AP-Foundation Concepts and Foundation

8/20/2019 AP-Foundation Concepts and Foundation

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Overview of FoundationConcepts and FoundationAlternatives

Amit PrashantIndian 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

Assumption L/B ratio is large plain strain problem Df 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 overburdenq = γ .Df

Strip Footing

’

’ 45− ’/2 45− ’/2

ShearPlanes

a b

de f

g i

j k

qu

Rough FoundationSurface

c’- ’ soilB

I

II II

III III


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. . 0.5 .u c q

q c N q N B N γ γ Terzaghis bearingcapacity equation

Terzaghis bearing capacity factors

Local Shear Failure:

Modify the strength parameters such as:2

3m

c c 12

tan tan3

mφ φ

Square and circular footing:

1.3 . . 0.4 .u c q

q c N q N B N γ γ

1.3 . . 0.3 .u c q

q c N q N B N γ γ

For square

For circular


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Total and Effective Stress Analysis

Total stress parameters

c and φ

From UC or UU test

In bearing capacity equation, use total overburden andbulk/saturated density.

Effective stress parameters

c’ and φ’

From direct shear test, CU or CD test

In bearing capacity equation, use effective overburden andbulk/submerged density.

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Effect of water table:

B

B

Dw

Df

Limit of influence

Case I: D w D f

Surcharge, . w f wq D D Dγ γ

Case II: D f D w ≤ (D f + B)

Surcharge, . F q Dγ

In bearing capacity equationreplace γ by-

w f D D

Bγ γ γ γ

Case III: D w > (D f + B)

No influence of water table.


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IS:6403-1981 Recommendations

ShapeFactors

1 0.2 tan 45

2

f

c

Dd

L

φ

1 0.1 tan 45

2

f

q

Dd d

Lγ

φ

InclinationFactors

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 γ γ

. . . .nu u c c c cq c N s d i 5.14

c N For cohesive soils where,

, ,c q

N N N γ as per Vesic(1973) recommendations

1 0.2c

Bs

L 1 0.2

q

Bs

L 1 0.4

Bs

Lγ For rectangle,

1.3c

s 1.2qs

0.8 for square, 0.6 for circles sγ γ

For square and circle,

for 10oφ

1q

d d γ for 10oφ

2

190

o

c qi i

β

2

1iγ β

φ


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Bearing Capacity

Correlations withSPT-value

Peck, Hansen, andThornburn (1974)

&

IS:6403-1981Recommendation


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Bearing Capacity Correlations with SPT-value

Teng (1962):

2 21

3 . . 5 100 . .6

nu w f wq N B R N D R For Strip Footing:

2 21

. . 3 100 . .3

nu w f wq N B R N D R For Square andCircular Footing:

For Df > B, take Df = B

0.5 1 1ww w f

D R R

D

0.5 1 1w f w w f

D D R 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)

1 f

D

B0.5

0

IS:6403-1981 Recommendation:

Cohesionless Soil

1.5Bto

2.0B

Bqc value istaken as

average forthis zone

Schmertmann (1975):

2

kg in

0.8 cm

cq

q N N γ


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Bearing Capacity Correlations with CPT-valueIS:6403-1981 Recommendation:

Cohesive Soil

Soil TypePoint Resistance Values

( qc ) kgf/cm2

Range of UndrainedCohesion (kgf/cm2)

Normally consolidated

clays

qc < 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-2eyAF=B’L’

y

x

V

M e

F

x y

V

M eF

In case of Moment loading

In case of Horizontal Force atsome height but the column is

centered on the foundation

. y Hx FH M F d

. x Hy FH

M 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 ofstresscalculation

Depth = z1

Depth = z2


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ApproximateMethods

.

