Overview of Foundation Concepts and Foundation Alternatives

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Overview of FoundationConcepts and FoundationAlternatives

Amit Prashant

Indian Institute of Technology Gandhinagar

Short Course on

Geotechnical Investigations for Structural Engineering

13– 15 November, 2014

IITGN Short Course on Geotechnical Investigations for Structural Engineering

Bearing Capacity

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Terzaghi’s Bearing Capacity Theory

AssumptionL/B ratio is large plain strain problemDf t B

Shear resistance of soil for Df depth is neglected

General shear failure

Shear strength is governed by Mohr-Coulomb Criterion

B

Df

neglected Effective overburden

q = γ’.Df

Strip Footing

f ’ f ’45− f ’/2 45− f ’/2

Shear

Planes

a b

de f

g i

j k

qu

Rough Foundation

Surface

c’- f ’ soilB

I

II IIIII III


4


. . 0.5 .u c qq c N q N B NγγTerzaghi’s bearing

capacity equation

Terzaghi’s bearing capacity factors

Local Shear Failure:

Modify the strength parameters such as:2

3mc c 1 2tan tan

3mφ φ

Square and circular footing:

1.3 . . 0.4 .u c qq c N q N B Nγγ

1.3 . . 0.3 .u c qq c N q N B Nγγ

For square

For circular

3


Total and Effective Stress Analysis

Total stress parameters

c and φFrom UC or UU test

In bearing capacity equation, use total overburden and

bulk/saturated density.

Effective stress parameters

c’ and φ’

From direct shear test, CU or CD test

In bearing capacity equation, use effective overburden and

bulk/submerged density.

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Effect of water table:

B

B

Dw

Df

Limit of influence

Case I: Dw t DfSurcharge, . w f wq D D Dγ γ

Case II: Df t Dw ≤ (Df + B)

Surcharge, . Fq Dγ

In bearing capacity equation

replace γ by-

w fD D

Bγ γ γ γ

Case III: Dw > (Df + B)

No influence of water table.

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IS:6403-1981 Recommendations

Shape

Factors

1 0.2 tan 452

fc

Dd

L

φ

1 0.1 tan 452

fq

Dd d

Lγφ

Inclination

Factors

Depth

Factors

Net Ultimate Bearing capacity: . . . . . 1 . . . 0.5 . . . . .nu c c c c q q q qq c N s d i q N s d i B N s d iγ γ γ γγ

. . . .nu u c c c cq c N s d i 5.14cNFor cohesive soils where,

, ,c qN N Nγ as per Vesic(1973) recommendations

1 0.2c

Bs

L1 0.2q

Bs

L1 0.4

Bs

LγFor rectangle,

1.3cs 1.2qs

0.8 for square, 0.6 for circles sγ γ

For square and circle,

for 10oφ

1qd dγ for 10oφ2

190

o

c qi iβ

2

1iγβφ


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Bearing CapacityCorrelations withSPT-value

Peck, Hansen, andThornburn (1974)

&

IS:6403-1981Recommendation

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Bearing Capacity Correlations with SPT-value

Teng (1962):

2 213 . . 5 100 . .

6nu w f wq N B R N D RFor Strip Footing:

2 21. . 3 100 . .

3nu w f wq N B R N D RFor Square and

Circular Footing:

For Df > B, take Df = B

0.5 1 1ww w

f

DR R

D

0.5 1 1w fw w

f

D DR R

D

Water Table Corrections:

B

B

Dw

Df

Limit of influence


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Bearing Capacity Correlations with CPT-value

0 100 200 300 4000

0.0625

0.1250

0.1675

0. 2500

nuq

qc

B (cm)

1fD

B

0.50

IS:6403-1981 Recommendation:

Cohesionless Soil

1.5B

to

2.0B

B

qc value is

taken as

average for

this zone

Schmertmann (1975):

2

kg in

0.8 cmc

q

qN Nγ

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Bearing Capacity Correlations with CPT-value

IS:6403-1981 Recommendation:

Cohesive Soil

Soil TypePoint Resistance Values

( qc ) kgf/cm2

Range of Undrained

Cohesion (kgf/cm2)

