Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially
loses strength and stiffness in response to an applied stress, usually earthquake shaking
or other sudden change in stress condition, causing it to behave like a liquid.
In soil mechanics the term "liquefied" was first used by Hazen in reference to the 1918
failure of the Calaveras Dam in California. He described the mechanism of flow liquefaction
of the embankment dam as follows:
If the pressure of the water in the pores is great enough to carry all the load, it
will have the effect of holding the particles apart and of producing a condition that is
practically equivalent to that of quicksand… the initial movement of some part of the material
might result in accumulating pressure, first on one point, and then on another, successively,
as the early points of concentration were liquefied.
The phenomenon is most often observed in saturated, loose, sandy soils. This is because a loose
sand has a tendency to compress when a load is applied; dense sands by contrast tend to
expand in volume or 'dilate'. If the soil is saturated by water, a condition that often
exists when the soil is below the ground water table or sea level, then water fills the gaps
between soil grains. In response to the soil compressing, this water increases in pressure
and attempts to flow out from the soil to zones of low pressure. However, if the loading
is rapidly applied and large enough, or is repeated many times such that it does not
flow out in time before the next cycle of load is applied, the water pressures may build
to an extent where they exceed the contact stresses between the grains of soil that keep
them in contact with each other. These contacts between grains are the means by which the
weight from buildings and overlying soil layers are transferred from the ground surface to
layers of soil or rock at greater depths. This loss of soil structure causes it to lose
all of its strength and it may be observed to flow like a liquid.
Although the effects of liquefaction have been long understood, it was more thoroughly
brought to the attention of engineers after the 1964 Niigata earthquake and 1964 Alaska
earthquake. It was also a major factor in the destruction in San Francisco's Marina
District during the 1989 Loma Prieta earthquake, and in Port of Kobe during the 1995 Great
Hanshin earthquake. More recently liquefaction was largely responsible for extensive damage
to residential properties in the eastern suburbs and satellite townships of Christchurch, New
Zealand during the 2010 Canterbury earthquake and more extensively again following the Christchurch
earthquakes that followed in early and mid-2011. The building codes in many developed countries
require engineers to consider the effects of soil liquefaction in the design of new
buildings and infrastructure such as bridges, embankment dams and retaining structures.
Technical definitions A state of 'soil liquefaction' occurs when
the effective stress of soil is reduced to essentially zero, which corresponds to a complete
loss of shear strength. This may be initiated by either monotonic loading or cyclic loading.
In both cases a soil in a saturated loose state, and one which may generate significant
pore water pressure on a change in load are the most likely to liquefy. This is because
a loose soil has the tendency to compress when sheared, generating large excess porewater
pressure as load is transferred from the soil skeleton to adjacent pore water during undrained
loading. As pore water pressure rises a progressive loss of strength of the soil occurs as effective
stress is reduced. It is more likely to occur in sandy or non-plastic silty soils, but may
in rare cases occur in gravels and clays A 'flow failure' may initiate if the strength
of the soil is reduced below the stresses required to maintain equilibrium of a slope
or footing of a building for instance. This can occur due to monotonic loading or cyclic
loading, and can be sudden and catastrophic. A historical example is the Aberfan disaster.
Casagrande referred to this type of phenomena as 'flow liquefaction' although a state of
zero effective stress is not required for this to occur.
The term 'cyclic liquefaction' refers to the occurrence of a state of soil when large shear
strains have accumulated in response to cyclic loading. A typical reference strain for the
approximate occurrence of zero effective stress is 5% double amplitude shear strain. This
is a soil test based definition, usually performed via cyclic triaxial, cyclic direct simple
shear, or cyclic torsional shear type apparatus. These tests are performed to determine a soil's
resistance to liquefaction by observing the number of cycles of loading at a particular
shear stress amplitude before it 'fails'. Failure here is defined by the aforementioned
shear strain criteria. The term 'cyclic mobility' refers to the mechanism
of progressive reduction of effective stress due to cyclic loading. This may occur in all
soil types including dense soils. However on reaching a state of zero effective stress
such soils immediate dilate and regain strength. Thus shear strains are significantly less
than a true state of soil liquefaction whereby a loose soil exhibits flow type phenomena.
