate the following:
(i)
Where the level of the foundations of the adjoining structure is at or
above the level of the bottom of the proposed excavation, the vertical load
of the adjoining structure shall be supported by proper foundations,
underpinning, or other equivalent means.
(ii) Where the level of the foundations of the adjoining structure is below the
level of the bottom of the proposed excavation, provision shall be made to
support any increased vertical or lateral load on the existing adjoining
structure caused by the new construction.
If on giving the required notice, incorporating or proposing to incorporate the
protective provisions which have duly been approved by the Authority, the owner of
the adjoining property refuses to permit the proposed excavation or to allow
necessary access and other facilities to the person undertaking the excavation for
providing the necessary and approved protection to the adjoining property, the
responsibility for any damage to the adjoining property due to excavation shall be
that of the owner of the adjoining property.
#### 3.12.2 Excavation Work
Every excavation shall be provided with safe means of entry and exit kept available
at all times. When an excavation has been completed, or partly completed and
discontinued, abandoned or interrupted, or the required permits have expired, the lot
shall be filled and graded to eliminate all steep slopes, holes, obstructions or similar
sources of hazard. Fill material shall consist of clean, noncombustible substances.
The final surface shall be graded in such a manner as to drain the lot, eliminate
pockets, prevent accumulation of water, and preclude any threat of damage to the
foundations on the premises or on the adjoining property.
##### 3.12.2.1 Methods of protection
Shoring, Bracing and Sheeting
With the exception of rock cuts, the sides of all excavations, including related or
resulting embankments, 1.5 m or greater in depth or height measured from the level
of the adjacent ground surface to the deepest point of excavation, shall be protected
and maintained by shoring, bracing and sheeting, sheet piling, or other retaining
structures. Alternatively, excavated slopes may be inclined not steeper than 1:1, or
stepped so that the average slope is not steeper than forty five degrees with no step
more than 1.5 m high, provided such slope does not endanger any structure,
including subsurface structures. All sides or slopes of excavations or embankments
shall be inspected after rainstorms, or any other hazard increasing event, and safe
conditions shall be restored. Sheet piling and bracing needed in trench excavations
shall have adequate strength to resist possible forces resulting from earth or
surcharge pressure. Design of Protection system shall be checked by a qualified
Geotechnical Engineer.
Guard Rail
A guard rail or a solid enclosure at least 1 m high shall be provided along the open
sides of excavations, except that such guard rail or solid enclosure may be omitted
from a side or sides when access to the adjoining area is precluded, or where side
slopes are one vertical to three horizontal or flatter.
##### 3.12.2.2 Placing of construction material
Excavated materials and superimposed loads such as equipment, trucks, etc. shall not
be placed closer to the edge of the excavation than a distance equal to one and one-
half times the depth of such excavation, unless the excavation is in rock or the sides
have been sloped or sheet piled (or sheeted) and shored to withstand the lateral force
imposed by such superimposed load. When sheet piling is used, it shall extend at
least 150 mm above the natural level of the ground. In the case of open excavations
with side slopes, the edge of excavation shall be taken as the toe of the slope.
##### 3.12.2.3 Safety regulations
Whenever subsurface operations are conducted that may impose loads or movement
on adjoining property, such as driving of piles, compaction of soils, or soil
densification, the effects of such operations on adjoining property and structures shall
be considered. The owner of the property that may be affected shall be given 48
hours written notice of the intention to perform such operations. Where construction
operations will cause changes in the ground water level under adjacent buildings, the
effects of such changes on the stability and settlement of the adjacent foundation
shall be investigated and provision made to prevent damage to such buildings. When
a potential hazard exists, elevations of the adjacent buildings shall be recorded at
intervals of twenty four hours or less to ascertain if movement has occurred. If so,
necessary remedial action shall be undertaken immediately.
Whenever, an excavation or fill is to be made that will affect safety, stability, or
usability of, the adjoining properties or buildings shall be protected as required by the
provisions of Sec 3.3 Part 7.
On excavation, the soil material directly underlying footings, piers, and walls shall be
inspected by an engineer/architect prior to construction of the footing. If such
inspection indicates that the soil conditions do not conform to those assumed for the
purposes of design and described on the plans, or are unsatisfactory due to
disturbance, then additional excavation, reduction in allowable bearing pressure, or
other remedial measures shall be adopted.
