Civil Engineering Reference
In-Depth Information
Figure 11.2
Behaviour of soil during drained constant
p
shearing.
→
Notice also that the loading path D
P for the overconsolidated soil before the peak
p
=
state is linear and vertical (i.e.
0).
Figure 11.2 shows the same two initial states but with the paths for drained shearing
with constant
p
. Again both paths must move towards, and ultimately reach, the crit-
ical state line. The soil initially on the wet side compresses on shearing and ultimately
fails at F
w
. The soil initially on the dry side first shears at constant volume to the peak
state at P but then it dilates and the shear stress reduces as the specific volume increases.
The shear stresses at the failure points, F
w
and F
d
, are different because the effective
stresses and specific volumes are different.
The principal distinction between soils that compress on drained shearing or where
pore pressures increase on undrained shearing and soils that dilate or where pore
pressures decrease, is in whether the initial state lies to the right (i.e. on the wet side)
or to the left (i.e. on the dry side) of the critical state line as illustrated in Fig. 11.3.
Soils initially on the wet side compress during drained shearing or the pore pressures
increase during undrained shearing, while soils initially on the dry side dilate or pore
pressures decrease.
The distinction between the dry side and the wet side of critical is very important
in determining the basic characteristics of soil behaviour. Soils must be heavily over-
consolidated (
R
p
δ
3) to be on the dry side while soils that are normally or lightly
overconsolidated (
R
p
<
>
2) will be on the wet side.
