Pile Design and Construction Practice, Fifth edition

264 Pile groups under compressive loading
Provided that the individual pile has an adequate safety factor against failure under
compressive loading there can be no risk of the block failure of a pile group terminated in
and applying stress to a coarse soil. As in the case of piles terminated in a clay, there is a
risk of differential settlement between adjacent piles in small groups if the toe loads of a
small group become concentrated in a small area when the piles deviate from their intended
line. The best safeguard against this occurrence is to adopt a reasonably wide spacing
between the piles. Methods of checking the deviation of piles caused by the installation
method are described in Chapter 11.
The immediate settlement of the pile group due to ‘elastic’ deformation of the coarse soil
beneath the equivalent flexible raft foundation must be calculated. Equation 5.22 is applicable
to this case and the deformation modulus E� v is substituted for E u as obtained from plate loading
tests in trial pits, or from standard penetration, pressuremeter or Camkometer tests, made
in boreholes. Schultze and Sherif (5.18) used case histories to establish a method for predicting
foundation settlements from the results of standard penetration tests using the equation:
s. p
� �
N 0.87 (1 � 0.4D/B)
(5.25)
where s is a settlement coefficient
p is the applied stress at foundation level
N is the average SPT N-value over a depth of 2B below foundation level, or d s if
the depth of cohesion-less soil is less than 2B
D and B are the foundation depth and width respectively
Values of the coefficient s and d s are obtained from Figure 5.25.
Coefficient of settlement s (mm/kNm 2 )
10.0
1.0
s.p
r �
N (1 + 0.4D/B)
ds /B ≥ 2
0.87
0.1
0.5 1 2 3 4 5 10 20 30 40 50
Breadth B (m)
L/B�100
5
2 D
1
d s
B
P
Reduction factors for d s/B < 2
L/B
ds /B
1 2 5 100
1.5 0.91 0.89 0.87 0.85
1.0 0.76 0.72 0.69 0.65
0.5 0.52 0.48 0.43 0.39
Figure 5.25 Determining foundation settlements from results of standard penetration tests (after
Schultze and Sherif (5.18) ).
L