Environmental Engineering Reference
In-Depth Information
Void ratio
Notation:
SS: Strain softening response
SH: Strain hardening response
LSS: Limited strain softening response
q
ST
: Static gravitational shear stress
Su: Ultimate undrained shear
strength
US: Ultimate State
e
SS
USL
(Ultimate State Line)
LSS
SH
Mean effective strees, p
Shear
stress
q
Collapse surface
Strain softening (SS)
q
US
S
u
q
ST
SS
, shear strain
p
q
q
US
Limited strain softening (LSS)
q
ST
LSS
p
Strain hardening
(SH)
Peak strength
envelope
q
q
Phase transformation
SH
q
ST
p
Figure 12.7.
Schematic of undrained monotonic behaviour of sand in triaxial compression (Robertson
1994).
“
Cyclic mobility
” is a form of temporary liquefaction where the shear stresses are always
greater than zero.
These are discussed further below.
12.4.1.2
Some consideration of the mechanics of undrained shear of granular soils,
and of liquefaction
12.4.1.2.1 Undrained shear of saturated granular soils
Figure 12.7 shows schematically the undrained behaviour of saturated sand in triaxial com-
pression.
A soil which has an initial void ratio higher than the ultimate state (steady state, criti-
cal state/line) will tend to contract (densify) in drained loading, reaching a void ratio equal
to that at the ultimate state (critical state) line at that mean effective stress. In saturated
undrained loading the soil is unable to contract, so positive pore pressures are developed,
which will give strain weakening behaviour, reaching the steady state undrained strength
S
us
. If the static gravitational shear stresses q
st
are greater than S
us
, flow liquefaction will
result.
A soil which has an initial void ratio just above the ultimate state line may show limited
strain softening, or temporary liquefaction, with the soil wanting to contract on initial
