CE 632 Pile Foundations Part-2 Handout

Foundation Analysis and Design: Dr. Amit Prashant

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.

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Foundation Analysis and Design: Dr. Amit Prashant

Load Tests on Piles

Note: Æ 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. Æ These piles may be used in the final construction 44

Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Maintained Load Test „

The test can be initial or routine test

„

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 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 diameter of pile or the settlement keeps on increasing at constant load.

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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Maintained Load Test „

After reaching ultimate load, the load is released in decrements of 1/6th of the total load and recovery 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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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Maintained Load Test Æ Ultimate Load De Beer (1968): Load settlement curve is plotted in a loglog 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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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Maintained Load Test Æ Ultimate Load Mazurkiewicz method:

ƒ Assumes parabolic curve. ƒ After initial straight portion EQUAL settlement lines are dra n to intersect load a drawn axis. is

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

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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Maintained Load Test Æ Safe Load as per IS: 2911 Safe Load for Single Pile:

Safe Load for Pile Group:

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Foundation Analysis and Design: Dr. Amit Prashant

Elastic Settlement of Piles „

Total settlement of pile under vertical working load

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.

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Foundation Analysis and Design: Dr. Amit Prashant

Elastic Settlement of Piles

Vesic’s (1977) semi-empirical method

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Foundation Analysis and Design: Dr. Amit Prashant

Elastic Settlement of Piles

Empirically by Vesic (1977)

I ws = 2 + 0.35 0 35

L D

Vesic’s (1977) semi-empirical method

⎛ L⎞ Cs = ⎜⎜ 0.93 + 0.16 ⎟⎟ .C p D ⎝ ⎠ 52

Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Constant Rate of Penetration Test „

This test is only used as initial test to determine rapidly the ultimate bearing capacity of the pile and can not be performed as routine test.

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

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

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

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

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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Cyclic Load Test „

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

„

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

„

Safe load for pile is determined as

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Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Cyclic Load Test „

During this test, loading stages are performed as in the maintained load test.

„

After each loading, the pile is again unloaded to previous stage and deformation is measured for 15 min. Then, load is again increased up to next loading step. The process continues until failure load.

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The recovered settlement is treated as elastic component and the permanent deformation as plastic. 55

Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Cyclic Load Test „

Elastic recovery in each step is plotted against the load which comprises of the elastic deformation (a) for mobilizing friction, (b) for mobilizing bearing, and (c) due to the deformation of the pile itself. Æ 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.

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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. 56

Foundation Analysis and Design: Dr. Amit Prashant

Vertical Load Test: Cyclic Load Test „

Elastic compression of pile may be determined as

F is taken as varying linearly from top to g = F/2 bottom, so average

„

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Elastic compression of sub-grade can be obtained by subtracting the elastic compression of pile from total elastic recovery. If this value as calculated comes out to be negative it is ignored. This new value of deformation is plotted against the load Æ Curve C2. Bearing and frictional resistance are again evaluated as described on the last slide. This process is repeated 3 to 4 times to obtain reasonable values of frictional and bearing resistance of pile 57

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Foundation Analysis and Design: Dr. Amit Prashant

Tapered Piles „

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Driven tapered piles with larger dimension at the top are believed to be more effective in sand deposits. Force components acting ti on the th pile il are given below.

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Foundation Analysis and Design: Dr. Amit Prashant

Tapered Piles „

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

„

The frictional resistance of these piles is relatively larger than that of straight piles as indicated in the adjacent plot.

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Foundation Analysis and Design: Dr. Amit Prashant

Stepped Tapered Pile

Aledg =

π 4

(r

2 i −1

− ri 2

)

Asi = π Di Li K o = 1 − sin φi′

Lledg

Li

Di

β = 2 K o .tan φi′

Qledg = Aledg .γ .Lledg .N q

Qsi = Asi .q.β 60

6

Qu

Foundation Analysis and Design: Dr. Amit Prashant

Uplift Piles in Clays „

Uplift resistance of pile is mainly provided by its friction resistance and self weight.

fs

Qu = f s . As + W p „

Wp

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

D

Qu = cu . As .K + Ws + W p

(

Qu

)

