CE 632 Pile Foundations Part-2 PPT

Foundation Analysis and Design: Dr. Amit Prashant

Load Tests on PilesLoad Tests on PilesLoad Tests on PilesLoad Tests on Piles

Note:

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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.


Load Tests on PilesLoad Tests on PilesLoad Tests on PilesLoad Tests on Piles

Note:Note:During this test pile should be loaded upto one and half times the

working (design) load and the maximum settlement of the test should not exceed 12 mm

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not exceed 12 mm.These piles may be used in the final construction


Vertical Load Test: Maintained Load TestVertical Load Test: Maintained Load Test

The test can be initial or routine testtest

The load is applied in increments of 20% of the estimated safe load. Hence the failure load is reached in 8-10 increments.

Settlement is recorded for eachSettlement is recorded for each increment until the rate of settlement is less than 0.1 mm/hr.

The ultimate load is said to have reached when the final settlement is more than 10% of the diameteris more than 10% of the diameter of pile or the settlement keeps on increasing at constant load.

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Vertical Load Test: Maintained Load TestVertical Load Test: Maintained Load Test

After reaching ultimate load theAfter reaching ultimate load, the load is released in decrements of 1/6th of the total load and recovery is measured until fullrecovery is measured until full rebound is established and next unload is done.

After final unload the settlement is measured for 24 hrs to estimate full elastic recovery.

Load settlement curve depends on the type of pile

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Vertical Load Test: Maintained Load Test Vertical Load Test: Maintained Load Test Ultimate LoadUltimate Load

De Beer (1968):

Load settlement curve is plotted in a log-p glog plot and it is assumed to be a bilinear relationship with its intersection as failure loadload

Chin Fung Kee (1977):

A h b liAssumes hyperbolic curve. Relationship between settlement and its division with load is taken

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as to be bilinear with its intersection as failure load


Vertical Load Test: Maintained Load Test Vertical Load Test: Maintained Load Test Ultimate LoadUltimate Load

Mazurkiewicz method:

Assumes parabolic curve.

After initial straight portion EQUAL settlement lines are drawn to intersect load axisdrawn to intersect load axis.

Intersection of lines at 45º from points on load axis and nextpoints on load axis and next settlement line are joined to form a straight line which intersects the load axis as failure loadthe load axis as failure load.

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

Safe Load for Single Pile:

Safe Load for Pile Group:

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Elastic Settlement of PilesElastic Settlement of PilesElastic Settlement of PilesElastic Settlement of PilesTotal settlement of pile under vertical working load

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ξ depends on the distribution of frictional resistance over the length of pile. ξ =0.5 for uniform or parabolic (peak at mid point) and 0.67 for

triangular distribution.


Elastic Settlement of PilesElastic Settlement of PilesElastic Settlement of PilesElastic Settlement of Piles

Vesic’s (1977) semi-empirical methodVesic s (1977) semi empirical method

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Elastic Settlement of PilesElastic Settlement of PilesElastic Settlement of PilesElastic Settlement of Piles

2 0 35 LI +

V i ’ (19 ) i i i l h d

2 0.35wsID

= +Empirically by Vesic (1977)

Vesic’s (1977) semi-empirical method

L⎛ ⎞0.93 0.16 .s p

LC CD

⎛ ⎞= +⎜ ⎟⎜ ⎟⎝ ⎠

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Vertical Load Test: Constant Rate of Penetration TestVertical Load Test: Constant Rate of Penetration TestVertical Load Test: Constant Rate of Penetration TestVertical Load Test: Constant Rate of Penetration Test

This test is only used as initial test to determine rapidly h l i b i i f h il d bthe ultimate bearing capacity of the pile and can not be performed as routine test.

Load-settlement curve can not be used to predict the settlement under working load conditions.

The rate of penetration is taken as 0.75 mm/min for friction piles and 1.5 mm/min for predominantly end bearing pilesbearing piles.

Test is continued until the deformation reaches 0.1D or a stage where further deformation does not increasea stage where further deformation does not increase load significantly.

The final load at the end of test is taken as ultimate load

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The final load at the end of test is taken as ultimate load capacity of pile.


Vertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load Testyy

Proposed by Van Weele(1957) with the aim of(1957) with the aim of determining strength in friction and bearing separatelyseparately.

Generally performed as initial test by loading the pile to lti t itultimate capacity

Safe load for pile is determined asdetermined as

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Vertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestyyDuring this test, loading stages are performed as in the maintained load test.

fAfter each loading, the pile is again unloaded to previous stage and deformation is measureddeformation is measured for 15 min. Then, load is again increased up to next loading step. The

ti tilprocess continues until failure load.