Normally consolidated

claysqc < 20 qc/18 to qc/15

Over consolidated clays qc > 20 qc/26 to qc/22

. . . .nu u c c c cq c N s d i


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Effective Area Method for Eccentric Loading

B

Df

eyex

L’=L-2ey

B’=B-2ey

AF=B’L’

yx

V

Me

F

xy

V

Me

F

In case of Moment loading

In case of Horizontal Force at

some height but the column is

centered on the foundation

.y Hx FHM F d

.x Hy FHM F d

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Settlement

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Pressure BulbSquare Footing Strip Footing

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Newmark’s Chart

Determine the depth, z, where youwish to calculate the stress increase

Adopt a scale as shown in the figure

Draw the footing to scale and placethe point of interest over the centerof the chart

Count the number of elements thatfall inside the footing, N

Calculate the stress increase as:

Point of

stress

calculation

Depth = z1

Depth = z2


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ApproximateMethods

.

.z

B Lq

B z L zσ

2

2z

Bq

B zσ

z

Bq

B zσ

Rectangular Foundation:

Square/Circular Foundation:

Strip Foundation:

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Settlement

Immediate Settlement: Occurs immediately after the construction.This is computed using elasticity theory (Important for Granular soil)

Primary Consolidation: Due to gradual dissipation of pore pressureinduced by external loading and consequently expulsion of water fromthe soil mass, hence volume change. (Important for Inorganic clays)

Secondary Consolidation: Occurs at constant effective stress withvolume change due to rearrangement of particles. (Important forOrganic soils)

SettlementS = Se + Sc + Ss

ImmediateSettlement

Se

PrimaryConsolidation

Sc

SecondaryConsolidation

Ss

For any of the above mentioned settlement calculations, we first need vertical stress

increase in soil mass due to net load applied on the foundation


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Elastic settlement of Foundation

0 0

1H H

e z z s x s ys

S dz dzE

ε σ µ σ µ σ

sE Modulus of elasticity

H Thickness of soil layer

sµ Poisson’s ratio of soil

Elastic settlement:

Elastic settlement for Flexible Foundation:

21e s fs

qBS I

Eµ

fI = influence factor: depends on the rigidity and shape of the foundation

sE = Avg elasticity modulus of the soil for (4B) depth below foundn level

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Steinbrenner’s Influence Factors for Settlement of the Corners of

loaded Area LxB on Compressible Stratus of m = 0.5, and Thickness Ht


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Elastic settlement of FoundationSoil Strata withSemi-infinite depth

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E in kPa

Several other sets of

correlations available


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Strain Influence Factor Method for Sandy Soil: Schmertmannand Hartman (1978)

2

1 20

zz

e fs

IS C C q D z

Eγ

1C Correction factor for foundation depth

1 0.5 f fD q Dγ γ

2C Correction factor for creep effects

q For square and circular foundation:

For foundation with L/B >10:

Interpolate the values for 1 < L/B < 10

1 0.2log time in years 0.1


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Settlement due to Primary Consolidation

log log1 1

s c c c c o avc

o o o c

C H C HS

e e

σ σ σσ σ

log1

s c o avc

o o

C HS

e

σ σσ

log1

c c o avc

o o

C HS

e

σ σσ

For NC clay

For OC clayo av cσ σ σ

For OC clayo c o avσ σ σ σ

c o o avσ σ σ σ

oσ Average effective vertical stress before construction

avσ Average increase in effective vertical stress

cσ Effective pre-consolidation pressure

oe Initial void ratio of the clay layer

cC Compression Index

sC Swelling Index

cH Thickness of the clay layer

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6av t m bσ σ σ σ

tσ

bσ

mσ

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Settlement Due to Secondary Consolidation

2

1

log1

cs

p

C H tS

e tα

pe

Cα Secondary Compression Index

2 1log

e

t t

Time, t (Log scale)

Void

Ratio, e

pe

1t 2teVoid ratio at the end of primary consolidation

cH Thickness of Clay Layer

Secondary consolidation settlement is more important in the case of

organic and highly-compressible inorganic clays


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Total Settlementfrom SPT Datafor Cohesionlesssoil

Multiply the settlementby factor W’

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Total Settlement from CPT Data for Cohesionless soil

3

2c

o

qC

σ

lnt ot

o

HS

C

σ σσ

Depth profile of cone resistancecan be divided in severalsegments of average coneresistance

Average cone resistance canbe used to calculate constant ofcompressibility.