Except in cases where a proposed excavation will extend less than 1.5 m below
grade, all underpinning operations and the construction and excavation of temporary
or permanent cofferdams, caissons, braced excavation surfaces, or other
constructions or excavations required for or affecting the support of adjacent
properties or buildings shall be subject to controlled inspection. The details of
underpinning, and construction of cofferdams, caissons, bracing or other
constructions required for the support of adjacent properties or buildings shall be
shown on the plans or prepared in the form of shop or detail drawings and shall be
approved by the engineer who prepared the plans.
3.13
Dewatering
All excavations shall be drained and the drainage maintained as long as the
excavation continues or remains. Where necessary, pumping shall be used. No
condition shall be created as a result of construction operations that will interfere
with natural surface drainage. Water courses, drainage ditches, etc. shall not be
obstructed by refuse, waste building materials, earth, stones, tree stumps, branches,
or other debris that may interfere with surface drainage or cause the impoundment of
surface water.
3.14
Slope Stability of Adjoining Buildings
The possibility of overturning and sliding of the building shall be considered. The
minimum factor of safety against overturning of the structure as a whole shall be 1.5.
Stability against overturning shall be provided by the dead load of the building, the
allowable uplift capacity of piling, anchors, weight of the soil directly overlying
footings provided that such soil cannot be excavated without recourse to major
modification of the building, or by any combination of these factors.
The minimum factor of safety against sliding of the structure under lateral load shall
be 1.5. Resistance to lateral loads shall be provided by friction between the
foundation and the underlying soil, passive earth pressure, batter piles or by plumb
piles, subject to the following:
(i)
The resistance to lateral loads due to passive earth pressure shall not be
taken into consideration where the abutting soil could be removed
inadvertently by excavation.
(ii) In case of pile supported structures, frictional resistance between the
foundation and the underlying soil shall be discounted.
(iii) The available resistance to friction between the foundation and the
underlying soil shall be predicted on an assumed friction factor of 0.5. A
greater value of the coefficient of friction may be used subject to
verification by analysis and test.
The faces of cut and fill slopes shall be prepared and maintained to control erosion.
The control may consist of effective planting. The protection for slopes shall be
installed as soon as practicable. Where cut slopes are not subject to erosion due to
erosion resistant character of the materials, such protection may be omitted. Where
necessary, check dams, cribbing, riprap or other devices or methods shall be
employed to control erosion.
3.15
Fills
#### 3.15.1 Quality of Fill
The excavation outside the foundation shall be backfilled with soil that is free of
organic material, construction debris and large rocks. The backfill shall be placed in
lifts and compacted in a manner which does not damage foundation, the
waterproofing or damp-proofing material.
#### 3.15.2 Placement of Fill
Fills to be used to support the foundation of any building or structure shall be placed
in accordance with established engineering principle. Before placement of the fill,
the existing ground surface shall be stripped off all organic growth, timber, rubbish
and debris. After stripping, the ground surface shall be compacted. Materials for fill
shall consist of sand, gravel, crushed stone, crushed earth, or a mixture of these. The
fill material shall contain no particles exceeding 100 mm in the largest dimension. A
soil investigation report and a report of satisfactory placement of fill, both acceptable
to the Building Official shall be submitted. In an uncontrolled fill, the soil within the
building area shall be explored using test pits. At least one test pit penetrating at least
2 m below the level of the bottom of the proposed foundation shall be provided for
every 200 m2 of building area. Wherever such test pits consistently indicate that the
fill is composed of material that is free of voids and free of extensive inclusion of
mud, organic materials such as paper, garbage, cans, metallic objects, or debris, the
fill material shall be acceptable. Where the fill shows voids or inclusions as described
above, either the fill shall be treated as having no presumptive bearing capacity, or
the building shall incorporate adequate strength and stiffness to bridge such voids or
inclusions or shall be articulated to prevent damage due to differential or localized
settlement of the fill.
#### 3.15.3 Specifications
Where foundations are to be placed on controlled fill materials, the fill must be
compacted in layers not exceeding 300 mm. Clear specifications shall be provided
for the range of water content, the degree of compaction to be achieved and the
method of compaction that shall be followed. Such specifications shall be based on
the shear strength requirement for the fill soil and allowable settlement estimate. The
minimum density of controlled fill shall be 95% of the optimum density obtained
from "Standard Test Methods for Laboratory Compaction Characteristics of Soil
Using Modified Effort ", (ASTM D1557).
