? .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 XDД −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 XDД 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 ?DД = 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 XDД = 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
(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 than0.075 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, G12 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)
Group
Symbol
Group Name B
Laboratory Classification
Percent
finer than
0.075 mm
Other Criteria Fine grained soils (Over 50% of the material smaller than0.075 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.
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: <vÒ<Ú 1³<Ú ≤0.10 (6.3.2) A consolidation test is to be performed on an undisturbed specimen at natural সড়রংঃঁৎব পড়হঃবহঃ ধহফ:ড় ৎবপড়ৎফ:যব:যরপশহবংং, ুঐচ্ ড়হ পড়হংড়ষরফধঃরড়হ ঁহফবৎ ধ ঢ়ৎবংংঁৎব ুঢ়চ্ বয়ঁধষ:ড় ড়াবৎনঁৎফবহ ঢ়ৎবংংঁৎব ঢ়ষঁং:যব বীঃবৎহধষ ঢ়ৎবংংঁৎব ষরশবষু:ড় নব বীবৎঃবফ ড়হ the soil. The specimen is then submerged under the same pressure and the final ঃযরপশহবংং ঐ্থ ৎবপড়ৎফবফ. জবষধঃরাব ংঁনংরফবহপব, .উ৪ রং ভড়ঁহফ ধং: .D4 5 GÒG′ G (6.3.3) Soils having .D4 0.02 are considered to be collapsible. 3.5.7 Identification and Classification of Dispersive Soils Dispersive nature of a soil is a measure of erosion. Dispersive soil is due to the dispersed structure of a soil matrix. An identification of dispersive soils can be made on the basis of pinhole test. The pinhole test was developed to directly measure dispersive potential of compacted fine grained soils in which water is made to flow through a small hole in a soil specimen, where water flow through the pinhole simulates water flow through a crack or other concentrated leakage channel in the impervious core of a dam or other structure. The test is run under 50, 180, 380 and 1020 mm heads and the soil is classified as follows in Table 6.3.4. Table 6.3.4: Classification of Dispersive Soil on the Basis of Pinhole Test (Sherard et. al. 1976) Test Observation Type of Soil Class of Soil Fails rapidly under 50 mm head. Dispersive soils D1 and D2 Erode slowly under 50 mm or 180 mm head Intermediate soils ND4 and ND3 No colloidal erosion under 380 mm or 1020 mm head Non-dispersive soils ND2 and ND1 Another method of identification is to first determine the pH of a 1:2.5 soil/water suspension. If the pH is above 7.8, the soil may contain enough sodium to disperse the mass. Then determine: (i) total excahangable bases, that is, 0³, L³, 9Q³and Na+ (milliequivalent per 100g of air dried soil) and (ii) cation exchange capacity (CEC) of soil (milliequivalent per 100g of air dried soil). The Exchangeable Sodium Percentage ESP is calculated from the relation: %C; 5 +H+ × 100(%) (6.3.4) %9; is given by: %9; 5 2 +H+ × 100(%) (6.3.5) If the %C; is above 8 percent and %C; plus %9Q; is above 15, dispersion will take place. The soils with %C; =7 to 10 are moderately dispersive in combination with reservoir waters of low dissolved salts. Soils with %C; greater than 15 have serious piping potential. Dispersive soils do not actually present any problems with building structures. However, dispersive soil can lead to catastrophic failures of earth embankment dams as well as severe distress of road embankments. 3.5.8 Identification and Classification of Soft Inorganic Soils No standard definition exists for soft clays in terms of conventional soil parameters, mineralogy or geological origin. It is, however, commonly understood that soft clays give shear strength, compressibility and severe time related settlement problems. In near surface clays, where form a crust, partial saturation and overconsolidation occur together and the overconsolidation is a result of the drying out of the clay due to changes in water table.
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 Structure0.0033 L
1/3000.0033 L
1/3000.0033 L
1/3000.0033 L
1/300 RCC Structures0.0015 L
1/6660.0015 L
1/6660.0021 L
1/5000.002 L
1/500 Multistoried Building (a) RCC or steel framed building with panel walls0.002 L
1/5000.002 L
1/5000.0025 L
1/4000.0033 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 Silos0.0015 L
1/6660.0015 L
1/6660.0025 L
1/4000.0025 L
1/400 Water Tank0.0015 L
1/6660.0015 L
1/6660.0025 L
1/4000.0025 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.
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
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 Foundations3.10.1 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, ?DД, 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 (?DД) of driven piles shall be determined in accordance with the following for compression loading. ?DД 5 ? + ?4 −I (6.3.11) For uplift loading; ?DД ≤0.7? + I (6.3.12) The allowable or working axial load shall be determined as: ?’ÐÐ> 5 ?DД/&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 methods3.10.1.11 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′ ±Lk′. The lateral effective stress, D′ is proportional to vertical effective stress, D/′ by a co-efficient, K. As such, B 5 0D/′ ±Lk′ 5 cD/′ (6.3.19a) Where, c 5 0±Lk′ 5 0±Lk′ 5 (1 −Zk′)√@ @ (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)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 sand3.10.1.15 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 sand3.10.1.16 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 Monument100 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 NoItem 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 MPa3.10.2 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 area0.035 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 sand3.10.4.7 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:
- = 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 0.03.10.6 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 ?DД 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) Method3.10.6.9 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 ?DД. 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, ?DД 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 ?DД, the total base area of the equivalent pier shall be used to determine the QT component of ?DД 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) pH9.5 to 12
pH indicator paper strips or electrical pH meter (ASTM D4972) Liquid limit450% 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