Settlement of each layer iscalculated separately due tofoundation loading and thenadded together


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Fox’s Depth CorrectionFactor for RectangularFootings of (L)x(B) atDepth (D)

Depth factorc Embedded

c Surface

S

S

Rigidity Factor as perIS:8009-1976

Total settlement of

rigid foundation

Total settlement at the center

of flexible foundation

Rigidity factor 0.8

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ShallowFoundation Design

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Common Types of Footing

Strip footing

Spread Footing

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Common Types of Footing

Combined Footing

Raft or Mat footing


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Location and depth of Foundation

IS:1904-1986: Minimum depth of foundation = 0.50 m.

Foundation shall be placed below the zone ofExcessive volume change due to moisture variation (usually

exists within 1.5 to 3.5 m depth)

Topsoil or organic material

Unconsolidated material such as waste dump

Foundations adjacent to flowing water (flood water,

rivers, etc.) shall be protected against scouring.

A raised water table may cause damage to the

foundation byFloating the structure

Reducing the effective stress beneath the foundation

Water logging around the building: proper drainage system

around the foundation may be required so that water does not

accumulate.

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Location and depth of Foundation

Footings on surface rock or sloping rock faces

Shallow rock beds: foundation on the rock surface after chipping

Rock bed with slope: provide dowel bars of minimum 16 mm

diameter and 225 mm embedment into the rock at 1 m spacing.

Footings adjacent to existing structures

Minimum horizontal distance between the foundations shall not be

less than the width of larger footing. Otherwise, the principal of

2H:1V distribution be used to minimize influence to old structure

Proper care is needed during excavation phase of foundation

construction beyond merely depending on the 2H:1V criteria.

Excavation may cause settlement to old foundation due to lateral

bulging in the excavation and/or shear failure due to reduction in

overburden stress in the surrounding of old foundation


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Plate Load Test – IS:1888-1982

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Plate Load Test: Bearing Capacity

uf f

up p

q B

q B

For cohesioless soil

uf upq q

For cohesive soil


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Modulus ofSub-gradeReaction

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Differential Settlement

Caused by variations in soil profile and structural loads

Consider construction tolerances

Consider best case/worst case scenarios

Use D/ ratios

d D

DD

L

δ = maximum settlement

D= differential settlement

D/ d = angular distortion

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Total and Differential Settlement for Clays


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Total and Differential Settlement for Sands

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Design values of δD/δ Ratios


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How Accurateare our

SettlementPredictions?

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Allowable Bearing Pressure

Maximum bearing pressure that can be applied on thesoil satisfying two fundamental requirements

Bearing capacity with adequate factor of safety– net safe bearing capacity

Settlement within permissible limits (critical in most cases)– net safe bearing pressure


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Teng’s (1962) Correlation:

depth correction factor 1 2fD

DC

B

Sa in mm and all other

dimensions in meter.

.cor NN C N

1.75 for 0 1.05

0.7N o ao a

C PP

σσ

3.5 for 1.05 2.8

0.7N o ao a

C PP

σσ

oσ Effective Overburden stress

20.3

1.4 32n cor w D a

Bq N R C S

Bρ

Net safe bearing pressure

2kN m

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Meyerhof’s (1974) Correlation:

210.49 for 1.2 mn D aq N R S kN m Bρ

1 depth correction factor

1 0.2 1.2

D

f

R

D

B

22

2

0.30.32 for 1.2 mn D a

Bq N R S kN m B

Bρ

Net safe bearing pressure

2 depth correction factor

1 0.33 1.33

D

f

R

D

B

Bowel’s (1982) Correlation:

210.73 for 1.2 mn D aq N R S kN m Bρ

22

2

0.30.48 for 1.2 mn D a

Bq N R S kN m B

Bρ

N-value corrected for overburden using bazaraa’s equation, but

the N-value must not exceed field value


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IS Code recommendation: Use total settlement correlations with SPT

data to determine safe bearing pressure.

Correlations for raft foundations:

Rafts are mostly safe in bearing capacity and they do not show much

differential settlements as compared to isolated foundations.

