Civil Engineering Reference
In-Depth Information
26.7 Undrained loading of unsaturated soil
A very important difference between the behaviours of saturated and unsaturated soil
is the response to undrained loading. Strictly undrained means no drainage of water
and no change of water content. In saturated soil this means there is no change in
volume either because both soil grains and water can be assumed to be incompressible
over changes of stress common in ground engineering. In unsaturated soil, however,
the air or gas is highly compressible and so constant water content does not mean
constant volume.
If soil is loaded isotropically and undrained at constant water content the change of
pore water pressure
δ
u
w
which occurs during a change of total stress
δσ
is given by
δ
u
w
=
B
δσ
(26.12)
where
B
is called a pore pressure parameter. For saturated soil there is no change of
volume and so there is no change of effective stress, in which case
1.
For unsaturated soil
B
is less than 1: the value of
B
decreases with decreasing degree
of saturation and becomes
B
δ
u
w
=
δσ
and
B
=
0.
In Sec. 9.4 I showed that the strength of soil depends on its specific volume and for
saturated soil this remains constant for undrained (constant water content) loading.
This is the reason why we can use the undrained strength
s
u
for undrained loading of
saturated soil. For unsaturated soil, however, constant water content does not mean
constant specific volume and the concept of undrained strength cannot be used.
=
0 in dry soil when
S
r
=
26.8 Strength of unsaturated soil in slope stability
The critical state strength of unsaturated soil cannot be described by an undrained
strength or by an effective stress strength. The best we can do is relate the strength of
unsaturated soil to the net stress (
σ
−
u
a
) and the matrix suction (
u
a
−
u
w
).
τ
f
of an unsat-
urated soil, the net stress and the matrix suction. Instead of matrix suction we could
plot degree of saturation as these are related, as shown in Fig. 26.5, but the relation-
ships between them are different for wetting and drying. If the soil is saturated with
S
r
Figure 26.6 shows the relationship between the critical state strength
0, total and effective stresses are equal and the strength envelope is
defined by the critical friction angle
=
1 and
u
w
=
φ
c
. As the degree of saturation reduces and the
matrix suction increases the strength increases and there will be a surface, as shown in
Fig. 26.6, which describes the strength at any net stress and matrix suction.
The lines on the surface represent the strength at constant matrix suction and if
they are assumed to be linear each can be represented by the Mohr-Coulomb criterion
given by
τ
f
=
c
+
σ
tan
φ
(26.13)
In Eq. 26.13 the cohesion intercept
c
depends on the matrix suction while the total
stress friction angle
φ
c
.
The arrow on the surface marked wetting shows that the strength at a given net
stress decreases with decreasing suction and increasing degree of saturation. The arrow
φ
may or may not be related to
