Environmental Engineering Reference
In-Depth Information
COMPRESSION CURVE
EQUIVALENT VOLUME CHANGE
OF GRAIN STRUCTURE DUE TO
CYCLIC STRAIN APPLICATIONS
DURING DRAINED LOADING
A
C
e
o
∆
e
B
σ
o
,
σ
f
EFFECTIVE
INDUCED
PORE WATER
PRESSURE
PRESSURES,
INITIAL AND
FINAL STAGES
∆
u
σ
o
σ
f
PRESSURE
Figure 12.9.
Schematic illustration of mechanism of pore pressure generation during cyclic loading (Seed
& Idriss 1982; USNRC, 1985, reproduced with permission of ASCE).
static stress conditions, e.g. a rise in pore pressures or strains induced for example by an
earthquake.
12.4.1.2.2 The effects of cyclic loading
Cyclic loading, such as that from an earthquake, causes densification of dry granular soils
by particle rearrangement due to the back and forth straining. If, however, the soil is sat-
urated and not allowed to drain during cyclic loading, just as in monotonic loading the
decreases in volume cannot occur and the tendency to decrease volume is counteracted by
an increase in pore pressure and decrease in effective stress - see Figure 12.9.
Hence, soil starting at A and subject to cyclic loading, which would otherwise have
ended at B, will in fact have stresses represented by C where total stress
o
is taken by
f
and
u
. The pore pressures must increase to maintain equilibrium in this undrained
condition.
The pore pressures build up gradually with the number of cycles of loading and only if
the pore pressures build up to equal the total stress, does the “initial liquefaction” (effec-
tive stress
0) condition occur.
The stress path followed by the soil affects the liquefaction potential.
Figure 12.10
shows the p
-q stress paths during a cyclic triaxial test, the effective stress path moving
from cycle 1 to the left before reaching the failure envelope on the k
f
line in cycle 21.
Once the failure line is reached, pore pressures and strain development accelerates.
After
0 is reached the stress paths are up and down the failure lines passing
through or near the origin twice in each cycle as shown. This is the cyclic liquefaction
condition.