The degree of compaction achieved in a fill shall be obtained from in-situ density
measurements. No new layer shall be placed unless a satisfactory density is attained
in each layer.
3.16
Protective Retaining Structures for Foundations/ Shore Piles
A retaining wall is a wall designed to resist lateral earth and/or fluid pressures,
including any surcharge, in accordance with accepted engineering practice. Retaining
walls for foundations shall be designed to ensure stability against overturning,
sliding, excessive foundation pressure and water uplift; and that they be designed for
a safety factor of 1.5 against lateral sliding and overturning. Generally sheet pile
retaining walls are used for construction raft foundations for buildings. Taller sheet
piles may need a tie back anchor driven and anchored behind the soil of the sheet pile
retaining wall.
3.17
Waterproofing and Damp-Proofing
#### 3.17.1 General
Walls or portions thereof that retain earth and enclose interior spaces, and floors
below grade shall be waterproofed and damp-proofed, with the exception of those
spaces where such omission is not detrimental to the building or occupancy. The roof
is also required to be waterproofed. The owner shall perform a subsurface
investigation to determine the possibility of the ground water table rising above the
proposed elevation of the floor or floors below grade unless satisfactory data from
adjacent areas demonstrate that ground water has not been a problem.
There may arise two situations: (i) where no hydrostatic pressure occurs and
(ii) where hydrostatic pressure occurs. Where hydrostatic pressure conditions exist,
floors and walls below finished ground level shall be waterproofed in accordance
with Sec 3.17.1.1 below. Where hydrostatic pressure conditions do not exist, damp-
proofing and perimeter drainage shall be provided in accordance with Sec 3.17.1.2
below. In addition, the damp-proofing and waterproofing shall also meet the
requirements of Sec 3.13.3. All damp-proofing and waterproofing materials shall
conform to the requirements of Sec 2.16.7 of Part 5.
##### 3.17.1.1 Waterproofing where hydrostatic pressure occurs
Where ground water investigation indicates that a hydrostatic pressure condition
exists, or is likely to occur, walls and floors shall be waterproofed in accordance with
the provisions stated as under.
##### 3.17.1.2 Floor waterproofing
Floors required to be waterproofed shall be of concrete and shall be designed and
constructed to withstand the anticipated hydrostatic pressure. Waterproofing of the
floor shall be accomplished by placing under the slab a membrane of rubberized
asphalt, or butyl rubber, or polymer modified asphalt, or neoprene, or not less than
### 0.15 mm polyvinyl chloride or polyethylene, or other approved materials, capable of
bridging nonstructural cracks. Joints in the membrane shall be lapped not less than
150 mm and sealed in an approved manner.
##### 3.17.1.3 Wall waterproofing
Walls required to be waterproofed shall be of concrete or masonry designed to
withstand the anticipated hydrostatic pressure and other lateral loads. Prior to the
application of waterproofing materials on concrete walls, all holes and recesses
resulting from the removal of form ties shall be sealed with a bituminous material or
other approved methods or materials. Unit masonry walls shall be pargeted on the
exterior surface below ground level with not less than 10 mm of Portland cement
mortar. The pargeting shall be continued to the foundation. Pargeting of unit masonry
walls is not required where a material is approved for direct application to the
masonry.
Waterproofing shall be applied from a point 300 mm above the maximum elevation
of the ground water table down to the top of the spread portion of the foundation.
The remainder of the wall up to a level not less than 150 mm above finished grade
shall be damp-proofed.
Wall waterproofing materials shall consist of two-ply hot-mopped felts, not less than
### 0.15 mm polyvinylchloride, 1.0 mm polymer modified asphalt, 0.15 mm
polyethylene or other approved methods or materials capable of bridging
nonstructural cracks. Joints in the membrane shall be lapped not less than 150 mm
and sealed in an approved manner. Joints in walls and floors, joints between the wall
and the floor, and penetrations of the wall and floor shall be made watertight utilizing
established methods and materials.
##### 3.17.1.4 Damp-proofing with no hydrostatic pressure
Where hydrostatic pressure will not occur, floors and walls shall be damp-proofed
and a subsoil drainage system shall be installed as described below:
##### 3.17.1.5 Floor damp-proofing
For floors, damp-proofing materials shall be installed between the floor and base
materials. The base material shall not be less than 100 mm in thickness consisting
of gravel or crushed stone containing not more than 10 percent material that passes a
### 4.75 mm sieve. Where a site is located in well drained gravel or sand/gravel mixture,
a floor base is not required. When the finished ground level is below the floor level
for more than 25 percent of the perimeter of the building, the base material need not
be provided. Where a separate floor is provided above a concrete slab the damp-
proofing may be installed on top of the slab.
