Pile Design and Construction Practice, Fifth edition

252 Pile groups under compressive loading

The third step is to calculate the settlement of the foundation which takes place in two
phases. The first is immediate settlement (i) caused by elastic compression of the soil
without dissipation of pore pressure. It is followed by consolidation settlement (c) which
takes place over the period of pore pressure dissipation at a rate which depends upon the
permeability of the soil. There is also the possibility of very long-term secondary settlement
() or creep of the soil. In the case of the very soft soils referred to in the previous
paragraph, secondary settlement could be a significant proportion of the total. The equation
for calculating it is given at the end of Section 5.2.
The net immediate settlement of foundations on clays is calculated from the equation:

(5.14)

where qn net foundation pressure

B foundation width

 Poisson’s ratio

Eu undrained deformation modulus

Ip influence factor

Ev(or for drained conditions Ev) can be obtained by one or more of the following methods:

(1) From the stress–strain curves established in the field by plate-bearing tests
(Figure 5.15)
(2) From drained triaxial compression tests on good quality samples (to obtain Ev)
(3) From oedometer tests to obtain the modulus of volume compressibility (mv), when Ev
is the reciprocal of mvand

i

qnB(1^2 )Ip

Eu

qn

Overburden

pressure,  (^9) vo
Lowest level
considered in
settlement analysis
Combined
 (^9) vo and z
 (^9) vo z
z³0.2 (^9) vo
Figure 5.14Vertical pressure and stress distribution for deep clay layer.