Civil Engineering Reference
In-Depth Information
factor decreases and the foundation settles due to increased shear straining. On the
other hand reduction of mean effective stress will cause plastic clay to swell and the
foundation will then heave. Rising or falling groundwater can cause either heave or
settlement of foundations.
Foundations which are lightly loaded with relatively large load factors, such as
house foundations, are susceptible to subsidence due to changes in groundwater which
cause high plasticity clay to shrink or swell. A major cause of changes of groundwater
near houses is vegetation or leaking drains. Trees and plants remove water from the
ground and so reduce the pore pressures in the ground within the influence of their
roots. Allowing vegetation to grow can cause subsidence while removing vegetation or
allowing drains to leak can cause heave. If the soil is unsaturated and the foundation
has a relatively heavy loading, flooding can cause collapse settlements, as described in
Chapter 26.
22.12 Summary
1. Foundations transmit loads to the ground. As the load increases the foundation
settles and it fails when the settlements become very large. Foundations may be
shallow or they may be deep to take advantage of the general increase of strength
and stiffness of soils with depth.
2. The bearing pressure
q
is the contact stress between the foundation and the soil.
The net bearing pressure of a deep foundation is the change of bearing pressure; this
may be positive so the foundation settles or it may be negative so it heaves. The
bearing pressure
q
and the net bearing pressure
q
n
are given by
V
+
W
q
=
(22.1)
A
=
−
q
n
q
p
0
(22.3)
3. Under a foundation pore pressures generally increase with undrained loading and,
with time, these dissipate as the soil consolidates. As a result further settlements
occur but effective stresses and safety factors increase.
4. The bearing pressure when the foundation fails is the bearing capacity
q
c
given by
q
c
=
s
u
N
c
+
p
0
(22.6)
2
(
1)
D
=
γ
−
γ
w
)
BN
γ
+
γ
−
γ
−
+
q
c
(
w
)(
N
q
p
0
(22.10)
for undrained and drained loading respectively, where
N
c
,
N
γ
and
N
q
are bearing
capacity factors.
5. An important criterion for foundation design is the need to limit the settlements.
This may be done by applying a load factor to the net bearing pressure. Alter-
natively, settlements may be calculated assuming that the soil in the foundation
is elastic. For foundations on sand settlements are related to the relative density
which may be estimated from the results of SPT tests.