Damp-proofing materials, where installed beneath the slab, shall consist of not less
than 0.15 mm polyethylene with joints lapped not less than 150 mm, or other
approved methods or materials. Where permitted to be installed on top of the slab,
damp-proofing shall consist of mopped on bitumen, not less than 0.1 mm
polyethylene, or other approved methods or materials. Joints in membranes shall be
lapped not less than 150 mm and sealed in an approved manner.
##### 3.17.1.6 Wall damp-proofing
For walls, damp-proofing materials shall be installed and shall extend from a point
150 mm above grade, down to the top of the spread portion of the foundation.
Wall damp-proofing material shall consist of a bituminous material, acrylic modified
cement base coating, rubberized asphalt, polymer-modified asphalt, butyl rubber, or
other approved materials capable of bridging nonstructural cracks.
##### 3.17.1.7 Perimeter drain
A drain shall be placed around the perimeter of a foundation that consists of gravel or
crushed stone containing not more than 10 percent material that passes through a
### 4.76 mm sieve. The drain shall extend a minimum of 300 mm beyond the outside
edge of the foundation. The thickness shall be such that the bottom of the drain is not
higher than the bottom of the base under the floor, and that the top of the drain is not
less than 150 mm above the top of the foundation. The top of the drain shall be
covered with an approved filter membrane material. Where a drain tile or perforated
pipe is used, the invert of the pipe or tile shall not be higher than the floor elevation.
The top of joints or the top of perforations shall be protected with an approved filter
membrane material. The pipe or tile shall be placed on not less than 50 mm of gravel
or crushed stone complying with this section, and shall be covered with not less than
150 mm of the same material.
The floor base and foundation perimeter drain shall discharge by gravity or
mechanical means into an approved drainage system. Where a site is located in well
drained gravel or sand/gravel mixture, a dedicated drainage system is not
required. When the finished ground level is below the floor level for more than
25 percent of the perimeter of the building, the foundation drain need be provided
only around that portion of the building where the ground level is above the floor
level.
#### 3.17.2 Other Damp-proofing and Waterproofing Requirements
##### 3.17.2.1 Placement of backfill
The excavation outside the foundation shall be backfilled with soil that is free of
organic material, construction debris and large rocks. The backfill shall be placed in
lifts and compacted in a manner which does not damage the waterproofing or damp-
proofing material or structurally damage the wall.
##### 3.17.2.2 Site grading
The ground immediately adjacent to the foundation shall be sloped away from the
building at a slope not less than 1 unit vertical in 12 units horizontal (1:12) for a
minimum distance of 2.5 m measured perpendicular to the face of the wall or an
alternative method of diverting water away from the foundation shall be used.
Consideration shall be given to possible additional settlement of the backfill when
establishing the final ground level adjacent to the foundation.
##### 3.17.2.3 Erosion protection
Where water impacts the ground from the edge of the roof, down spout, scupper,
valley or other rainwater collection or diversion device, provisions shall be used to
prevent soil erosion and direct the water away from the foundation.
3.18
Foundation on Slopes
Where footings are to be founded on a slope, the distance of the sloping surface at
the base level of the footing measured from the centre of the footing shall not be less
than twice the width of the footing.
When adjacent footings are to be placed at different levels, the distance between the
edges of footings shall be such as to prevent undesirable overlapping of structures in
soil and disturbance of the soil under the higher footing due to excavation of the
lower footing.
On a sloping site, footing shall be on a horizontal bearing and stepped. At all changes
of levels, footings shall be lapped for a distance of at least equal to the thickness of
foundation or three times the height of step, whichever is greater. Adequate
precautions shall be taken to prevent tendency for the upper layers of soil to move
downhill.
3.19
Foundations on Fills and Problematic Soils
#### 3.19.1 Footings on Filled up Ground
Footings shall not be constructed on loosely filled up ground with non-uniform
density or consistency, unless adequate strengthening of the soil is made by applying
ground improvement techniques.
