CE 632 Pile Foundations Part-2 Handout

(1)

Foundation Analysis and Design: Dr. Amit Prashant

Load Tests on Piles

43 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 Piles

44 Note:

ÆDuring 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.

ÆThese piles may be used in the final construction

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

45

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.

(2)

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.

46

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

on the type of pile

Vertical Load Test: Maintained Load Test

Æ

Ultimate 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

47 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

Vertical Load Test: Maintained Load Test

Æ

Ultimate Load

Mazurkiewicz method:

Assumes parabolic curve.

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

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.

(3)

Vertical Load Test: Maintained Load Test

Æ

Æ Safe

Safe

Load as per IS: 2911

Safe Load for Single Pile:

49 Safe Load for Pile Group:

Elastic Settlement of Piles

Total settlement of pile under vertical working load

50

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 Piles

51 Vesic’s (1977) semi-empirical method

(4)

Elastic Settlement of Piles

2 0 35

L

I

+

52 Vesic’s (1977) semi-empirical method

2 0.35

ws

I

D

= +

Empirically by Vesic (1977)

0.93 0.16

.

s p

L

C

D

⎛

⎞

=

⎜

_⎜

+

⎟

_⎟

⎝

⎠

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.

Load-settlement curve can not be used to predict the

settlement under working load conditions.

53

The rate of penetration is taken as 0.75 mm/min for

friction piles and 1.5 mm/min for predominantly end

bearing piles.

Test is continued until the deformation reaches 0.1D or

a stage where further deformation does not increase

load significantly.

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

capacity of pile.

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

lti t it ultimate capacity Safe load for pile is

(5)

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

55 deformation is measured

for 15 min. Then, load is again increased up to next loading step. The process continues until failure load.

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

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

56 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.

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.

Vertical Load Test: Cyclic Load Test

Elastic compression of pile may be determined as

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

57 g

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

(6)

Tapered

Piles

Driven tapered piles with larger dimension at the top are believed to be more effective in sand deposits. Force components ti th il 58 acting on the pile are

given below.

Tapered

Piles

Value of K for tapered piles is recommended between 1.7Koto 2.2Koby Bowels. Meyerhof (1976) suggested K≥1 5 59 suggested K≥1.5. 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.

Stepped Tapered Pile

(

2 2

)

1

4

ledg i i

A

=

π

r

₋

−

r

L

ledg

D

si i i

A

=

π

D L

. .

.

ledg ledg ledg q

Q

=

A

γ

L

N

i

L

D

i

1 sin

o i

K

= −

φ

′

β

=

2 K

o

. tan

φ

i

′

. .

si si

Q

=

A q

β

(7)

Uplift Piles in Clays

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

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

.

u s s p

Q

=

f A

+

W

u

Q

p W s

f

D

Q

A K

+

W

+

W

Q

61

.

u u s s p

Q

=

c A K

+

W

+

W

(

2 2

)

2.25 .

u b u p

Q

=

π

D

−

D

c

+

W

p W s

f

s W D u

Q

b D

Uplift Piles in Other Soils

Meyerhof and Adams (1968): Minimum of the three equations below 2 . . . tan 2 u b b u p Q =πc D L′ +πsγ′D L K φ′+W 2 ⎡ ⎤

(

. tan

)

. . . u h b p Q = c′+σ′ φ π′ D L W+ L ≤ H Æ

1 with its maximum value of 1

b b mL mH s D D ⎛ ⎞ = + ⎜ + ⎟ ⎝ ⎠ 62

(

)

2 2 . . . tan u b b u p Q =πc D H′ +sγ′D _⎣⎡L− L−H ⎤_⎦K φ′+W

(

2 2

)(

)

. . . 4 b u c v q s s p Q =π D −D c N′ +σ′N +A f +W L > H Æ Bearing capacity failure Æ

Dynamic Pile Formula

Sanders (1850):

W

=

H

=

S

=

u

Q

=

Weight of hammer Height of fall

Pile resistance or Pile capacity Pile penetration for the last blow Wellington (1898):

63 Engineering News Formula

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

(8)

