Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter
4 SOIL SETTLEMENT
Raft foundations are relatively
large in size. Hence,
the bearing capacity is generally not the
controlling factor in
design. Differential and
total settlements usually
govern the
design. A common
approach for estimating the settlement of
a raft foundation is to
model the
ground support as
springss using the subgrade reaction method. This method suffers from a number of drawbacks.
Firstly, the modulus
of subgrade reaction is
not an intrinsic soil property. It depends upon not only
the stiffness of the soil, but also
the dimensions of the foundation.
Secondly, there is no interaction
between the springs. They are
assumed to
be independent of each other and can only respond in
the direction of the
loads. BSI (2004) cautions that the subgrade
reaction model is generally not
appropriate for estimating the total and differential settlement of a raft foundation. Finite element
analysiss or elastic continuum method is preferred for the design of
raft foundations (French, 1999;
Poulos,
2000).
Figure 4.9 Common Types of Raft Foundation
March 2009
4-13