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,
1
3 � 1935) � 2465� 3110 kN,
Safety factor � (1935�2465)/3156 � 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
� 4 � 5.5 � ( 2
3 � 10.5) � 16.5 m
The dimensions of the equivalent raft are:
L � 48.75 � ( 2
3 � 10.5 � 2 � 1
4) � 52.25 m
B � 18.75 � ( 2
3 � 10.5 � 2 � 1
4) � 22.25 m
Pressure on base of equivalent raft due to building load �
Calculating the immediate settlement
At a level of equivalent raft, E u � E f � 65 MN/m 2
At rockhead, E u � 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 26.1�11.1 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, I� p is 0.14.
Settlement at corner of rectangle �
139 � 11.1 � 0.14 � 1000
65 � 1000
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 � 1000
� � 57.8 mm
1000
48 � 21 � 160
52.25 � 22.25 � 139 kN/m 2
� 3.3 mm