20.7 3n w D aq N R C S kN mρTeng’s Correlation:

Peck, Hanson, and Thornburn (1974):20.88a net w aq C N S kN m

Correlations using CPT data:

Meyerhof’s correlations may be used by substituting qc/2 for N,

where qc is in kg/cm2.

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Net vs. Gross Allowable Bearing Pressure

2 2g c c c f cQ Q B D B D Dγ γ

Gross load

2 2

g cg f c c

Q Qq D D

B Bγ γ γ

2c

n g f c c

Qq q D D

Bγ γ γ

cγ γ is small, so it may be neglected

2c

n

Qq

B

Soil Soil

cD

fD

2c

a net

Qq

B

2c

g c c c

Qq D t

Bγ γ

t

2c

n g f c c f

Qq q D D t D

Bγ γ γ

Usually Dc+t is much smaller than Df

2c

n f

Qq D

Bγ

2c

a net f

Qq D

Bγ

2c

a gross

Qq

B


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SUMMARY of Terminology

Net Loading IntensityPressure at the level of foundation causing actual

settlement due to stress increase. This includes

the weight of superstructure and foundation only.

Ultimate Bearing capacity:

Maximum gross intensity of loading that the

soil can support against shear failure is

called ultimate bearing capacity.

Net Ultimate Bearing Capacity:

Maximum net intensity of loading that the

soil can support at the level of foundation.

Gross Loading IntensityTotal pressure at the level of foundation

including the weight of superstructure,

foundation, and the soil above foundation.

superstructure Foundation soil

Foundationg

Q Q Qq

A

n g fq q Dγ

from

Bearing capacity calculationuq

nu u fq q Dγ

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SUMMARY of Terminology

Gross Safe Bearing capacity:

Maximum gross intensity of loading that the soil

can safely support without the risk of shear failure.

Safe Bearing Pressure:

Maximum net intensity of loading that can be

allowed on the soil without settlement

exceeding the permissible limit.

Allowable Bearing Pressure:

Maximum net intensity of loading that can

be allowed on the soil with no possibility of

shear failure or settlement exceeding the

permissible limit.

Net Safe Bearing capacity:

Maximum net intensity of loading that the soil can

safely support without the risk of shear failure.nu

ns

qq

FOS

gs ns fq q Dγ

from settlement analysissqρ

Minimum of

bearing capacity and

settlement analysis

a netq


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Loads on Foundation

Permanent Load: This is actual service load/sustained loads of astructure which give rise stresses and deformations in the soil belowthe foundation causing its settlement.

Transient Load: This momentary or sudden load imparted to astructure due to wind or seismic vibrations. Due to its transitorynature, the stresses in the soil below the foundation carried by suchloads are allowed certain percentage increase over the allowablesafe values.

Dead Load: It includes the weight of the column/wall, footings,foundations, the overlaying fill but excludes the weight of thedisplaced soil

Live Load: This is taken as per the specifications of IS:875 (pt-2) –1987.

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Loads for Proportioning and Design of FoundationIS:1904 - 1986

Following combinations shall be used

Dead load + Live load

Dead Load + Live load + Wind/Seismic load

For cohesive soils only 50% of actual live load is consideredfor design (Due to settlement being time dependent)

For wind/seismic load < 25% of Dead + Live load

Wind/seismic load is neglected and first combination is used tocompare with safe bearing load to satisfy allowable bearing pressure

For wind/seismic load ≥ 25% of Dead + Live load

It becomes necessary to ensure that pressure due to secondcombination of load does not exceed the safe bearing capacity bymore than 25%. When seismic forces are considered, the safebearing capacity shall be increased as specified in IS: 1893 (Part-1)-2002 (see next slide). In non-cohesive soils, analysis for liquefactionand settlement under earthquake shall also be made.


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Other considerations for Shallow Foundation Design

For economical design, it is preferred to have square footing for

vertical loads and rectangular footing for the columns carrying

moment

Allowable bearing pressure should not be very high in comparison

to the net loading intensity leading to an uneconomical design.

It is preferred to use SPT or Plate load test for cohesionless soils

and undrained shear strength test for cohesive soils.

In case of lateral loads or moments, the foundation should also be

checked to be safe against sliding and overturning. The FOS shall

not be less than 1.75 against sliding and 2.0 against overturning.

