Pile groups under compressive loading 259Ground levelBase of equivalent raft
foundation for Layer 1Ditto for Layer 3Layer 3Layer 2L a y e r 1(^30) ̊ Ditto for Layer 2
4
1
Figure 5.21Load distribution beneath pile group in layered soil formation.
The consolidationsettlement cis calculated from the results of oedometer tests made on
clay samples in the laboratory. The curves for the pressure/voids ratio obtained from these
tests are used to establish the coefficient of volume compressibility mv.
In hard glacial tills or weak highly weathered rock it may be difficult to obtain satisfac-
tory undisturbed samples for oedometer tests. If the results of standard penetration tests
are available, values of mv(and also cu) can be obtained from empirical relationships
established by Stroud(5.7)shown in Figure 5.22.
Having obtained a representative value of mvfor each soil layer stressed by the pile group,
the oedometer settlement oedfor this layer at the centre of the loaded area is calculated from
the equation:
oed (5.23)
where dis a depth factor, (^) zis the average effective vertical stress imposed on the soil layer
due to the net foundation pressure qnat the base of the equivalent raft foundation and His
the thickness of the soil layer. The depth factor dis obtained from Fox’s correction
curves(5.15)shown in Figure 5.23. To obtain the average vertical stress (^) zat the centre of each
soil layer the coefficients in Figure 5.13 should be used. The oedometer settlement must now
be corrected to obtain the field value of the consolidation settlement. The correction is made
by applying a ‘geological factor’gto the oedometer settlement, where
(5.24)
Published values of ghave been based on comparisons of the settlement of actual
structures with computations made from laboratory oedometer tests. Values established by
Skempton and Bjerrum(5.16)are shown in Table 5.2.
c goeddmv (^) zH