Efficiency of Pile Driving

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

(

)

1 2 . . W v = W+P v v₁ W P .v₂ W + ⎛ ⎞ = ⎜_⎝ ⎟_⎠ Efficiency as the ration on energy 0.2

0.3 0.4 0.5 0.6 0.7 η Heavier hammer or lighter piles give better efficiency e = 0 64

Efficiency as the ration on energy after and before the impact

2 2 2 2 2 1 . 2 1 . 2 W P v g W W P W W P v g W η ⎛ + ⎞ ⎜ ⎟ ⎝ ⎠ = = + ⎛ ⎞⎛ + ⎞ ⎜ ⎟ ⎜ ⎟_⎝ _⎠ ⎝ ⎠ 0 0.1 0 1 2 W/P efficiency

Efficiency of blow with a non-zero value of the coefficient of restitution e. 2 2 For P W W Pe W Pe W P W P η + ⎛ − ⎞ > → = _{− ⎜} _⎟ + ⎝ + ⎠ 2 For W P W Pe W P η + > → = + negligible

Dynamic Pile Formula: Modified

Dynamic Pile Formula: Modified Hiley

Hiley Formula

Formula

W

=

H

=

S

=

u

Q

=

Weight of hammer Height of fall

Pile resistance or Pile capacity Pile penetration for the last blow

α =

Hammer fall efficiency Efficiency of blow

η =

65

Efficiency of blow

η =

Sum of temporary elastic compression of pile, dolly, packing, and ground

C

=

Hammer Fall Efficiency:

Dynamic Pile Formula: Modified Hiley Formula

Coefficient of Restitution:

(9)

Dynamic Pile Formula: Modified Hiley Formula

Temporary Elastic Compression

Driving without helmet or dolly but only a cushion or pad f 25 thi k h d 1 1 761 R C = 67 of 25 mm thick on head. 1 1.761 3.726 5.509 C A R A R A = =

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.

2 . 0.657R L C A = 3 0.073 2.806 p R C A = + p

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

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.

68

Ultimate driving resistance in kN R

Length of pile in meters. Weight of hammer in kN. W H p N L

Total number of blows to drive the pile

s

Height of free fall in meters.

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

Dynamic Pile Formula: Janbu Formula

Ultimate capacity (FS) U R

(

)

η

(

1 1

)

k C λ C

Efficiency factor (0.7 to 0.4, depending on driving conditions) . .W H α λ Units: kN and m. 69 Weight of hammer/ram

(

)

0.75 0.15 d C = + P W W

(

1 1

)

U d c d k =C + +λ C P Weight of pile Area of pile H A 2 . . . . c W H A E S α λ = E

Height of free fall in meters.

Elastic modulus of pile

Set per blow as for Simplex formula s

L Length of pile

CE 632 Pile Foundations Part-2 Handout

Load Tests on Piles

Load Tests on Piles

Load Tests on Piles

Load Tests on Piles

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Æ

Æ

Ultimate Load

Ultimate Load

Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Æ

Æ

Ultimate Load

Ultimate Load







Vertical Load Test: Maintained Load Test

Vertical Load Test: Maintained Load Test

Æ

Æ Safe

Safe

Load as per IS: 2911

Load as per IS: 2911

Elastic Settlement of Piles

Elastic Settlement of Piles

Elastic Settlement of Piles

Elastic Settlement of Piles

Elastic Settlement of Piles

Elastic Settlement of Piles

2 0 35

L

I

+

2 0.35

I

D

= +

0.93 0.16

.

L

C

C

D

⎛

⎞

=

⎜

⎜

+

⎟

⎟

⎝

⎠

Vertical Load Test: Constant Rate of Penetration Test

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.

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 piles.

Test is continued until the deformation reaches 0.1D or

a stage where further deformation does not increase

load significantly.

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

capacity of pile.

Vertical Load Test: Cyclic Load Test

Vertical Load Test: Cyclic Load Test

Vertical Load Test: Cyclic Load Test

Vertical Load Test: Cyclic Load Test

_⎜

_⎟