12626795 analysis of bearing capacity driven pile
TRANSCRIPT
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 1
STRUCTURAL ENGINEER : SYAIFUL ASHARI, ST
8.1 INTRODUCTION
Driven pile is a type of the deep foundation. This foundation is driven to the ground using a hammer
which is dropped from the prescribed height. The hammer is introduces energy to push the pile to the
soil.
The followings are several reason to use the driven pile foundation (deep foundation),as follows :
� The upper soil condition is so bad so the use of spread footing is very un economically.
� Large uplift capacity is required.
� Large lateral capacity is required.
� Requirement for pier foundation and abutment foundation in bridge structure.
This chapter describes the analysis of bearing capacity for driven pile foundation based on the soil
properties and in situ test, dynamic formula to predict the bearing capacity, lateral bearing capacity and
analysis of group pile.
8.2 TYPE OF PILE FOUNDATION
8.2.1 GENERAL
Load transfer from the super structure to the pile foundation is depends to the type of soil. The bearing
capacity of pile foundation is from the end bearing capacity and skin friction capacity .
Cohesionless soil provides the end bearing capacity and cohesive soil provide skin friction
capacity . For general soil condition the bearing capacity is provided by the end bearing capacity plus
with skin friction capacity.
The ultimate bearing capacity of pile foundation can be written as :
usupu QQQ += [8.1]
where :
Qu = ultimate bearing capacity
Qup = ultimate end bearing capacity
Qus = ultimate skin friction capacity
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CAPACICAPACICAPACICAPACITYTYTYTY –––– DRIVEN DRIVEN DRIVEN DRIVEN PILEPILEPILEPILE
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ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 2
And the allowable bearing capacity is :
s
us
p
upa SF
QSF
QQ += [8.2]
where :
SFp = safety factor for end bearing capacity (2.0 – 4.0)
SFs = safety factor for skin friction capacity (2.0 – 4.0)
8.2.2 END BEARING PILE
End bearing pile is pile foundation that the major of bearing capacity is provided by end bearing
capacity . The skin friction capacity in end bearing pile can be neglected because it has small
influence.
End bearing capacity is calculated as follows :
pupup AqQ = [8.3]
where :
Qup = ultimate end bearing capacity
qup = ultimate end bearing pressure
Ap = end bearing contact area
8.2.3 FRICTION PILE
Friction pile is pile foundation that the major of bearing capacity is provided by skin friction capacity
(provided by adhesion) . The end bearing capacity in end bearing pile can be neglected because it
has small influence.
Ultimate skin friction capacity is calculated as follows :
∑= ssus AfQ [8.4]
where :
Qus = ultimate skin friction capacity
fs = ultimate skin friction stress
As = skin friction contact area
8.3 LOAD TRANSFER MECHANISM
8.3.1 GENERAL
Pile foundation almost to carry the moment load, this moment is transfer becomes compressive axial
load and tensile axial load . The design of bearing capacity of pile foundation must consider the type
of load acts in the pile.
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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8.3.2 COMPRESSIVE AXIAL CAPACITY
The bearing capacity of pile foundation due to axial compressive load is provided by the end bearing
capacity and skin friction capacity .
8.3.3 UPLIFT AXIAL CAPACITY
The bearing capacity of pile foundation due to axial tensile load is provided only by the skin friction
capacity .
8.4 ANALYSIS OF AXIAL BEARING CAPACITY – SOIL PROPE RTIES
8.4.1 GENERAL
Basic bearing capacity formula can be used for bearing capacity analysis for deep foundation with
several modifications. Analysis based on soil properties is using internal friction angle and
undrained shear strength .
8.4.2 CONTACT AREA
A. General
Contact area is the important thing to be considered in the pile foundation design. The contact area
may be different for different type of pile foundation.
B. Open Ended Steel Pipe Pile
When the pipe pile is driven the inside of the pipe will be plugged with the soil .
For condition of full plug , the end bearing contact area is the same with area of pipe if it is open
ended , as follows :
soilsteelp AAA += [8.5]
where :
Ap = end bearing contact area
Asteel = area of steel profile
Asoil = area of plug soil
For condition of partial plug , the end bearing contact area is half of the area of pipe if it is open
ended , as follows :
( )soilsteelp AA5.0A += [8.6]
where :
Ap = end bearing contact area
Asteel = area of steel profile
Asoil = area of plug soil
The skin friction contact area is the perimeter of the profile .
