Environmental Engineering Reference
In-Depth Information
Effective Stress
Failure Line at High
Pressure
Instability
Line
I
G
H
E
Effective Stress Paths
for Undrained
Compression Tests
C
D
LOW PRESSURES
HIGH PRESSURES
A
F
B
Static
Liquefaction
O
PA RTICLE REARRANGING
PA RTICLE CRUSHING
p, Effective Mean Normal Stress
Figure 12.8.
Undrained effective stress behaviour of loose silty sands (Lade and Yamamuro, 1997).
shear and to dilate at larger strains, so developing positive, then negative pore pressures
in undrained loading. A soil with an initial void ratio below the ultimate state line will
tend to dilate, or develop negative pore pressures in undrained loading, giving a strain
“hardening” response.
The susceptibility of a soil to static liquefaction is dependent on the particle size distri-
bution, void ratio (or density index), the initial stress conditions and the stress path of
loading, e.g. triaxal compression or extension.
Lade and Yamamuro (1997), and Yamamuro and Lade (1997), show that there are four
different types of undrained stress paths after the instability line (also known as collapse
surface). These are shown in Figure 12.8:
-
Static liquefaction
occurs at low stresses and is characteristed by large pore pressure
development, resulting in zero effective stresses at low axial strain levels (stress path
AO). Increasing confining pressures result in increasing effective stress friction angles in
this stress region;
-
Temporary liquefaction
occurs at higher stresses than the static liquefaction region and
is characterized by stress path BCDE. Hence the behavior is contractant from C to D,
then dilatant from D to E which occurs at large axial strains. The tendency to dilate
(and less likely to contract) increases as the initial confining stresses increase;
-
Temporary instability
(stress path FGHI) is similar to temporary liquefaction except
that the amount the stress difference increases beyond the initial peak (i.e. I vs G), is not
as large as that exhibited by temporary liquefaction;
-
Instability
- at higher stresses, due partly to particle crushing.
It should be noted that a soil may reach the instability line (collapse surface) at stress
conditions less than the effective stress failure line. Also, a soil can be in a stress condition
just to the right (stable) of the instability line and cross over the line by change in
