256 Pile groups under compressive loading
increases with depth. For materials with a linear increase, Butler(5.12)developed a method based
on the research of Brown and Gibson(5.13), for calculating settlements where Euor Evincreases
linearly with depth through a layer of finite thickness. The value of the modulus at any depth z
below the base of the equivalent block foundation is given by the equation:
E Ef(1 kz/B) (5.21)where Efis the modulus at the base of the equivalent foundation.
To obtain k, values of Euor Evobtained by one or more of the methods listed above are
plotted against depth and a straight is drawn through the plotted points. The value of kis then
obtained using Figure 5.19 which also shows the values of the influence factor Ip. The curves
in this figure are based on normally consolidated clays having a Poisson’s ratio of 0.5 and
are appropriate to a compressible layer of thickness not greater than 9 times the breadth of
the foundation. For a rigid pile group the immediate settlement as calculated for a flexible
pile group is multiplied by a factor of 0.8 to obtain the averagesettlement of the rigid group,
and a depth factor is applied using the curves in Figure 5.20.
Where a piled foundation consists of a number of small clusters of piles or individual
piles connected by ground beams or a flexible ground floor slab the foundation arrangement
can be considered as flexible.
L/B `L/B 10 0 2 4 6 8100.1L/B 1L/B 10 L/B^5L/B 0.2 0.3 0.4 0.5 0.6 0.7 0.8Values of F 1 and F 2F 1F 2( ) ( )L/B 2
L/B 5L/B L/B^2 L/B^10Figure 5.18Values of Steinbrenner’s influence factor Ip(for vof 0.5,Ip Fl, for v 0 ,Ip F 1 F2.
Note
When using this diagram to calculate at the centre of a rectangular area take Bas half the foundation width to
obtain H/B and L/B.
i