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 4
C. H Steel Pile
When the pipe pile is driven the inside of the pipe will be half plugged with the soil .
The end bearing contact area is :
( )soilsteelp AA5.0A += [8.7]
where :
Ap = end bearing contact area
Asteel = area of steel profile
Asoil = area of plug soil
The skin friction contact area is the perimeter of the pile with full plug .
8.4.3 END BEARING CAPACITY
A. General
The ultimate bearing capacity of pile foundation can be computed using the bearing capacity formula
as described in the previous chapter.
B. Bearing Capacity Formula
In general basic bearing capacity formula can be written as follows :
**q
*cu BNqNcNq γγ++= [8.8]
Because the width of pile foundation is small (B is small) , so the end term of the equation can be
neglected, so the end bearing pressure can be written as :
*q
*cup qNcNq += [8.9]
Ultimate end bearing capacity is computed as follows :
( ) p*q
*cup AqNcNQ += [8.10]
For the condition of cohesive soil (c=s u and φφφφ=0), the formula becomes :
uup s9q = [8.11]
where :
qp = ultimate end bearing pressure
su = undrained shear strength
*q
*c N,N = bearing capacity factor (include depth factor, inclination factor)
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 5
Ultimate end bearing capacity is computed as follows :
( ) puup As9Q = [8.12]
C. Vesic’s Method
Vesic propose the end bearing capacity formula based on the expansion of cavities theory .
The end bearing capacity can be calculated as follows :
( ) p*
0*cup AN'cNQ σσ+= [8.13]
where :
Qup = ultimate end bearing capacity
σ’0 = mean normal ground stress at the level of pile point
**c N,N σ = bearing capacity factor
The variables above is defined as :
'q3K21
' 00
+=σ
( )φ−= sin1K0
[8.14]
where :
q’ = effective vertical stress at the pile point
K0 = coefficient of earth pressure
φ = internal friction angle
The bearing capacity factor is defined as :
( ) ( )φ−= cot1NN *q
*c
( )0
*q*
K21
N3N
+=σ
[8.15]
D. Janbu’s Method
The end bearing capacity can be calculated as follows :
( ) p*q
*cup AN'qcNQ += [8.16]
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 6
where :
Qup = ultimate end bearing capacity
q’ = effective vertical stress at the pile point
*q
*c N,N = bearing capacity factor
The bearing capacity factor is defined as :
( ) ( )φ−= cot1NN *q
*c
( ) ( ) ( )φη
φ++φ= tan'22
2*q etan1tanN
[8.17]
E. Coyle & Castello’s Method
Coyle & Castello’s method is used for cohesionless soil .
The end bearing capacity can be calculated as follows :
( ) p*qup AN'qQ = [8.18]
where :
Qup = ultimate end bearing capacity
q’ = effective vertical stress at the pile point
*qN = bearing capacity factor
F. Meyerhof’s Method
Meyerhof proposes two formula can be used for cohesionless soil and cohesive soil.
The table below shows the Meyerhof’s end bearing capacity formula, as follows :
TABLE 8.1 END BEARING CAPACITY – MEYERHOF
COHESIONLESS
SOIL
COHESSIVE
SOIL
( ) ( )( ) p*qp
*qup AtanN50AN'qQ φ≤= ( ) pu
*cup AcNQ =
where :
cu = undrained cohesion
8.4.4 SKIN FRICTION CAPACITY
A. General
The ultimate bearing capacity of pile foundation can be computed using the bearing capacity formula
as described in the previous chapter.
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 7
B. αααα Method
The α method is calculates the skin friction resistance for cohesive soil based on the adhesion
factor αααα.
The skin friction capacity can be calculated as follows :
( ) suus AsQ α=
csu = [8.19]
where :
Qus = ultimate skin friction capacity
α = adhesion factor
su = undrained shear strength
c = cohesion
As = skin friction contact area
The adhesion factor α is determined based on the undrained shear strength s u usually use the
graph .
C. ββββ Method
The β method is calculates the skin friction resistance for cohesionless soil based on the coefficient
of lateral earth pressure .
The skin friction capacity can be calculated as follows :
( ) svus A'Q βσ=
( )stanK φ=β [8.20]
where :
Qus = ultimate skin friction capacity
σ’v = vertical effective stress at measured point
K = coefficient of lateral earth pressure
φs = friction angle of soil versus pile
Conservatively the lateral earth pressure can be computed, as follows :
Normally Consolidated Clays
( )ssin1K φ−= [8.21]
Over Consolidated Clays
( )( ) OCRsin1K sφ−= [8.22]
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 8
Bhushan propose the following equation to calculate β factor :
( )rD65.018.0 +=β [8.23]
where :
Dr = relative density
D. λλλλ Method
The λ method is calculates the skin friction resistance for cohesive soil based on the coefficient of
lateral earth pressure .