Occurrence Liquefaction is more likely to occur in loose
to moderately saturated granular soils with poor drainage, such as silty sands or sands
and gravels capped or containing seams of impermeable sediments. During wave loading,
usually cyclic undrained loading, e.g. seismic loading, loose sands tend to decrease in volume,
which produces an increase in their pore water pressures and consequently a decrease in shear
strength, i.e. reduction in effective stress. Deposits most susceptible to liquefaction
are young sands and silts of similar grain size, in beds at least metres thick, and saturated
with water. Such deposits are often found along stream beds, beaches, dunes, and areas
where windblown silt and sand have accumulated. Some examples of soil liquefaction include
quicksand, quick clay, turbidity currents, and earthquake induced liquefaction.
Depending on the initial void ratio, the soil material can respond to loading either strain-softening
or strain-hardening. Strain-softened soils, e.g. loose sands, can be triggered to collapse,
either monotonically or cyclically, if the static shear stress is greater than the ultimate
or steady-state shear strength of the soil. In this case flow liquefaction occurs, where
the soil deforms at a low constant residual shear stress. If the soil strain-hardens,
e.g. moderately dense to dense sand, flow liquefaction will generally not occur. However,
cyclic softening can occur due to cyclic undrained loading, e.g. earthquake loading. Deformation
during cyclic loading will depend on the density of the soil, the magnitude and duration of
the cyclic loading, and amount of shear stress reversal. If stress reversal occurs, the effective
shear stress could reach zero, then cyclic liquefaction can take place. If stress reversal
does not occur, zero effective stress is not possible to occur, then cyclic mobility takes
place. The resistance of the cohesionless soil to
liquefaction will depend on the density of the soil, confining stresses, soil structure,
the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal
occurs. Earthquake liquefaction
The pressures generated during large earthquakes with many cycles of shaking can cause the
liquefied sand and excess water to force its way to the ground surface from several metres
below the ground. This is often observed as "sand boils" also called "sand blows" or "sand
volcanoes" at the ground surface. The phenomenon may incorporate both flow of already liquefied
sand from a layer below ground, and a quicksand effect whereby upward flow of water initiates
liquefaction in overlying non-liquefied sandy deposits due to buoyancy.
The other common observation is land instability – cracking and movement of the ground down
slope or towards unsupported margins of rivers, streams, or the coast. The failure of ground
in this manner is called 'lateral spreading', and may occur on very shallow slopes of angles
of only 1 or 2 degrees from the horizontal. One positive aspect of soil liquefaction is
the tendency for the effects of earthquake shaking to be significantly damped for the
remainder of the earthquake. This is because liquids do not support a shear stress and
so once the soil liquefies due to shaking, subsequent earthquake shaking is not transferred
to buildings at the ground surface. Studies of liquefaction features left by prehistoric
earthquakes, called paleoliquefaction or paleoseismology, can reveal a great deal of information about
earthquakes that occurred before records were kept or accurate measurements could be taken.
Soil liquefaction induced by earthquake shaking is also a major contributor to urban seismic
risk. Effects
The effects of soil liquefaction on the built environment can be extremely damaging. Buildings
whose foundations bear directly on sand which liquefies will experience a sudden loss of
support, which will result in drastic and irregular settlement of the building causing
structural damage, including cracking of foundations and damage to the building structure itself,
or may leave the structure unserviceable afterwards, even without structural damage. Where a thin
crust of non-liquefied soil exists between building foundation and liquefied soil, a
'punching shear' type foundation failure may occur. The irregular settlement of ground
may also break underground utility lines. The upward pressure applied by the movement
of liquefied soil through the crust layer can crack weak foundation slabs and enter
buildings through service ducts, and may allow water to damage the building contents and
electrical services. Bridges and large buildings constructed on
pile foundations may lose support from the adjacent soil and buckle, or come to rest
at a tilt after shaking. Sloping ground and ground next to rivers and
lakes may slide on a liquefied soil layer, opening large cracks or fissures in the ground,
and can cause significant damage to buildings, bridges, roads and services such as water,
natural gas, sewerage, power and telecommunications installed in the affected ground. Buried tanks
and manholes may float in the liquefied soil due to buoyancy. Earth embankments such as
flood levees and earth dams may lose stability or collapse if the material comprising the
embankment or its foundation liquefies. Mitigation methods
Methods to mitigate the effects of soil liquefaction have been devised by earthquake engineers
and include various soil compaction techniques such as vibro compaction, dynamic compaction,
and vibro stone columns. These methods result in the densification of soil and enable buildings
to withstand soil liquefaction. Existing buildings can be mitigated by injecting
grout into the soil to stabilize the layer of soil that is subject to liquefaction.