When wind/seismic loads are considered the FOS is taken as 1.5

for both the cases.


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Combined Footings

Combined footing is preferred whenThe columns are spaced too closely that if isolated footing isprovided the soil beneath may have a part of common influencezone.

The bearing capacity of soil is such that isolated footing designwill require extent of the column foundation to go beyond theproperty line.

Types of combined footingsRectangular combined footing

Trapezoidal combined footing

Strap beam combined footing

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Rectangular Combined Footing

If two or more columns are carryingalmost equal loads, rectangularcombined footing is provided

Proportioning of foundation willinvolve the following steps

Area of foundation

Location of the resultant force

For uniform distribution of pressure under the foundation, theresultant load should pass through the center of foundation base.

Length of foundation,

Offset on the other side,

The width of foundation,

1 2

a net

Q QA

q2

1 2

Q Sx

Q Q

12L L S

2 1 0L L S L

B A L

Q1 Q2

Q1+Q2

x

L1 S L2


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Trapezoidal Combined Footing

If one of the columns is carrying muchlarger load than the other one,trapezoidal combined footing is provided

Proportioning of foundation will involvethe following steps if L, and L1 are known

Area of foundation

Location of the resultant force

For uniform distribution of pressure under the foundation, theresultant load should pass through the center of foundation base.This gives the relationship,

Area of the footing,

1 2

a net

Q QA

q2

1 2

Q Sx

Q Q

1 21

1 2

2

3

B B Lx L

B B1 2

2

B BL A

Solution of thesetwo equations

gives B1 and B2

Q1 Q2

Q1+Q2

x

L1 S L2

B1 B2

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Strap Combined Footing

Strap footing is used toconnect an eccentrically loadedcolumn footing to an interiorcolumn so that the moment canbe transferred through thebeam and have uniform stressdistribution beneath both thefoundations.

This type of footing is preferredover the rectangular ortrapezoidal footing if distancebetween the columns isrelatively large.

Some design considerations:

Strap must be rigid: Istrap/Ifooting > 2.

Footings should be proportioned to have approximately equal soil

pressure in order to avoid differential settlement

Strap beam should not have contact with soil to avoid soil reaction to it.

Q1 Q2

M2


Pile FoundationDesign

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When is it needed

Top layers of soil are highly compressible for it to supportstructural loads through shallow foundations.

Rock level is shallow enough for end bearing pilefoundations provide a more economical design.

Lateral forces are relatively prominent.

In presence of expansive and collapsible soils at the site.

Offshore structures

Strong uplift forces on shallow foundations due to shallowwater table can be partly transmitted to Piles.

For structures near flowing water (Bridge abutments, etc.)to avoid the problems due to erosion.


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Types of Piles Based on Their Function and Effectof Installation

Piles based on their functionEnd Bearing Piles

Friction Piles

Compaction Piles

Anchor Piles

Uplift Piles

Effect of InstallationDisplacement Piles

Non-displacement Piles

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Displacement PilesIn loose cohesionless soils

Densifies the soil upto a distance of 3.5 times the pile diameter(3.5D) which increases the soil’s resistance to shearing

The friction angle varies from the pile surface to the limit ofcompacted soil

In dense cohesionless soilsThe dilatancy effect decreases the friction angle within the zone ofinfluence of displacement pile (3.5D approx.).

Displacement piles are not effective in dense sands due to abovereason.

In cohesive soilsSoil is remolded near the displacement piles (2.0 D approx.) leadingto a decreased value of shearing resistance.

Pore-pressure is generated during installation causing lowereffective stress and consequently lower shearing resistance.

Excess pore-pressure dissipates over the time and soil regains itsstrength.

Example: Driven concrete piles, Timber or Steel piles


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Non-displacement Piles

Due to no displacement during installation, there is no heave inthe ground.

Cast in-situ piles may be cased or uncased (by removingcasing as concreting progresses). They may be provided withreinforcement if economical with their reduced diameter.

Enlarged bottom ends (three times pile diameter) may beprovided in cohesive soils leading to much larger point bearingcapacity.

Soil on the sides may soften due to contact with wet concreteor during boring itself. This may lead to loss of its shearstrength.