The skin friction capacity can be calculated as follows :
{ }( ) suvus As2'Q +σλ= [8.24]
where :
Qus = ultimate skin friction capacity
λ = friction capacity coefficient
v'σ = average vertical stress of ground surface and pile tip
us = average undrained shear strength of ground surface and pile tip
The factor λ is depended to the embedment length of the pile usually use the graph .
8.5 ANALYSIS OF AXIAL BEARING CAPACITY – IN SITU TE ST
8.5.1 GENERAL
Most practical method to obtain the bearing capacity is based on the in situ test such as standard
penetration test (SPT) and cone penetration test (CPT).
8.5.2 END BEARING CAPACITY – SPT
A. General
If the SPT data is used to obtain the bearing capacity it is recommended to use higher factor of safety
because inconsistency of the SPT test result.
B. Meyerhof’s Method
End bearing capacity based on the Meyerhof is :
corpcorup N400ABD
N40Q ≤
= [8.25]
where :
Qup = ultimate end bearing capacity (kPa)
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 9
Ncor = corrected N SPT value
D = embedment length
B = pile diameter
Ap = end bearing contact area
Ncor must be taken as average value in the range of 8B above pile tip and 3B below pile tip .
8.5.3 SKIN FRICTION CAPACITY – SPT
A. General
If the SPT data is used to obtain the bearing capacity it is recommended to use higher factor of safety
because inconsistency of the SPT test result.
B. Meyerhof’s Method
The following is the skin friction capacity based on SPT test according to Meyerhof.
TABLE 8.2 SKIN FRICTION CAPACITY – MEYERHOF
LARGE DISPLACEMENT
PILE
SMALL DISPLACEMENT
PILE
( ) scorus AN2Q = ( ) scorus ANQ =
where :
Qus = ultimate skin friction capacity (kPa)
Ncor = corrected N SPT value
As = skin friction contact area
C. Vesic’s Method
The following is the skin friction capacity based on SPT test according to Vesic.
TABLE 8.3 SKIN FRICTION CAPACITY – VESIC
LARGE DISPLACEMENT
PILE
OPEN ENDED,
H PILE
sD54.1
us A80Q4r
= s
D54.1us A25Q
4r
=
where :
Qus = ultimate skin friction capacity (kPa)
Dr = relative density
As = skin friction contact area
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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8.5.4 END BEARING CAPACITY – CPT
A. General
The CPT data can be used to predict the bearing capacity based on the cone resistance and side
friction .
B. LCPC’s Method
End bearing capacity based on the LCPC method is :
( ) pcceup AkqQ = [8.26]
where :
Qup = ultimate end bearing capacity (kPa)
qce = equivalent cone resistance at pile tip (kPa)
kc = cone end bearing factor
Ap = end bearing contact area
qce is taken as the average in the range of 1.5B above pile tip and 1.5B below pile tip .
The factor of cone end bearing is taken as :
TABLE 8.4 CONE END BEARING FACTOR
TYPE kc
Clay & Silt 0.600
Sand & Gravel 0.375
Chalk 0.400
8.5.5 SKIN FRICTION CAPACITY – CPT
A. General
The CPT data can be used to predict the bearing capacity based on the cone resistance and side
friction .
B. Meyerhof’s Method
Skin friction capacity based on cone resistance according to the Meyerhof is :
( ) scus Aq005.0Q = [8.26]
where :
Qus = ultimate skin friction capacity (kPa)
qc = cone resistance (kPa)
As = skin friction contact area
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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If the side friction is used the following equation can be used :
TABLE 8.5 SKIN FRICTION CAPACITY – MEYERHOF
LARGE DISPLACEMENT
PILE
SMALL DISPLACEMENT
PILE
( ) ssus Aq0.25.1Q −= ( ) ssus AqQ =
where :
Qus = ultimate skin friction capacity (kPa)
qs = side friction (kPa)
As = skin friction contact area
C. Nottingham & Schmertmann’s Method
Skin friction capacity based on cone resistance is :
TABLE 8.6 SKIN FRICTION CAPACITY – NOTTINGHAM & SCHMERTMANN
COHESIONLESS SOIL
z < 8B z ≥≥≥≥ 8B COHESSIVE SOIL
sscsus AfBz
'Q
α= ( ) sscsus Af'Q α= ( ) ssccus Af'Q α=
where :
Qus = ultimate skin friction capacity (kPa)
qs = side friction (kPa)
z = depth to mid point of soil layer
B = pile diameter
αααα’s and αααα’c is determined based on the graph.
