Environmental Engineering Reference
In-Depth Information
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Deformations during cyclic loading will stabilize, unless the soil is very loose and
flow liquefaction is triggered. The resulting movements are due to external causes
and occur only during the cyclic loading;
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Can occur in almost any saturated sand provided that the cyclic loading is suffi-
ciently large in magnitude and duration, but no shear stress reversal occurs;
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Cohesive soils can experience cyclic mobility, but rate effects (creep) usually
control deformations.
Note that strain softening soils also experience cyclic softening (cyclic liquefaction or
cyclic mobility) depending on the ground geometry.
In
Figure 12.14
the first step is to evaluate the material characteristics in terms of a
strain softening or strain hardening response. If the soil is strain softening, flow liquefac-
tion is possible if the soil can be triggered to collapse and if the gravitational shear stresses
are larger than the ultimate or minimum strength. The trigger mechanism can be either
monotonic or cyclic. Whether a slope or soil structure will fail and slide will depend on
the amount of strain softening soil relative to strain hardening soil within the structure,
the brittleness of the strain softening soil and the geometry of the ground. The resulting
deformations of a soil structure with both strain softening and strain hardening soils will
depend on many factors, such as distribution of soils, ground geometry, amount and type
of trigger mechanism, brittleness of the strain softening soil and drainage conditions.
If the soil is strain hardening, flow liquefaction will generally not occur. However, cyclic
softening can occur due to cyclic undrained loading, such as earthquake loading. The
amount and extent of deformations during cyclic loading will depend on the density of the
soil, the magnitude and duration of the cyclic loading and the extent to which shear stress
reversal occurs. If extensive shear stress reversal occurs, it is possible for the effective
stresses to reach zero and hence, cyclic liquefaction can take place. When the condition of
essentially zero effective stress is achieved, large deformations can result. If cyclic loading
continues, deformations can progressively increase. If shear stress reversal does not take
place, it is generally not possible to reach the condition of zero effective stress and defor-
mations will be smaller, i.e. cyclic mobility will occur.
Both flow liquefaction and cyclic liquefaction can cause very large deformations. Hence
it can be very difficult to clearly identify the correct phenomenon based on observed
deformations following earthquake loading. Earthquake-induced flow liquefaction
movements tend to occur after the cyclic loading ceases, due to the progressive nature of
the load redistribution. However, if the soil is sufficiently loose and the static shear
stresses are sufficiently large, the earthquake loading may trigger essentially “flow lique-
faction” within the first few cycles of loading. Also, if the soil is sufficiently loose, the
ultimate undrained strength may be close to zero with an associated effective confining
stress very close to zero (Ishihara, 1993). Cyclic liquefaction movements, on the other
hand, tend to occur during the cyclic loading since it is the inertial forces that drive the
phenomenon.
12.4.2
Soils susceptible to liquefaction
It has long been recognized that saturated sands, silty sands, and gravelly sands are sus-
ceptible to liquefaction.
Figure 12.15
shows the boundaries suggested in 1985 by
USNRC.
For mine tailings, USNRC recognized that soils with high silt and even clay size parti-
cles would be liquefiable as shown in
Figure 12.16
.
Hunter and Fell (2003a and 2003b) gathered data from case studies where flow lique-
faction had occurred, mainly under static loading conditions. These would also apply to
earthquake loading.
Figure 12.17
shows the data separated into classes of slope, with a
