Environmental Engineering Reference
In-Depth Information
Figure 6.8.
Steady state or critical state concept (Leroueil et al., 1996).
A
Sucs
Su
O
B
σ
1
σ
3
p
2
Figure 6.9.
Stress path for saturated loose sand which displays collapse behaviour.
The line through these points is termed the collapse surface (Sladen et al., 1985;
Dawson et al., 1998), or critical stress ratio (Vaid and Eliadorani, 1998) or the instability
line (Lade, 1992). At these stress conditions the soil wishes to contract as it shears but, being
constrained by the no volume change and conditions of saturated undrained conditions,
develops positive pore pressures resulting in weakening to the ultimate strength S
u
at B.
The important point here is that the collapse surface strength may be lower than the
peak, or critical state (constant volume) strength, which is commonly used in stability
analysis, so failure may occur at stresses lower than expected (e.g. Dawson et al., 1998).
As pointed out by Sladen et al. (1985), for very loose sands whose state lies on the collapse
surface, only minimal excess pore pressure is necessary to trigger collapse. Loading may
be essentially drained up to the point of reaching the collapse surface.
The degree of strain weakening has been characterized in terms of the Brittleness Index
I
B
(Bishop, 1971):
p
r
(6.2)
I
100%
B
p