8.6 ANALYSIS OF AXIAL BEARING CAPACITY – DYNAMIC TE ST
8.6.1 GENERAL
The pile bearing capacity can be predicted using the driving energy transferred to the pile using a
hammer. This method is known as dynamic method and can be used simply based on the final blow
count (final set) .
8.6.2 SANDER’S METHOD
Sander propose dynamic formula to predict the axial load capacity of the driven pile as follows :
( )FSshW
Q ra = [8.27]
where :
Qa = allowable axial bearing capacity
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 12
Wr = weight of hammer
h = hammer stroke / hammer fall distance
s = final penetration per blow at end of driving
FS = factor of safety (FS = 8)
8.6.3 ENGINEERING NEW’S METHOD
The most popular dynamic formula used is proposed by Engineering News (Wellington, 1888), as
follows :
( )( )FSCshW
Q ra +
= [8.28]
where :
Qa = allowable axial bearing capacity
Wr = weight of hammer
h = hammer stroke / hammer fall distance
s = final penetration per blow at end of driving
FS = factor of safety (FS = 6)
C = constant (drop hammer = 25 mm)
(single acting hammer = 2.5 mm)
8.6.4 MODIFIED ENGINEERING NEW’S METHOD
The following is dynamic formula which is modification of Engineering News Formula, as follows :
( )
++
+=
pr
p2
rrha WW
WnW
CshWe
Q [8.29]
where :
Qa = allowable axial bearing capacity
Wr = weight of hammer
Wp = weight of pile + hammer
h = hammer stroke / hammer fall distance
s = final penetration per blow at end of driving
FS = factor of safety (FS = 6)
C = constant (C = 2.5 mm)
eh = efficiency of hammer
n = coefficient of restitution
The following is the hammer efficiency and coefficient of restitution, as follows :
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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TABLE 8.7 HAMMER EFFICIENCY
HAMMER
TYPE eh
Drop Hammer 0.75 – 1.00
Single Acting Hammer 0.75 – 0.85
Double Acting Hammer 0.85
Diesel Hammer 0.85 – 1.00
TABLE 8.8 COEFFICIENT OF RESTITUTION
MATERIAL N
Wood Pile 0.00
Compact Wood On Steel Pile 0.25
Compact Wood Over Steel Pile 0.32
Steel On Steel Pile / Concrete Pile 0.50
Cast Iron Hammer On Concrete Pile 0.40
8.7 ANALYSIS OF UPLIFT CAPACITY
8.7.1 GENERAL
The uplift capacity of the driven pile is only provided by the skin friction capacity .
8.7.2 SKIN FRICTION CAPACITY
The skin friction capacity to determine the uplift capacity can be calculated with the similar procedure
as previously explained.
8.8 ANALYSIS OF LATERAL BEARING CAPACITY
8.3.4 GENERAL
During earthquake the pile foundation is take the lateral load from the result of super structure load.
When the pile subjected to lateral load the pile can be divided into two major categories, as follows :
� Rigid Pile , the pile length is short.
� Elastic Pile , the pile length is long.
8.3.5 MATLOCK & REESE’S METHOD
D. General
Matlock and Reese propose the elastic solution to analyze laterally loaded pile.
Due to the lateral load the following reactions can be calculated, as follows :
� Pile Deflection.
� Pile Slope.
� Bending Moment.
� Shear Force.
� Soil Reaction.
This method is can be used for pile embedded in granular soil .
