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Liquefaction
 Soil
liquefaction
is a condition in which the strength and stiffness of a soil is reduced by
earthquake shaking or other rapid loading. Liquefaction and related phenomena
have been responsible for tremendous amounts of damage in historical earthquakes
around the world.
Liquefaction occurs in saturated soils, that is, soils in which the space
between individual particles is completely filled with water. This water exerts
a pressure on the soil particles that influences how tightly the particles
themselves are pressed together. Prior to an earthquake, the water pressure is
relatively low. However, earthquake shaking can cause the water pressure to
increase to the point where the soil particles can readily move with respect to
each other.
Earthquake shaking often
triggers this increase in water pressure, but construction related activities
such as blasting can also cause an increase in water pressure.
When liquefaction occurs, the
strength of the soil decreases and, the ability of a soil deposit to support
foundations for buildings and bridges is reduced as seen in the photo of the
overturned apartment complex buildings in Niigata in 1964.
Liquefied soil also exerts higher pressure on
retaining walls, which can cause them to tilt or slide. This movement can cause
settlement of the retained soil and destruction of structures on the ground
surface. Increased water pressure can also trigger landslides and cause the
collapse of dams. Lower San Fernando dam suffered an underwater slide during the
San Fernando earthquake, 1971. Fortunately, the dam barely avoided collapse,
thereby preventing a potential disaster of flooding of the heavily populated
areas below the dam.
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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
 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. |
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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 landslide,
Alaska 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 (Seed,
1968).
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
state criteria
section, the disturbance needed to trigger flow liquefaction can, in some
instances, be very small. Read more about the
initiation of flow liquefaction.
Cyclic
Mobility
Cyclic mobility is a liquefaction phenomenon, triggered by cyclic loading,
occurring 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
pore water pressure caused by liquefaction can cause pore water to flow
rapidly to the ground surface.
This flow can occur both during and after an earthquake. If the flowing
pore water 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. |
source :
http://www.ce.washington.edu/~liquefaction/html/main.html
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