The term liquefaction has actually been used to describe
a number of related phenomena. Because the phenomena can have similar effects, it can
be difficult to distinguish between them. The mechanisms causing them, however, are
different. These phenomena can be divided into two main categories: flow liquefaction
and cyclic mobility.
Flow liquefaction is a phenomenon in which the static
equilibrium is destroyed by static or dynamic loads in a soil deposit with low residual strength.
Residual strength is the strength of a liquefied soil.
Static loading, for example, can be applied by new buildings on a slope that exert additional
forces on the soil beneath the foundations. Earthquakes, blasting, and pile driving are all example of
dynamic loads that could trigger flow liquefaction. Once triggered,
the strength of a soil susceptible to flow liquefaction is no longer sufficient to withstand the
static stresses that were acting on the soil before the disturbance.
An analogy can be seen in the picture above, where the static stability of a ski jumper in the
starting gate is disturbed when the jumper pushes himself from the start seat. After this
relatively small disturbance, the static driving force caused by gravity, being greater
than the frictional resisting force between the ski and snow, causes the skier to
accelerate down the ramp. The path that brings the ski jumper to an unstable state is
analogous to the static or dynamic disturbance that triggers flow liquefaction - in both
cases, a relatively small disturbance proceeds an instability that allows gravity to
take over and produce large, rapid movements.
Failures caused by flow
liquefaction are often characterized by large and rapid movements which can produce the type of
disastrous effects experienced by the Kawagishi-cho apartment buildings, which suffered a
remarkable bearing capacity failure during the
Niigata Earthquake 1964. The Turnagain Heights
Earthquake 1964 which is thought
to be triggered by liquefaction of sand lenses in the 130-acre slide area
provides another example of flow liquefaction.
Sheffield Dam suffered a flow failure triggered by the Santa
Barbara Earthquake in 1925. A 300 ft section (of the 720 feet long dam) moved as
much as 100 ft downstream. The dam consisted mainly of silty sands and
sandy silts excavated from the reservoir and compacted by routing
construction equipment over the fill
As these case histories illustrate, flow failures, can
involve the flow of considerable volumes of material, which undergoes very large
movements that are actually driven by static stresses. As described in the
section, the disturbance needed to trigger flow liquefaction can, in some instances, be
very small. Read more about the initiation of
Cyclic mobility is a liquefaction phenomenon,
triggered by cyclic loading, occuring in soil deposits with static shear stresses
lower than the soil strength. Deformations due to cyclic mobility develop
incrementally because of static and dynamic stresses that exist during an earthquake.
Lateral spreading, a common
result of cyclic mobility, can occur on gently sloping and on flat ground close to
rivers and lakes. The 1976 Guatemala earthquake caused lateral spreading along the
Motagua river. Observe the cracks parallel to the river in the picture to the right.
On level ground, the high porewater pressure caused by
liquefaction can cause porewater to flow rapidly to the ground surface.
This flow can occur both during and after an earthquake. If the flowing porewater rises
quickly enough, it can carry sand particles through cracks up to the surface, where
they are deposited in the form of sand volcanoes or sand boils. These features can often
be observed at sites that have been affected by liquefaction, such as in the field
along Hwy 98 during the 1979 El Centro earthquake shown above.