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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E. Pile Deflection
The pile deflection at any depth of pile can be calculated, as follows :
( )
+
=
pp
2
xpp
3
x IEMT
BIE
QTAzx [8.30]
where :
x(z) = deflection at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
Ax, Bx = constant
F. Pile Slope
The pile slope at any depth of pile can be calculated, as follows :
( )
+
=θ θθ
pppp
2
IEMT
BIE
QTAz [8.31]
where :
θ(z) = slope at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
Aθ, Bθ = constant
G. Pile Bending Moment
The pile bending moment at any depth of pile can be calculated, as follows :
( ) ( ) ( )MBQTAzM mm += [8.32]
where :
M(z) = bending moment at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
Am, Bm = constant
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 15
H. Pile Shear Force
The pile shear force at any depth of pile can be calculated, as follows :
( ) ( )
+=TM
BQAzV vv [8.33]
where :
V(z) = shear force at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
Av, Bv = constant
I. Soil Reaction
The soil reaction at any depth of pile can be calculated, as follows :
( )
+
=2pp
T
MB
TQ
Azp [8.34]
where :
p(z) = soil reaction at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
Ap, Bp = constant
J. Characteristic Length of Soil-Pile System
The T variable is the characteristic length of soil-pile system, as follows :
5
h
pp
n
IET = [8.35]
where :
T = characteristic length
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
nh = constant of horizontal modulus of subgrade reaction
The pile of rigid if L ≤≤≤≤ 2T and the pile is elastic if L ≥≥≥≥ 5T.
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
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The value of nh is as follows :
TABLE 8.9 NH
SOIL nh
(kN/m 3)
Loose Sand 1800 – 2200
Medium Sand 5500 – 7000
Dense Sand 15000 – 18000
Loose Submerged Sand 1000 – 1400
Medium Submerged Sand 3500 – 4500
Dense Submerged Sand 9000 – 12000
K. A & B Constant
The following table shows the A and B constant.
TABLE 8.10 A CONSTANT
A COEFFICIENT z/T
Ax Ax Aθθθθ Am Av
0.0 2.435 -1.623 0.000 1.000 0.000
0.1 2.273 -1.618 0.100 0.989 -0.227
0.2 2.112 -1.603 0.198 0.956 -0.422
0.3 1.952 -1.578 0.291 0.906 -0.586
0.4 1.796 -1.545 0.379 0.840 -0.718
0.5 1.644 -1.503 0.459 0.764 -0.822
0.6 1.496 -1.454 0.532 0.677 -0.897
0.7 1.353 -1.397 0.595 0.585 -0.947
0.8 1.216 -1.335 0.649 0.489 -0.973
0.9 1.086 -1.268 0.693 0.392 -0.977
1.0 0.962 -1.197 0.727 0.295 -0.962
1.2 0.738 -1.047 0.767 0.109 -0.885
1.4 0.544 -0.893 0.772 -0.056 -0.761
1.6 0.381 -0.741 0.746 -0.193 -0.609
1.8 0.247 -0.596 0.696 -0.298 -0.445
2.0 0.142 -0.464 0.628 -0.371 -0.283
3.0 -0.075 -0.040 0.225 -0.349 0.226
4.0 -0.050 0.052 0.000 -0.106 0.201
5.0 -0.009 0.025 -0.033 0.015 0.046
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 17
TABLE 8.11 B CONSTANT
B COEFFICIENT z/T
Bx Bx Bθθθθ Bm Bv
0.0 1.623 -1.750 1.000 0.000 0.000
0.1 1.453 -1.650 1.000 -0.007 -0.145
0.2 1.293 -1.550 0.999 -0.028 -0.259
0.3 1.143 -1.450 0.994 -0.058 -0.343
0.4 1.003 -1.351 0.987 -0.095 -0.401
0.5 0.873 -1.253 0.976 -0.137 -0.436
0.6 0.752 -1.156 0.960 -0.181 -0.451
0.7 0.642 -1.061 0.939 -0.226 -0.449
0.8 0.540 -0.968 0.914 -0.270 -0.432
0.9 0.448 -0.878 0.885 -0.312 -0.403
1.0 0.364 -0.792 0.852 -0.350 -0.364
1.2 0.223 -0.629 0.775 -0.414 -0.268
1.4 0.112 -0.482 0.688 -0.456 -0.157
1.6 0.029 -0.354 0.594 -0.477 -0.047
1.8 -0.030 -0.245 0.498 -0.476 0.054
2.0 -0.070 -0.155 0.404 -0.456 0.140
3.0 -0.089 0.057 0.059 -0.213 0.268
4.0 -0.028 0.049 -0.042 0.017 0.112
5.0 0.000 -0.011 -0.026 0.029 -0.002
8.3.6 DAVISSON & GILL ’S METHOD
A. Pile Deflection
Davisson and Gill propose the elastic solution to analyze laterally loaded pile.
This method is can be used for pile embedded in cohesive soil .