Quicksand
Quicksand forms when water saturates an area of loose sand and the ordinary sand is agitated.
When the water trapped in the batch of sand cannot escape, it creates liquefied soil that
can no longer support weight. Quicksand can be formed by standing or flowing underground
water, or by earthquakes. In the case of flowing underground water, the force of the water
flow opposes the force of gravity, causing the granules of sand to be more buoyant. In
the case of earthquakes, the shaking force can increase the pressure of shallow groundwater,
liquefying sand and silt deposits. In both cases, the liquefied surface loses strength,
causing buildings or other objects on that surface to sink or fall over.
The saturated sediment may appear quite solid until a change in pressure or shock initiates
the liquifaction causing the sand to form a suspension with each grain surrounded by
a thin film of water. This cushioning gives quicksand, and other liquefied sediments,
a spongy, fluidlike texture. Objects in the liquefied sand sink to the level at which
the weight of the object is equal to the weight of the displaced sand/water mix and the object
floats due to its buoyancy. Quick clay
Quick clay, also known as Leda Clay in Canada, is a water-saturated gel, which in its solid
form resemble a unique form of highly sensitive clay. This clay has a tendency to change from
a relatively stiff condition to a liquid mass when it is disturbed. This gradual change
in appearance from solid to liquid is a process known as spontaneous liquefaction. The clay
retains a solid structure despite the high water content, because surface tension holds
water-coated flakes of clay together in a delicate structure. When the structure is
broken by a shock or sufficient shear, it turns to a fluid state.
Quick clay is only found in the northern countries such as Russia, Canada, Alaska in the U.S.,
Norway, Sweden, and Finland, which were glaciated during the Pleistocene epoch.
Quick clay has been the underlying cause of many deadly landslides. In Canada alone, it
has been associated with more than 250 mapped landslides. Some of these are ancient, and
may have been triggered by earthquakes. Turbidity currents
Submarine landslides are turbidity currents and consist of water saturated sediments flowing
downslope. An example occurred during the 1929 Grand Banks earthquake that struck the
continental slope off the coast of Newfoundland. Minutes later, transatlantic telephone cables
began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve
cables were snapped in a total of 28 places. Exact times and locations were recorded for
each break. Investigators suggested that a 60-mile-per-hour submarine landslide or turbidity
current of water saturated sediments swept 400 miles down the continental slope from
the earthquake’s epicenter, snapping the cables as it passed.
Research The George E. Brown, Jr. Network for Earthquake
Engineering Simulation hosts two geotechnical centrifuges for studying soil behavior. The
NEES centrifuge at University of California Davis has radius of 9.1 m, maximum payload
mass of 4500 kg, and available bucket area of 4.0 m2. The centrifuge is capable of producing
75g's of centrifugal acceleration at its effective radius of 8.5 m. The centrifuge capacity in
terms of the maximum acceleration multiplied by the maximum payload is 53 g x 4500 kg
= 240 g-tonnes. The NEES centrifuge at the Center for Earthquake Engineering Simulation
at Rensselaer Polytechnic Institute has a nominal radius, 2.7 m, which is the distance
between the center of payload and the centrifuge axis. The space available for the payload
is a depth of 1,000 mm, width of 1,000 mm, height of 800 mm, and a maximum height of
1,200 mm. The performance envelope is 160 g, 1.5 tons, and 150 g-tons.
See also Paleoseismology
Dry quicksand Atterberg limits
Mud volcano Sand volcano or sand blow
Thixotropy Earthquake engineering
Events attributed to liquefaction Aberfan disaster
References
Further reading Seed et al., Recent Advances in Soil Liquefaction
Engineering: A Unified and Consistent Framework, 26th Annual ASCE Los Angeles Geotechnical
Spring Seminar, Long Beach, California, April 30, 2003, Earthquake Engineering Research
Center PDF External links
Media related to Soil liquefaction at Wikimedia Commons
Soil Liquefaction Shaking, Liquefaction on Harbor Island, one
of the few known live observations of an earthquake liquefaction event by a seismologist
Video showing liquefaction process