Th dThe recovered settlement is treated as elastic component and the permanent

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the permanent deformation as plastic.


Vertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestElastic recovery in each step is plotted against the load which comprises of the elastic deformationcomprises of the elastic deformation (a) for mobilizing friction, (b) for mobilizing bearing, and (c) due to the deformation of the pile itself Curve C(c) due to the deformation of the pile itself. Curve C1.

Assuming that elastic shortening of pile is zero, draw a line from the origin parallel to the straight portion of the curve, which gives approximate value of the bearing and frictional resistance, as shown in the adjacent figure.j g

Assuming that elastic shortening of pile is zero, draw a line from th i i ll l t th t i ht ti f th hi h ithe origin parallel to the straight portion of the curve, which gives approximate value of the bearing and frictional resistance, as shown in the adjacent figure.

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Vertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestVertical Load Test: Cyclic Load TestElastic compression of pile may be determined as

F is taken as varying linearly from top to

bottom, so average = F/2 g

Elastic compression of sub-grade can be obtained by subtracting the elastic compression of pile from total elastic recovery. If this value as

l l d b i i i i dcalculated comes out to be negative it is ignored.This new value of deformation is plotted against the load Curve C2. Bearing and frictional resistance are again evaluated as described on the

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g glast slide. This process is repeated 3 to 4 times to obtain reasonable values of frictional and bearing resistance of pile


TaperedTaperedTapered Tapered PilesPiles

Driven tapered pilesDriven tapered piles with larger dimension at the top are believed to be more effective in sand deposits.Force components

ti th ilacting on the pile are given below.

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TaperedTaperedTapered Tapered PilesPilesValue of K forValue of K for tapered piles is recommended between 1 7K tobetween 1.7Ko to 2.2Ko by Bowels. Meyerhof (1976) suggested K≥1 5suggested K≥1.5. Blanchet (1980) suggested K=2Ko.

The frictional resistance of these piles isthese piles is relatively larger than that of straight piles as

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st a g t p es asindicated in the adjacent plot.


Stepped Tapered PileStepped Tapered PileStepped Tapered PileStepped Tapered Pile

( )2 214ledg i iA r rπ−= − ledgL

Dsi i iA D Lπ= iL iD

′ ′1 sino iK φ′= − 2 .tano iKβ φ′=

. . .ledg ledg ledg qQ A L Nγ=

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. .si siQ A q β=


Uplift Piles in ClaysUplift Piles in ClaysuQ

p yp yUplift resistance of pile is mainly provided by its friction resistance and self weight.

Q f A

sf

Uplift capacity of pile with bottom bulb is taken

.u s s pQ f A W= + pW

Uplift capacity of pile with bottom bulb is taken as minimum of the following two equations by Meyerhof and Adams (1968) D

Q A K W W+ + Q. .u u s s pQ c A K W W= + +

( )2 22.25 .u b u pQ D D c Wπ= − + sf

uQ

( )u b u pQ

pWsf

WsW

D

61bD


Uplift Piles in Other SoilsUplift Piles in Other Soilspp

Meyerhof and Adams (1968): Minimum of the three equations below

( ). tan . . .u h b pQ c D L Wσ φ π′ ′ ′= + +

Meyerhof and Adams (1968): Minimum of the three equations below2. . . . . . . tan

2u b b u pQ c D L s D L K Wππ γ φ′ ′ ′= + +L ≤ H

L H⎛ ⎞

2⎡ ⎤

1 with its maximum value of 1b b

mL mHsD D

⎛ ⎞= + +⎜ ⎟

⎝ ⎠

( )22. . . . . . . . tanu b b u pQ c D H s D L L H K Wπ γ φ⎡ ⎤′ ′ ′= + − − +⎣ ⎦

( )( )2 2Q D D c N N A f Wπ σ′ ′= + + +

L > H

Bearing capacity ( )( ). . .4 bu c v q s s pQ D D c N N A f Wσ= − + + +failure

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Dynamic Pile FormulaDynamic Pile Formulayy

Sanders (1850): W = Weight of hammerSanders (1850): W =H =Q =

Weight of hammer

Height of fall

Pile resistance or Pile capacity

S =uQ Pile resistance or Pile capacity

Pile penetration for the last blowWellington (1898): Engineering News Formula

C = A constant accounting for energy loss C g gyduring driving[1 in. or 25.4 mm for drop hammer][0.1 in or 2.54 mm for steam hammer]