Qu = 2.25π D − D .cu + W p 2 b

2

fs Wp Ws D

Db

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Foundation Analysis and Design: Dr. Amit Prashant

Uplift Piles in Other Soils Qu = ( c′ + σ h′ .tan φ ′ ) .π .Db .L + W p Meyerhof and Adams (1968): Minimum of the three equations below

„

L≤H

Æ

Qu = π .c′.Db .L + s = 1+

L>H

Æ

Bearing capacity failure Æ

π 2

s.γ ′.Db .L2 K u .tan φ ′ + W p

⎛ mH ⎞ mL with its maximum value of ⎜1 + ⎟ Db Db ⎠ ⎝

2 Qu = π .c′.Db .H + s.γ ′.Db . ⎡ L2 − ( L − H ) ⎤ .K u .tan φ ′ + W p ⎣ ⎦

Qu =

π 4

(D

2

b

)

− D 2 ( c′.N c + σ v′ .N q ) + As . f s + W p

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Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula „

Sanders (1850):

„

Wellington (1898): Engineering News Formula

W = Weight of hammer H = Height of fall Qu = Pile resistance or Pile capacity S = Pile penetration for the last blow

C = A constant accounting for energy loss

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

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Foundation Analysis and Design: Dr. Amit Prashant

0.7

Based on the Newton’s law of conservation of momentum. Assuming that coefficient of restitution of hammer to pile is zero and hammer moves along the pile after impact

0.6

⎛W + P ⎞ v1 = ⎜ ⎟ .v2 ⎝ W ⎠

W .v1 = (W + P ) .v2 „

Efficiency as the ration on energy after and before the impact

0.4 0.3 0.2

Heavier hammer or lighter piles give better efficiency

0.1

0 W = 0 1 2 ⎞⎛W + P ⎞ 2 W + P W/P ⎟ ⎜ W ⎟ .v2 ⎠ ⎠⎝ Efficiency of blow with a non-zero value of the coefficient of restitution e.

η=

„

1 ⎛W + P ⎞ 2 .v2 2 ⎜⎝ g ⎟⎠

e=0

0.5

η

„

Efficiency of Pile Driving

1 ⎛W 2 ⎜⎝ g

For W > P → η =

W + Pe 2 W +P

For P > W → η =

W + Pe 2 ⎛ W − Pe ⎞ −⎜ ⎟ W +P ⎝ W +P ⎠

2

2

negligible

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Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula: Modified Hiley Formula W = Weight of hammer H = Height of fall Qu = Pile resistance or Pile capacity S = Pile penetration for the last blow α = Hammer fall efficiency η = Efficiency of blow C = Sum of temporary elastic compression of pile, dolly, packing, and ground

Hammer Fall Efficiency:

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Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula: Modified Hiley Formula Coefficient of Restitution:

Factor of Safety for Hiley’s Formula:

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Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula: Modified Hiley Formula Temporary Elastic Compression

Driving without helmet or dolly but only a cushion or pad off 25 mm thick thi k on head. h d

R A R = 3.726 A R = 5.509 A

C1 = 1.761 1 761

C2 = 0.657

Driving of concrete or steel piles with helmet and short dolly without cushion. Concrete pile driven with only 75 mm packing under helmet and without dolly.

R.L A

C3 = 0.073 + 2.806

R Ap

Ap = Overall cross-sectional area of pile at toe in cm2 67

Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula: Simplex Formula for Frictional Piles Frictional 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 kN

R Np

Total number of blows to drive the pile Length of pile in meters.

L W H s

Weight of hammer in kN. Height of free fall in meters. Average set i.e. penetration in cm for last blow being the average of last four blows. 68

Foundation Analysis and Design: Dr. Amit Prashant

Dynamic Pile Formula: Janbu Formula Units: kN and m. Ultimate capacity (FS)

RU

η

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

(

kU = Cd 1 + 1 + λc Cd W

)

Cd = 0.75 + 0.15 ( P W )

λc =

α .W .H A.E.S 2

Weight of hammer/ram

P

Weight of pile

H

Height of free fall in meters.

α

Hammer fall efficiency as mentioned for modified Hiley’s formula Area of pile

A E s

Elastic modulus of pile Set per blow as for Simplex formula

L

Length of pile

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