B. Pile Deflection
The pile deflection at any depth of pile can be calculated, as follows :
( )
+
=
pp
2
xpp
3
x IEMR
'BIE
QR'Azx [8.36]
where :
x(z) = deflection at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
A’x, B’x = constant
C. Pile Bending Moment
The pile bending moment at any depth of pile can be calculated, as follows :
( ) ( ) ( )M'BQR'AzM mm += [8.37]
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 18
where :
M(z) = bending moment at any depth of pile
Q = shear force at top of pile
M = bending moment at top of pile
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
A’m, B’m = constant
D. R Coefficient
The variable R is defined as follows :
4
s
pp
k
IER = [8.38]
where :
Ep = modulus of elasticity of pile
Ip = moment of inertia of pile
ks = modulus of subgrade reaction
8.3.7 BROM’S METHOD
A. General
Brom divide the condition as free head condition and restrained head condition .
Brom’s method only can be used for homogeneous soil , purely cohesive soil or purely
cohesionless soil .
B. Cohesive Soil
The following figure is the pressure diagram proposed by Brom for cohesive soil for free head
condition .
FIGURE 8.1 COHESIVE SOIL – FREE HEAD CONDITION
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 19
The minimum embedment depth of the pile due to shear force Q is :
( ) ( )fB5.1
Bs25.2f5.0B5.1eQFS
Du
min ++++=
( )Bs9QFS
fu
=
[8.39]
where :
Dmin = minimum embedment depth
Q = lateral shear force
e = eccentricity of lateral load
B = pile diameter
su = undrained shear strength
FS = safety factor (FS = 3)
The following figure is the pressure diagram proposed by Brom for cohesive soil for restrained head
condition .
FIGURE 8.2 COHESIVE SOIL – RESTRAINED HEAD CONDITION
The minimum embedment depth of the pile due to shear force Q is :
( )B5.1
Bs9FSQ
Du
min +
= [8.39]
where :
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 20
Dmin = minimum embedment depth
Q = lateral shear force
B = pile diameter
su = undrained shear strength
FS = safety factor (FS = 3)
C. Cohesionless Soil
The following figure is the pressure diagram proposed by Brom for cohesionless soil for free head
condition .
FIGURE 8.3 COHESIONLESS SOIL – FREE HEAD CONDITION
The minimum embedment depth of the pile due to shear force Q is :
( )eDQ
KBD5.0FS
min
p3min
+γ
=
φ+=2
45tanK 2p
[8.40]
where :
Dmin = minimum embedment depth
Q = lateral shear force
B = pile diameter
su = undrained shear strength
e = eccentricity of lateral load
Kp = coefficient of passive lateral earth pressure
FS = safety factor (FS = 3)
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 21
The following figure is the pressure diagram proposed by Brom for cohesionless soil for restrained
head condition .
FIGURE 8.3 COHESIONLESS SOIL – RESTRAINED HEAD CONDITION
The minimum embedment depth of the pile due to shear force Q is :
( )p
min BK5.1FSQ
Dγ
= [8.40]
where :
Dmin = minimum embedment depth
Q = lateral shear force
B = pile diameter
su = undrained shear strength
Kp = coefficient of passive lateral earth pressure
FS = safety factor (FS = 3)
8.9 GROUP PILE FOUNDATION
8.9.1 GENERAL
When the load is becomes bigger the group pile must be used to carry the load. The design of group
pile must consider the efficiency of the group and the arrangement of the pile .
8.9.2 PILE CONFIGURATION
The minimum spacing between pile in group pile is :
( )D5.35.2s −= [8.41]
ANALYSIS OF BEARING CAPACITY – DRIVEN PILE FOUNDATION
8 - 22
where :
s = pile spacing
D = pile diameter
8.9.3 LOAD TRANSFER
The group pile may be subjected to centric load and eccentric load.
The load transfer to the pile due to centric load is :
nP
P1 = [8.42]
where :
P1 = vertical load in one pile
P = total centric vertical load
n = number of pile
The load transfer to the pile due to eccentric load is :
∑±
∑±=
2x
2y
1y
yM
x
xM
nP
P [8.43]
where :
P1 = vertical load in one pile
P = total centric vertical load
Mx = moment about X axis
My = moment about Y axis
x = x distance from center of pile cap
y = y distance from center of pile cap
n = number of pile
8.9.4 GROUP EFFICIENCY
The group efficiency of group pile can be calculated based on the Converse – Labarre formula, as
follows :
{ } { }
−+−θ−=mn90
n1mm1n1Eg
sD
tan 1−=θ
[8.44]
where :
Eg = efficiency of group pile
m = number of pile columns
n = number of pile rows