#### 3.19.2 Ground Improvement
In poor and weak subsoil, the design of shallow foundation for structures and
equipment may present problems with respect to both sizing of foundation as well as
control of foundation settlements. A viable alternative in certain situations developed
over recent years is to improve the subsoil to an extent that the subsoil would
develop an adequate bearing capacity and foundations constructed after subsoil
improvement would have resultant settlements within acceptable limits. Selection of
ground improvement techniques may be done in accordance with good practice.
#### 3.19.3 Soil Reinforcement
Use of suitable geo-synthetics/geo-textiles may be made in an approved manner for
ground improvement where applicable based on good practice.
3.20
Foundation Design for Dynamic Forces
#### 3.20.1 Effect of Dynamic Forces
Where machinery operations or other vibrations are transmitted through foundation,
consideration shall be given in the foundation design to prevent detrimental
disturbance of the soil. Impact forces shall be neglected in foundation design except
for foundations bearing on loose granular soils, foundations supporting cranes, heavy
machinery and moving equipment, or where ratio of live load causing the impact to
the dead load exceeds 50%.
#### 3.20.2 Machine Foundation
Machine foundations are subjected to the dynamic forces caused by the machine.
These dynamic forces are transmitted to the foundation supporting the machine.
Although the moving parts of the machine are generally balanced, there is always
some unbalance in practice which causes an eccentricity of rotating parts. This
produces an oscillating force. The machine foundation must satisfy the criteria for
dynamic loading in addition to that for static loading.
##### 3.20.2.1 Types of machine foundations
Basically, there are three types of machine foundation:
(i)
Machines which produce a periodic unbalanced force, such as
reciprocating engines and compressors. The speed of such machines is
generally less than 600 rpm. In these machines, the rotary motion of the
crank is converted into the translatory motion. The unbalanced force
varies sinusoidal.
(ii) Machines which produce impact loads, such as forge hammers and punch
presses. In these machines, the dynamic force attains a peak value in a
very short time and then dies out gradually. The response is a pulsating
curve. It vanishes before the next pulse. The speed is usually between 60
to 150 blows per minute.
(iii) High speed machines, such as turbines, and rotary compressors.
The speed of such machines is very high; sometimes, it is even more than
3000 rpm.
The following four types of machine foundations are commonly used.
(i)
Block Type: This type of machine foundation consists of a pedestal
resting on a footing (Figure 6.3.4a). The foundation has a large mass and
a small natural frequency.
(ii) Box Type: The foundation consists of a hollow concrete block (Figure
6.3.4b). The mass of the foundation is less than that in the block type and
the natural frequency is increased.
(iii) Wall Type: A wall type of foundation consists of a pair of walls having a
top slab. The machine rests on the top slab (Fig6.3.4c).
(iv) Framed Type: This type of foundation consists of vertical columns having
a horizontal frame at their tops. The machine is supported on the frame
(Figure 6.3.4d).
Machines which produce periodical and impulsive forces at low speeds are generally
provided with a block type foundation. Framed type foundations are generally used
for the machines working at high speeds and for those of the rotating types. Some
machines which induce very little dynamic forces, such as lathes, need not be
provided with a machine foundation. Such machines may be directly bolted to the
floor.
(a)
(b)
(c)
(d)
Figure 6.3.4. Types of machine foundations; (a) Block type; (b) Box type; (c) Wall type;
(d) Framed type
##### 3.20.2.2 Design considerations
For satisfactory performance, machine foundations should satisfy the following
requirements: (i) resonance is avoided, (ii) bearing capacity and settlement are safe,
and (iii) there is an adequate vibration and shock isolation. Avoidance of resonance is
discussed in this Section.
Resonance:
Based on their operating frequencies, the machines are classified as (i) low speed
having frequency less than 300 revolutions per minute (rpm), (ii) medium speed,
frequency 300 to 1000 rpm, and (iii) high speed, frequency greater than 1000 rpm.
To avoid resonance, the natural frequency (or the resonant frequency) of the machine
foundation-soil system must be either very large or very small compared to the
operating speed of the machine.
Low speed machines (B1 \< 300 rpm):
Provide a foundation with a natural frequency at least twice the operating frequency,
i.e., the frequency ratio Y (5 B1/B)) is less than 0.5. Natural frequency can be
increased (i) by increasing base area or reducing total static weight of the foundation,
(ii) by increasing modulus of shear rigidity of the soil by compaction, grouting or
injection, (iii) by using piles to provide the required foundation stiffness.
