Pile Design and Construction Practice, Fifth edition

Depth below ground level in m
5
10
15
20
0
Modulus of volume compressibility m v in MN/m 2
0.05 0.10 0.15 0.20 0.25
m v
Pile groups under compressive loading 291
Determining the stability of the group assuming that it acts as a block foundation, and
using the procedure, in Sections 5.1 and 5.2.1, the shaft resistance of the block is calculated
on the fissured strength of the clay as a result of general relaxation of the mass of soil
surrounding the group. Because there is a relatively small pile-to-soil contact around the
periphery of the group an adhesion factor will not be applied.
Unit shear resistance on sides of block � 0.75 � (60 � 110)/2 � 63.75 kN/m 2
Total shear resistance � 2 � 12.4 � (9.9 �6.6) � 63.75 � 26086 kN
For calculating the base resistance from equation 5.1, N c from Figure 5.6 is 5.14 (the classic
value for a shallow foundation on clay in undrained shear), s c is 1.3, d c for D/B � 1.5 and
� � 0� is 1.0 (Figure 5.9), i c is 1.0 for a centrally applied vertical load, and b c is 1.0.
Taking the intact shear strength of the clay at base level as 110 kN/m 2 :
Base resistance of foundation�110�5.14�1.3�1.5�1.0�1.0�65.34�72 039 kN
Total resistance of foundation � 26086 �72039 � 98125 kN
Factor of safety against general shear failure � 98125/70�360 � 4.0
E u
9.30
13.30
17.30
21.30
Base of
equivalent
raft
Layer 1
13.80 � 10.50
Layer 2
Layer 3
10 rows of 7 piles
in group 9.90 � 6.60
D � 13.90
9.30
18.40 � 15.20
23.00 � 19.90
25
25.30
29.30
30
0 20 40 60 80 100
Deformation modulus Eu in MN/m2 Layer 4
27.60 � 24.60
Layer 5
60º 32.20 � 29.30
Centre line
Incompressible stratum
Figure 5.43 Profiles of the undrained deformation modulus E u and the coefficient of compressibility m v.
1.5
1.4