Civil Engineering Reference
In-Depth Information
applications of physics and chemistry, but it is probably less approximate than, say,
sociology and economics. You can usually, but not always, improve a theory bymaking
it more complicated and by adding more variables. For example, if material strength
and stiffness parameters are allowed to vary with ambient temperature the theories
will become more complex but possibly more realistic. In this topic I shall be dealing
with fairly simple theories of soil mechanics and geotechnical engineering which are
suitable for most routine design problems.
Although it is always essential to consider the ultimate limit states of structures to
demonstrate that they will not collapse, the principal design criterion for many struc-
tures, particularly foundations, is the need to limit ground movements and settlements.
In practice this is often done by applying a factor to the collapse load. In my first job
as a young engineer I was involved in the design of a very large earthfill dam, where
the consequences of collapse would have been catastrophic and would certainly have
meant major loss of life: the chief engineer required a factor of safety of about 1.25
against slope failure. In my second job I was asked to design the foundations for a small
store shed which was part of a water treatment works: the chief engineer required a
factor of 3.
I was puzzled by this inconsistency until I discovered that the large factor required
for the foundations of the store shed was not really a factor of safety but was a factor to
limit the settlement. The chief engineer knew that if the collapse load of a foundation
was reduced by a factor of 3 the resulting settlements would be small. The point is
illustrated in Fig. 1.6, which shows the settlement
of a foundation with an increasing
vertical load
V
. In Fig. 1.6(b) there is a collapse load
V
c
and a safe load
V
s
that is
about 80 per cent of
V
c
, corresponding to a factor of safety of about 1.25. There is
also an allowable load
F
a
and for this load the settlements are small.
The safe load
V
s
is given by
ρ
1
F
s
V
c
V
s
=
(1.4)
where
F
s
is a factor of safety. Values of
F
s
in geotechnical engineering are normally in
the range 1.25 to 1.5, depending on the consequences of failure and the uncertainties
in the analyses and determination of the loads and the soil parameters.
Figure 1.6
Factor of safety and load factor for a foundation.