High speed machines (B1 > 1000 rpm):
Provide a foundation with natural frequency not higher than one-half of the operating
value, i.e., frequency ratio ≥2 . Natural frequency can be decreased by increasing
weight of foundation. During starting and stopping, the machine will operate briefly
at resonant frequency B7 of the foundation. Probable amplitude is computed at both
B7 and B1 and compared with allowable values to determine if the foundation
arrangement must be altered.
Types of foundations:
Considering their structural forms, the machine foundations, in general, are of the
following types: (i) box foundation consisting of a pedestal of concrete, (ii) box
foundation consisting of a hollow concrete block, (iii) wall foundation consisting of a
pair of walls supporting the machine. (iv) framed foundation consisting of vertical
columns and a top horizontal frame work which forms the seat of essential
machinery.
Low speed machines (e.g., forge hammers, presses, low speed reciprocating engines
and compressors) are generally supported on block foundation having a large contact
area with soil.
Medium speed machines (e.g., reciprocating diesel and gas engines) also have, in
general, block foundations resting on springs or suitable elastic pads.
High speed and rotating type of machines (e.g., internal combustion engines, electric
motors, and turbo generator machines) are generally mounted on framed foundations.
Other high speed machines are placed on block foundations.
As far as possible, the centre of gravity of the whole system and the centroid of the
base area should be on the same vertical axis. At the most an eccentricity of 5%
could be allowed.
Permissible amplitude:
Many times the permissible amplitude at operating speed is specified by the
manufactures. If not specified, the following values may be adopted for guidance (i)
low speed machines. (B1 \< 500 rpm), horizontal and vertical vibrations, A 5
### 0.25 mm. (ii) operating speed B1 5 500 to 1500 rpm, A 5 0.4 mm to 0.6 mm for
horizontal, and A 5 0.7 mm to 0.9 mm for vertical mode of vibration; (iii) operating
speed B1 up to 3000 rpm, A 5 0.2 mm for horizontal and A 5 0.5 mm for vertical
vibrations (iv) hammer foundations, A 5 10 mm.
##### 3.20.2.3 Design methods
The various design methods can be grouped as follows: (i) empirical and semi-
empirical methods, (ii) methods considering soil as a spring and (iii) methods
considering soil as a semi-infinite elastic mass (elastic half-space-approach) and its
equivalent lumped parameter method. The lumped parameter method is currently
preferred and will be described here. A good machine foundation should satisfy the
following criteria.
(i)
Like ordinary foundations, it should be safe against shear failure caused
by superimposed loads, and also the settlements should be within the safe
limits.
(ii) The soil pressure should normally not exceed 80% of the allowable
pressure for static loading.
(iii) There should be no possibility of resonance. The natural frequency of the
foundation should be either greater than or smaller than the operating
frequency of the machine.
(iv) The amplitudes under service condition should be within the permissible
limits for the machine.
(v)
The combined centre of gravity of the machine and the foundation should
be on the vertical line passing through the centre of gravity of the base
plane.
(vi) Machine foundation should be taken to a level lower than the level of the
foundation of the, adjacent buildings and should be properly separated.
(vii) The vibrations induced should neither be annoying to the persons nor
detrimental to other structures.
(viii) Richart (1962) developed a plot for vertical vibrations, which is generally
taken as a guide for various limits of frequency and amplitude which has
been presented in Figure 6.3.5(a). A modified chart suggested by IS:
2974-Part 1, Figure 6.3.5(b) may also be used.
(ix) The depth of the ground-water table should be at least one fourth of the
width of the foundation below the base place.
##### 3.20.2.4 Vibration analysis of a machine foundation:
Although a machine foundation has 6 degree of freedom, it is assumed to have a
single degree of freedom for a simplified analysis. Figure 6.3.6 shows a machine
foundation supported on a soil mass. In this case, the mass mf lumps together the
mass of the machine and the mass of foundation. The total mass mf acts at the centre
of gravity of the system. The mass is under the supporting action of the soil. The
elastic action can be lumped together into a single elastic spring with a stiffness k.
Likewise; all the resistance to motion is lumped into the damping coefficient c. Thus
the machine foundation reduces to a single mass having one degree of freedom. The
analysis of damped, forced vibration is, therefore, applicable to the machine
foundation.
(a)
(b)
Figure 6.3.5. Limits of frequency and amplitudes of foundation; (a) Richart (1962) chart;
(b) IS: 2974-Part 1, 1982
Figure 6.3.6. Machine foundation supported on a soil mass
3.20.2.5
Determination of parameters for vibration analysis
For vibration analysis of a machine foundation, the parameters m, c and k are required.
