Pile Design and Construction Practice, Fifth edition

For the arrangement of the piles shown in Figure 5.43, the overall dimensions of the group
are 91.1 9.9 m, 61.1 6.6 m. The spread of the load shown in Figure 5.3a applies.

Depth to centre of equivalent raft

Dimensions of equivalent raft

and

Pressure at level of equivalent raft

The settlements are calculated over the zone of soil down to the level of the incompressible
stratum, that is at a depth of 20 m below the base of the equivalent raft. It is convenient to
divide the soil into five 4 m layers commencing at 9.30 m and extending to 29.30 m. The
immediate and consolidation settlements are then calculated for each layer.

Immediate settlement in Layer 1

From Figure 5.43, average Eu 39 MN/m^2. From Figure 5.20, for H/B 4/10.5 0.38,
and L/B 13.8/10.5 1.3,  1 0.15, and for D/B 9.3/10.5 0.9, and L/B 1.3,
 0 0.93. Therefore from equation 5.22:

Immediate settlement

The settlements in the underlying four layers are calculated in a similar manner, the
calculations for all five layers being tabulated thus

i 0.150.93^174 10.51 000

39 1 000

6.5 mm

qn 70 ^360

10.513.8

174 kN/m^2

9.90(^14 7.802) 13.8 m

6.60(^14 7.802) 10.5 m

23 13.90 9.3 m

Pile groups under compressive loading 293

Layer B (m) L (m) qn (kN/m^2 )  1  0 Eu (MN/mv^2 ) i(mm)

1 10.5 13.8 174 0.15 0.93 39 6.5
2 15.1 18.4 90 0.06 0.93 52 1.5
3 19.7 23.0 55 0.03 0.92 64 0.5
4 24.3 27.6 37 0.02 0.92 76 0.2
5 28.9 32.2 27 0.01 0.93 88 0.1

Total immediate settlement 8.8

The immediate settlement can be checked from equation 5.21 because the deformation
modulus increases linearly with depth. At the level of equivalent raft Euis 32 MN/m^2 and at
20 m below this level it is 97 MN/m^2. Therefore from equation 5.21:

97 32(1 20 k/10.5)

k 1.1