Civil Engineering Reference
In-Depth Information
Bearing capacity and settlement
of shallow foundations
22.1 Types of foundations
Any structure that is not flying or floating rests on or in the ground and the base of the
structure and the soil together make up the foundation. Buildings and embankments
must have foundations and so must vehicles and people. The criteria for the design of
a foundation are that the settlements should be limited so that the building does not
become damaged, vehicles can still move about and you do not lose your boots in mud.
All foundations settle because nothing (not even tarmac or rock) is absolutely rigid, but
obviously some settle more than others; look at the Tower of Pisa for instance. When
you walk across the beach and leave a footprint it is simply a mark of the settlement
of a foundation and so too is a tyre track.
In civil engineering foundations are shallow, deep or piled, as illustrated in Fig. 22.1.
(The distinction
D
/
B
1 to 3 for a deep foundation is made for convenience.) We know
that, in general, the strength and stiffness of soil increases with depth (because effective
stresses increase with depth) and so one advantage of a deep foundation and a pile is
that they are founded in stronger and stiffer soil; often the tip of a pile rests on very stiff
and strong soil or rock. Another advantage is that shear stresses between the soil and
the sides of a deep foundation or a pile contribute to the load capacity; in a shallow
foundation the contribution of the side shear stresses is negligible.
The characteristics of a typical foundation are illustrated in Fig. 22.2. The weight
of the foundation is
W
and it supports a vertical load
V
, a horizontal load
H
and a
moment
M
. The force
V
is usually just the weight of the structure while the horizontal
force
H
and moment
M
arise from wind and wave loads and unexpected impacts.
Normally both
W
and
V
are known with some certainty but the loads
H
and
M
can
only be estimated. For most of this chapter I will consider simple foundations which
have only a vertical load
V
on them.
The base width is
B
; for unit length out of the page this is the base area, so the gross
bearing pressure
q
is
=
V
+
W
q
=
(22.1)
A
Many simple foundations, including piles, are constructed from solid concrete which
has unit weight
c
AD
) depends on the
size of the foundation. Some foundations are hollow, particularly where they are used
γ
c
only a little larger than that of soil, so
W
(
≈
γ