These parameters can be determined as under.
Mass (m):
When a machine vibrates, some portion of the supporting soil mass also vibrates. The
vibrating soil is known as the participating mass or in-phase soil mass. Therefore, the
total mass of the system is equal to the mass of the foundation block and machine Ô5Õ
and the mass (5) of the participating soil. Thus
5 5 5 + 5
(6.3.51)
Unfortunately, there is no rational method to determine the magnitude of 5. It is usually
related to the mass of the soil in the pressure bulb. The value of 5 generally varies
between zero and 5. In other words, the total mass (5) varies between 5 and 25 in
most cases.
Spring Stiffness (k):
The spring stiffness depends upon the type of soil, embedment of the foundation block,
the contact area and the contact pressure distribution. The following are the common
methods.
Laboratory Test:
অ:ৎরধীরধষ:বংঃ রিঃয াবৎঃরপধষ ারনৎধঃরড়হং রং পড়হফঁপঃবফ:ড় ফবঃবৎসরহব ণড়ঁহম্থং সড়ফঁষঁং (%).
Alternatively, the modulus of rigidity (,) is determined conducting the test under
torsional vibration, and % is obtained indirectly from the relation, % 5 2,(1 + C), where
ক্র রং চড়রংংড়হ্থং ৎধঃরড়. ঞযব ংঃরভভহবংং (স) রং ফবঃবৎসরহবফ ধং
m 5
w
TH
B
(6.3.52)
Where, ! = cross-sectional area of the specimen, and 6 = length of the specimen.
ইধৎশধহ্থং গবঃযড়ফ:
The stiffness can also be obtained from the value of % using the following relation
given by Barken.
m 5
1.12H
1Òê √
(6.3.53)
Where, = base area of the machine, i.e. area of contact.
Plate Load Test:
A repeated plate load test is conducted and the stiffness of the soil mW is found as the
slope of the load-deformation curve. The spring constant m of the foundation is as
under.
For cohesive soils:
m 5 mW n
A
Wy
(6.3.54)
For cohesionless soil:
m 5 mW n
A³=.2
W+0.3y
(6.3.55)
Where, is the width of foundation (in m), ! is the width of plate (in m).
Alternatively, spring constant can be obtained from the subgrade modulus m, as
m 5 mZ
(6.3.56)
Where, = area of foundation.
Resonance Test:
The resonance frequency B) is obtained using a vibrator of mass m set up on a steel
plate supported on the ground. The spring stiffness obtained from the relation
B) 5
\[
6 5
6 tm/5 5 4\_B)5
(6.3.57)
Where, F) is natural circular frequency.
Damping Constant (O):
Damping is due to dissipation of vibration energy, which occurs mainly because of
the following reasons.
(i)
Internal friction loss due to hysteresis and viscous effects.
(ii) Radiational loss due to propagation of waves through soil.
The damping factor D for an under-damped system can be determined in the
laboratory. Vibration response is plotted and the logarithmic decrement δ is found
from the plot, as
5
63
√1Ò3È # 5
&
6
(6.3.58)
The damping factor D may also be obtained from the area of hysteresis loop of the
load displacement curve, as
# 5
∆7
(6.3.59)
Where, I = total work done; and ∆I = work lost hysteresis. The value of # for
most soils generally varies between 0.01 and 0.1.
3.21
Geo-Hazard Analysis for Buildings
Geo-hazard analysis of buildings include design considerations for possible
landslides, ground subsidence, earthquakes and other seismic events, erosion and
scour,
construction
in
toxic
and/or
contaminated
landfills,
groundwater
contamination etc. A preliminary review of the selected site should be carried out for
existence of any of the above mentioned geo-hazard in the area. A detailed analysis
may be carried out only if the preliminary review indicates a significant threat for the
building which may exist from any of the above mentioned potential geo-hazard at
the selected location for the building. See relevant section for details.
3.22
List of Related Appendices
Appendix D
Methods of Soil Exploration, Sampling and Groundwater
Measurements
Appendix E
Recommended Criteria for Identification and Classification of
Expansive Soil
Appendix F
Construction of Pile Foundation
Appendix G
Other Methods of Estimating Ultimate Axial Capacity of Piles and
Drilled Shafts, and Design Charts for Settlement
Appendix H
References of Chapter 3 Part 6 (Soils and Foundations).