Pile Design and Construction Practice, Fifth edition

258 Pile groups under compressive loading

When making a settlement analysis for a pile group underlain by a layered soil strata
with different, but progressively increasing modulus values with depth, the strata are
divided into a number of representative horizontal layers. An average modulus value is
assigned to each layer. The dimensions Land Bin Figure 5.20 are determined for each
layer on the assumption that the vertical stress is spread to the surface of each layer at an
angle of 30 from the edges of the equivalent block foundation (Figure 5.21). The total
settlement of the piled foundation is then the sum of the average settlements calculated for
each layer.
The foregoing procedure is referred to in EC7, Annex F, as the Stress–Strainmethod.
The other procedure described in Annex F is the Adjusted Elasticitymethod. A typical
example of the latter is the use of the Christian and Carrier(5.14)influence factors shown in
Figure 5.20. These give the average settlement of the pile group from the equation:

Average settlement (5.22)

In the above equation Poisson’s ratio is taken as 0.5. The influence factors  1 , and  0 are
related to the depth and the length/breadth ratio of the equivalent block foundation, and
the thickness of the compressible layer as shown in Figure 5.20. Euis obtained by means of
one or more of the methods listed above.

i  1  0 qnB/Eu

1.0

2.0

1.5

1.0

0.5

0

0110

H/B

Circle

Square

L/B 5

L/B L/B 10

L/B 2

100 1 000

D

H B

μ 0

D/B

0 5 10 15 20

0.9

0.8


^0 
^1 qB

E

 1

Average settlement

r

Figure 5.20Influence factors for calculating immediate settlements of flexible foundations of width B
at depth Dbelow ground surface (after Christian and Carrier(5.14)).