Civil Engineering Reference
In-Depth Information
the dry side and are similar to Figs 9.1 and 10.2. In Sec. 10.5 I showed that at the peak
state the stress ratio was related to the rate of dilation by
q
p
=
d
ε
v
M
−
(11.5)
d
ε
s
Since elastic strains are relatively small compared to the plastic strains. Eq. (11.5)
also applies to states before and after the peak and to soils on the wet side and on
the dry side (except at states close to the start of the shearing where the behaviour is
essentially elastic). Figure 11.11(c) shows Eq. (11.5) as
q
/
p
against d
s
for the
normally consolidated soil and for the overconsolidated soil. There are two points,
D and F where the rates of volume change are zero and
q
/
p
=
ε
v
/d
ε
M. Consequently, by
plotting soil test data as
q
/
p
against d
s
the position of the critical state point F can
be found even if the loading is terminated before the samples have reached their critical
states. It is best to conduct tests on both normally consolidated and overconsolidated
samples of clay or on loose and dense samples of sand to obtain data on both sides of
the critical state.
ε
v
/d
ε
11.7 Slip planes and apparent errors in test results
During shearing at and beyond the peak state overconsolidated clays and dense sands
on the dry side of critical often have non-uniform strains and develop strong discon-
tinuities like those shown in Fig. 2.10. These are usually called slip planes although
they have finite thickness which may be only a few grains thick. Strains in slip planes
were described in Sec. 2.8. Soil in a slip plane has volumetric strains which are very
different to the mean volumetric strain in the whole sample. In a nominally undrained
test in which no water enters of leaves the sample water may move into a slip plane
from nearby soil so there is local drainage. Once slip planes appear you cannot
rely on measurements of volumetric strains made with the instruments described in
Chapter 7.
If a soil is on the dry side of critical it will dilate on shear and if a slip plane starts
to form the water content of the soil in it will increase, the soil will weaken and the
slip plane will grow. However, if the soil is on the wet side of critical it will compress
during shear and if a slip plane starts to form the soil will strengthen and the slip plane
will stop growing. Slip planes are seen most often in soils whose states are on the dry
side of critical. As there must be some drainage of water into the slip plane from the
surrounding soil, volumetric strains and water contents measured in the usual way
at the boundaries of the sample are different from those in the soil in the slip plane.
The quantity of this local drainage depends on the permeability of the soil and on the
rate of shearing in the test.
Figure 11.12 illustrates the behaviour of a sample of soil in a nominally undrained
triaxial compression test. The drainage tap was closed so no water could enter or leave
the sample but water could move by local drainage into a slip plane from nearby soil.
If the soil is fully undrained it would follow the path O
→
Y
→
F
u
in Fig. 11.12 and
if it is fully drained it would follow the path O
F
d
. (These are the same as
those shown in Figs. 11.1 and 11.2.) If there is partial local drainage the soil follows an
→
Y
→
