Pile Design and Construction Practice, Fifth edition

302 Pile groups under compressive loading

from which l 10.4 m (say 10.5 m) and the trial depth is satisfactory.
Checking the criterion of a safety factor of 3 in end bearing and unity in skin friction,
allowable load which roughly equals the building load
plus the negative skin friction. Checking the overall safety factor on the combined loading,

Safety factor (1 9352 465)/3 156 1.4

This is satisfactory since the negative skin friction on the piles will not contribute to the
settlement of the pile group.
The transfer of load from the pile group to the soil will be as shown in Figure 5.3b.

Depth below ground level to base of equivalent raft

The dimensions of the equivalent raft are:

Pressure on base of equivalent raft due to building load

Calculating the immediate settlement
At a level of equivalent raft, Eu Ef 65 MN/m^2
At rockhead, Eu 120 MN/m^2
From equation 5.21, 120 65(1 28 k/22.25)
k 0.7
Divide equivalent raft into four rectangles, each 26.111.1 m
From Figure 5.19 for L/B 26.1/11.1 2.3, H/B 28/11.1 2.5, and k 0.7, Ipis 0.14.

Settlement at corner of rectangle

Settlement at centre of equivalent raft 4 3.3 13.2 mm

Calculating the consolidation settlement
To calculate the settlement of the pile group due to the building loads only, the 28 m layer
of clay between the equivalent raft and rockhead is divided into four 7 m layers.

Oedometer settlement in Layer 1.
From Figure 5.13 with and z/B 3.5/22.2 0.16 and L/B 52.2/22.2 2.3, stress at centre
of rectangle 0.83 139 118 kN/m^2. Modulus of volume compressibility 0.07 m^2 /MN.
Then from equation 5.23:

oedometer settlement uncorrected by depth factor

0.07^118 ^7 1 000

(^1000)
57.8 mm

139 11.10.141 000

65 1 000

3.3 mm

48 ^21 ^160

52.2522.25

139 kN/m^2

B 18.75(^23 10.5 2 ^14 ) 22.25 m

L 48.75(^23 10.5 2 ^14 ) 52.25 m

4 5.5(^23 10.5) 16.5 m

(^13 1935) 24653110 kN,