. z

B Lq

B z L zσ

2

2 z

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 pressure

induced 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 stressincrease in soil mass due to net load applied on the foundation


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Elastic settlement of Foundation

0 0

1 H H

e z z s x s y

s

S dz dz E

ε σ µ σ µ σ

s E Modulus of elasticity

H Thickness of soil layer

sµ Poissons ratio of soil

Elastic settlement:

Elastic settlement for Flexible Foundation:

21e s f s

qBS I

E µ

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

s E = 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 ofloaded Area LxB on Compressible Stratus of

= 0.5 , and Thickness H t


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Elastic settlement of FoundationSoil Strata withSemi-infinite depth


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E in kPa

Several other sets ofcorrelations available


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Settlement Due to Secondary Consolidation

2

1

log1

cs

p

C H t S

e t

α

pe

C α Secondary Compression Index 2 1log

e

t t

Time, t (Log scale)

V o i d R a t i o , e

pe

1t 2t

eVoid ratio at the end of primary consolidation

c H Thickness of Clay Layer

Secondary consolidation settlement is more important in the case oforganic 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

2

c

o

qC

σ

lnt ot

o

H S

C

σ σ

σ

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 factor

c 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 of Excessive 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 thefoundation by Floating the structure

Reducing the effective stress beneath the foundation

Water logging around the building: proper drainage systemaround the foundation may be required so that water does notaccumulate.


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

Footings adjacent to existing structures

Minimum horizontal distance between the foundations shall not beless than the width of larger footing. Otherwise, the principal of2H: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 lateralbulging in the excavation and/or shear failure due to reduction inoverburden 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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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 Accurate

are ourSettlement

Predictions?


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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 2 f

D

D

C B

Sa in mm and all otherdimensions in meter.

.cor N N C N

1.75

for 0 1.050.7

N o a

o a

C PP

σσ

3.5

for 1.05 2.80.7

N o a

o a

C PP

σσ

oσ Effective Overburden stress

2

0.31.4 3

2n cor w D a

Bq N R C S

Bρ

Net safe bearing pressure

2kN m


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Allowable Bearing PressureMeyerhof’s (1974) Correlation:

2

10.49 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

2

2

2

0.30.32 for 1.2 m

n 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:

2

10.73 for 1.2 mn D aq N R S kN m Bρ

2

2

2

0.30.48 for 1.2 m

n D a

Bq N R S kN m B

Bρ

N-value corrected for overburden using bazaraas equation, butthe N-value must not exceed field value


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IS Code recommendation: Use total settlement correlations with SPTdata to determine safe bearing pressure.

Correlations for raft foundations:

Rafts are mostly safe in bearing capacity and they do not show muchdifferential settlements as compared to isolated foundations.

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

Peck, Hanson, and Thornburn (1974):2

0.88a net w aq C N S kN m

Correlations using CPT data:

Meyerhofs correlations may be used by substituting qc /2 for N,where qc is in kg/cm

2.


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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 cQq q D D B

γ γ γ

cγ γ is small, so it may be neglected

2

cn

Qq

B

Soil Soil

c D

f D

2

ca net

Qq

B

2

cg 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

2

cn f

Qq D

Bγ

2

ca net f

Qq D

Bγ

2

ca 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 includesthe weight of superstructure and foundation only.

Ultimate Bearing capacity:

Maximum gross intensity of loading that thesoil can support against shear failure iscalled ultimate bearing capacity.

Net Ultimate Bearing Capacity:

Maximum net intensity of loading that thesoil can support at the level of foundation.

Gross Loading IntensityTotal pressure at the level of foundationincluding the weight of superstructure,foundation, and the soil above foundation.

superstructure Foundation soil

Foundation

g

Q Q Qq

A

n g f q q Dγ

from

Bearing capacity calculation

uq

nu u f q q Dγ


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SUMMARY of Terminology

Gross Safe Bearing capacity:

Maximum gross intensity of loading that the soilcan safely support without the risk of shear failure.

Safe Bearing Pressure:

Maximum net intensity of loading that can beallowed on the soil without settlement

exceeding the permissible limit.

Allowable Bearing Pressure:

Maximum net intensity of loading that canbe allowed on the soil with no possibility ofshear failure or settlement exceeding thepermissible limit.

