Pile Design and Construction Practice, Fifth edition

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 26 086 kN

For calculating the base resistance from equation 5.1, Ncfrom Figure 5.6 is 5.14 (the classic
value for a shallow foundation on clay in undrained shear), scis 1.3, dcfor D/B 1.5 and
 0 is 1.0 (Figure 5.9), icis 1.0 for a centrally applied vertical load, and bcis 1.0.
Taking the intact shear strength of the clay at base level as 110 kN/m^2 :

Base resistance of foundation 110 5.141.31.51.01.065.34 72 039 kN

Total resistance of foundation 26 086 72 039 98 125 kN

Factor of safety against general shear failure 98 125/70 360 4.0

Modulus of volume compressibility mv in MN/m^2

0

5

0.05 0.10 0.15 0.20 0.25

9.30

13.30

17.30

21.30

25.30

29.30

Eu

mv

10

15

20

25

30

Depth below ground level in m

0 20 40 60 80 100

Deformation modulus Eu in MN/m^2

Base of

equivalent

raft

Layer 1

Layer 2

Layer 3

Layer 4

18.40  15.20

13.80  10.50

23.00  19.90

27.60  24.60

32.20  29.30

Layer 5

Centre line

Incompressible stratum

1.5

10 rows of 7 piles

in group 9.90  6.60

D^

13.909.301.4

60 º

Figure 5.43Profiles of the undrained deformation modulus Euand the coefficient of compressibility mv.