Concreting under water may be challenging and may resultingin waisting or necking of concrete in squeezing ground.

Example: Bored cast in-situ or pre-cast piles

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Load Transfer Mechanism of Piles

The frictional resistanceper unit area at anydepth

Ultimate skin frictionresistance of pile

Ultimate point load

Ultimate load capacityin compression

Ultimate load capacityin tension

.z

sz

Qq

S z perimeter of pileS

suQ

.pu pu pQ q A

u pu suQ Q Q

u suQ Q

bearing capacity of soilpuq bearing area of pilepA

upQ usQ

uQ

sQz

z


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IS:2911 Pile Load Capacity in Cohesionless Soils

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ForBoredPiles

ForDrivenPiles

66

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68


IS code recommends K-value to be chosen between 1 and 2 fordriven piles and 1 and 1.5 for bored piles. However, it is advisableto estimate this value based on the type of construction and fair

estimation of the disturbance to soil around pile. Typical values ofratio between K and Ko are listed below.

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IS:2911 Pile Load Capacity in Cohesive Soils

0.5For 1 0.5 , but 1v vu uc cσ α σ

0.25For 1 0.5 , but 0.5 and 1v vu uc cσ α σ


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IS:2911 Pile Load Capacity in Cohesive Soils

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Meyerhof’s Formula for Driven Piles based on SPT value

For L/D > 10

A limiting value of 1000 t/m2 for point bearing and 6 t/m2 is

suggested

For Sand:

For Non-plastic silt and fine sand:

For Clays:


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IS:2911 Pile Load Capacity in Non-CohesiveSoils Based on CPT data

The ultimate pointbearing capacity:

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IS:2911 Pile Load Capacity in Non-CohesiveSoils Based on CPT data

Correlation of SPT and CPT:

The ultimate skin friction resistance:


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Allowable Pile Capacity

Factor of Safety shall be used by giving due consideration to thefollowing points

Reliability of soil parameters used for calculation

Mode of transfer of load to soil

Importance of structure

Allowable total and differential settlement tolerated by structure

Factor of Safety as per IS 2911:

uall

QQ

FSu

all

QQ

FS

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Load Tests on Piles

Note: Piles used for initial testing are loaded to failure or at least twice the

design load. Such piles are generally not used in the final construction.

Note: During this test pile should be loaded upto 1.5 times the working

(design) load and the maximum settlement of the test should not exceed

12 mm. These piles may be used in the final construction.


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Vertical Load Test: Maintained Load Test

The test can be initial or routinetest

The load is applied in incrementsof 20% of the estimated safeload. Hence the failure load isreached in 8-10 increments.

Settlement is recorded for eachincrement until the rate ofsettlement is less than 0.1 mm/hr.

The ultimate load is said to havereached when the final settlementis more than 10% of the diameterof pile or the settlement keeps onincreasing at constant load.

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Vertical Load Test: Maintained Load Test

After reaching ultimate load, theload is released in decrements of1/6th of the total load andrecovery is measured until fullrebound is established and nextunload is done.

After final unload the settlementis measured for 24 hrs toestimate full elastic recovery.

Load settlement curve dependson the type of pile


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Vertical Load Test: Maintained Load TestUltimate Load

De Beer (1968):

Load settlement curve is plotted in a

log-log plot and it is assumed to be a

bilinear relationship with its

intersection as failure load

Chin Fung Kee (1977):

Assumes hyperbolic curve.

Relationship between

settlement and its division with

load is taken as to be bilinear

with its intersection as failure

load

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Vertical Load Test: Maintained Load Test SafeLoad as per IS: 2911

Safe Load for Single Pile:

Safe Load for Pile Group:


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Dynamic Pile Formula for Driven Piles:Modified Hiley Formula

. . .

/ 2u

W HQ

S C

α η. . .

/ 2u

W HQ

S C

α η

WH

SuQ

Weight of hammer

Height of fall

Pile resistance or Pile capacity

Pile penetration for the last blow

α Hammer fall efficiency

Efficiency of blowηSum of temporary elastic compressionof pile, dolly, packing, and ground

C

Note: Dynamic pile formula are not used for soft clays due to pore pressure evolution

Overview of Foundation Concepts and Foundation Alternatives

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