Civil Engineering Reference
In-Depth Information
be very small for an underground car park or a submerged tank. If
q
n
is positive
the foundation will settle, but if it is negative (i.e. the total stress at foundation level
reduces) the foundation will rise. By careful design of a compensated foundation it is
possible to have
q
n
≈
0 so that settlements are negligible.
22.2 Foundation behaviour
Figure 22.3(a) shows a simple shallow foundation with a gross bearing pressure
q
, a net
bearing pressure
q
n
and a settlement
ρ
. If the foundation is rigid (e.g. concrete) the
settlement
will be uniform and the bearing pressure will vary across the foundation.
If, on the other hand, the foundation is flexible (e.g. an earth embankment) the bearing
pressure will be uniform but the settlements will vary. Figure 22.3 illustrates mean
values of
q
and
ρ
for each case. Figure 22.3(b) shows the relationship between net
bearing pressure
q
n
and settlement
ρ
. The general form of Fig. 22.3(b) is the same
for drained and undrained loadings but the magnitudes of the stresses and settlements
will be different for each case. As the bearing pressure increases the settlements start
to accelerate and at some point the foundation can be said to have failed because the
settlements have become large. Foundations do not fail in the sense that they can no
longer support a load or the load on them has reached a maximum or starts to decrease.
Instead they continue to settle and the bearing pressure continues to increase slowly as
the depth of the foundation increases with further settlement. Sometimes a foundation
under eccentric loading starts to rotate like the leaning Tower of Pisa and then it can
reach a state where the mean bearing capacity starts to decrease.
Notice that I have defined the bearing capacity
q
c
as a net bearing pressure at which
settlements accelerate. Some other texts and codes define the bearing capacity as a
gross bearing pressure. You should watch out for this.
Obviously you cannot load a building foundation close to its bearing capacity
q
c
as
the settlements would then be too large and the building would probably be damaged
(although it may not fall down). To limit the settlements to some allowable value
ρ
ρ
a
it is necessary to reduce the bearing pressure to some allowable bearing pressure
q
a
,
as shown in Fig. 22.3(b). In practice this is usually achieved by applying a load factor
to the bearing capacity.
Figure 22.4(a) shows the net bearing pressure of a foundation increased to
q
a
slowly
so that the loading is drained. The foundation settlements increase in parallel with
Figure 22.3
Loading and settlement of a foundation.