Net Safe Bearing capacity:

Maximum net intensity of loading that the soil cansafely support without the risk of shear failure.

nuns

qq

FOS

gs ns f q q Dγ

from settlement analysissqρ

Minimum of

bearing capacity and

settlement analysis

a net q


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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 such

loads 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 Foundation

IS: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 second

combination 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 forvertical loads and rectangular footing for the columns carryingmoment

Allowable bearing pressure should not be very high in comparisonto the net loading intensity leading to an uneconomical design.

It is preferred to use SPT or Plate load test for cohesionless soilsand undrained shear strength test for cohesive soils.

In case of lateral loads or moments, the foundation should also bechecked to be safe against sliding and overturning. The FOS shallnot be less than 1.75 against sliding and 2.0 against overturning.When wind/seismic loads are considered the FOS is taken as 1.5for both the cases.


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Combined Footings

Combined footing is preferred when The columns are spaced too closely that if isolated footing is

provided 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 footings Rectangular combined footing

Trapezoidal combined footing

Strap beam combined footing


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Rectangular Combined Footing If two or more columns are carrying

almost 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 Q A

q

2

1 2

Q S x

Q Q

12 L L S

2 1 0 L L S L

B A L

Q1 Q2

Q1+Q2x

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 Q A

q

2

1 2

Q S x

Q Q

1 21

1 2

2

3

B B L x L

B B

1 2

2

B B L A

Solution of thesetwo equations

gives B1 and B2

Q1 Q2

Q1+Q2x

L1 S L2

B1 B2


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Strap Combined Footing Strap footing is used to

connect 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 is

relatively large. Some design considerations:

Strap must be rigid: Istrap /Ifooting > 2.

Footings should be proportioned to have approximately equal soilpressure 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 support

structural 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 shallow

water 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 function End Bearing Piles

Friction Piles

Compaction Piles

Anchor Piles Uplift Piles

Effect of Installation Displacement Piles

Non-displacement Piles


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Displacement Piles In loose cohesionless soils

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

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

In dense cohesionless soils The dilatancy effect decreases the friction angle within the zone of

influence of displacement pile (3.5D approx.).

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

In cohesive soils Soil is remolded near the displacement piles (2.0 D approx.) leading

to a decreased value of shearing resistance. Pore-pressure is generated during installation causing lower

effective 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 be

provided 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 resistance

per unit area at anydepth

Ultimate skin frictionresistance of pile

Ultimate point load

Ultimate load capacityin compression

Ultimate load capacityin tension

.

zsz

Qq

S z

perimeter of pileS

suQ

. pu pu pQ q A

u pu suQ Q Q

u suQ Q

bearing capacity of soil puq

bearing area of pile p

A

upQ

usQ

uQ

sQ z

z


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IS:2911 Pile Load Capacity in Cohesionless Soils


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ForBoredPiles

ForDrivenPiles

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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

0.5

For 1 0.5 , but 1v vu uc cσ α σ

0.25

For 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 issuggested

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-Cohesive

Soils 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

FS uall

QQ

FS


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Load Tests on Piles

Note: Piles used for initial testing are loaded to failure or at least twice thedesign 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 exceed12 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 Test Ultimate Load

De Beer (1968):

Load settlement curve is plotted in alog-log plot and it is assumed to be abilinear relationship with its

intersection as failure load

Chin Fung Kee (1977):

Assumes hyperbolic curve.Relationship betweensettlement and its division withload is taken as to be bilinearwith its intersection as failureload


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Vertical Load Test: Maintained Load Test Safe

Load as per IS: 2911Safe Load for Single Pile:

Safe Load for Pile Group:


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Dynamic Pile Formula for Driven Piles:

Modified Hiley Formula

. . .

/ 2u

W H Q

S C

α η

. . .

/ 2u

W H Q

S C

α η

W

H

S

uQ

Weight of hammer

Height of fall

Pile resistance or Pile capacity

Pile penetration for the last blow

α Hammer fall efficiencyEfficiency of blowη Sum 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

AP-Foundation Concepts and Foundation

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