Pile Design and Construction Practice, Fifth edition

260 Pile groups under compressive loading

(a) 10

(b)

8

6

f 1

f 2

4

2

0

0 102030

Plasticity index

Plasticity index

40 50

Mass shear strength

c f 1 N (kN/m^2 )

Modulus of volume compressibility

mv 1 (m^2 MN)

60 70

0 10203040506070

1.0

0.8

0.6

0.4

0.2

0

f 2 N

Figure 5.22Relationship between mass shear strength, modulus of volume compressibility, plasticity

index, and standard penetration test N-values (after Stroud(5.7)) (a) N-value versus

undrained shear strength (b) N-value versus modulus of volume compressibility.

The total settlement of the pile group is then the sum of the immediate and consolidation
settlements calculated for each separate layer. A typical case is a gradual decrease in com-
pressibility with depth. In such a case the stressed zone beneath the pile group is divided
into a number of separate horizontal layers, the value of mvfor each layer being obtained by
plotting mvagainst the depth as determined from the laboratory oedometer tests. The base
of the lowermost layer is taken as the level at which the vertical stress has decreased to
one-tenth of qn. The depth factor dis applied to the sum of the consolidationsettlements
calculated for each layer. It is not applied to the immediate settlement if the latter has been
calculated from the factors in Figure 5.20.
Another method of estimating the totalsettlement of a structure on an over-consolidated
clay is to use equation 5.14, making the substitution of a deformation modulus obtained for
loading under drained conditions. This modulus is designated by the term , which is
substituted for Euin the equation. It is approximately equal to 1/mv. The equation implies
a homogeneous and elastic material and thus it is not strictly valid when used to calculate
consolidation settlements. However, when applied to over-consolidated clays for which the
settlements are relatively small, the method has been found by experience to give reasonably
reliable predictions. Success in using the method depends on the collection of sufficient data

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