A factor of safety FS = 6 is recommended for estimating the allowable capacity

63Note: Dynamic pile formula are not used for soft clays due to pore pressure evolution


Efficiency of Pile DrivingEfficiency of Pile Driving 0.7y gy gBased on the Newton’s law of conservation of momentum. Assuming that coefficient of restitution 0 5

0.6 e = 0

Assuming that coefficient of restitution of hammer to pile is zero and hammer moves along the pile after impact

W P+⎛ ⎞0.4

0.5

η H i h( )1 2. .W v W P v= +1 2.W Pv v

W+⎛ ⎞= ⎜ ⎟

⎝ ⎠Efficiency as the ration on energy 0.2

0.3

η Heavier hammer or lighter piles

give better efficiencyEfficiency as the ration on energy

after and before the impact22

1 .2

W P vW

⎛ ⎞+⎜ ⎟⎝ ⎠ 0

0.1efficiency

2

222

21 .2

g WW PW W P v

g W

η⎜ ⎟⎝ ⎠= =

+⎛ ⎞ +⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

00 1 2

W/P2 g W⎝ ⎠⎝ ⎠

Efficiency of blow with a non-zero value of the coefficient of restitution e.22W P W P⎛ ⎞2W P

negligible

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2

For W Pe W PeP WW P W P

η + −⎛ ⎞> → = −⎜ ⎟+ +⎝ ⎠

2

For W PeW PW P

η +> → =

+


Dynamic Pile Formula: Modified Dynamic Pile Formula: Modified HileyHiley FormulaFormulayy yy

W = Weight of hammer

H =

SuQ =

Height of fall

Pile resistance or Pile capacity

S =Pile penetration for the last blow

α =Hammer fall efficiencyEfficiency of blowη = Efficiency of blowη =Sum of temporary elastic compression of pile, dolly, packing, and ground

C =

Hammer Fall Efficiency:

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Dynamic Pile Formula: Modified Hiley FormulaDynamic Pile Formula: Modified Hiley FormulaDynamic Pile Formula: Modified Hiley FormulaDynamic Pile Formula: Modified Hiley Formula

Coefficient of Restitution:

Factor of Safety for Hiley’s Formula:

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Dynamic Pile Formula: Modified Hiley FormulaDynamic Pile Formula: Modified Hiley FormulaTemporary Elastic CompressionTemporary Elastic Compression

Driving without helmet or dolly but only a cushion or pad f 25 thi k h d1 1 761 RC = of 25 mm thick on head.1 1.761

3.726

CARA

= Driving of concrete or steel piles with helmet and short dolly without cushion

5.509

ARA

=

dolly without cushion.Concrete pile driven with only 75 mm packing under helmet and without dolly.

2.0.657 R LCA

=

R

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3 0.073 2.806p

RCA

= +pA = Overall cross-sectional area of pile at toe in cm2


Dynamic Pile Formula: Simplex Formula for Dynamic Pile Formula: Simplex Formula for Frictional PilesFrictional Piles

Frictional resistance of the pile is brought into the empirical relationship inFrictional resistance of the pile is brought into the empirical relationship in this formula by measuring the total number of blows for driving the full length of pile.

Ultimate driving resistance in kNR U t ate d g es sta ce

Length of pile in meters.pN

LTotal number of blows to drive the pile

Weight of hammer in kN.WHs

Height of free fall in meters.Average set i e penetration in cm for last blow being the

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s Average set i.e. penetration in cm for last blow being the average of last four blows.


Dynamic Pile Formula: Janbu FormulaDynamic Pile Formula: Janbu FormulaDynamic Pile Formula: Janbu FormulaDynamic Pile Formula: Janbu Formula

R

Units: kN and m.

Ultimate capacity (FS)UR

( )

η

( )1 1k C Cλ

Efficiency factor (0.7 to 0.4, depending on driving conditions). .W Hαλ

Weight of hammer/ram

( )0.75 0.15dC P W= +

W

( )1 1U d c dk C Cλ= + + 2

. .

. .cW H

A E Sαλ =

P Weight of pile

H Height of free fall in meters.α Hammer fall efficiency as mentioned for modified Hiley’s formula

Area of pileAE Elastic modulus of pile

α Hammer fall efficiency as mentioned for modified Hiley s formula

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Set per blow as for Simplex formulasL Length of pile

Documents

CE 632 Pile Foundations Part-2 PPT