ISSN: 2277-5536 (Print); 2277-5641 (Online)
Liquefaction and studying comparison Method of soil liquefaction
1-Ali Alizadeh
Department of civil engineering, Jolfa Brnch, Islamic Azad University, Jolfa. Iran
2-M.yousefzade fard (phD Geotechnics)Jolfa
Corresponding author's: Ali Alizadeh
Page | 35 Abstract:
Liquefaction is a process by which sediments below the water table temporarily lose
strength and behave as a viscous liquid rather than a solid. The types of sediments most
susceptible are clay-free deposits of sand and silts; occasionally, gravel liquefies. The
actions in the soil which produce liquefaction are as follows: seismic waves, primarily
shear waves, passing through saturated granular layers, distort the granular structure, and
cause loosely packed groups of particles to collapse. These collapses increase the porewater pressure between the grains if drainage cannot occur. If the pore-water pressure
rises to a level approaching the weight of the overlying soil, the granular layer
temporarily behaves as a viscous liquid rather than a solid. Liquefaction has occurred.
In the liquefied condition, soil may deform with little shear resistance; deformations large
enough to cause damage to buildings and other structures are called ground failures. The
ease with which a soil can be liquefied depends primarily on the looseness of the soil, the
amount of cementing or clay between particles, and the amount of drainage restriction.
The amount of soil deformation following liquefaction depends on the looseness of the
material, the depth, thickness, and areal extent of the liquefied layer, the ground slope,
and the distribution of loads applied by buildings and other structures.
Liquefaction does not occur at random, but is restricted to certain geologic and
hydrologic environments, primarily recently deposited sands and silts in areas with high
ground water levels. Generally, the younger and looser the sediment, and the higher the
water table, the more susceptible the soil is to liquefaction. Sediments most susceptible to
liquefaction include Holocene (less than 10,000-year-old) delta, river channel, flood
plain, and aeolian deposits, and poorly compacted fills. Liquefaction has been most
abundant in areas where ground water lies within 10 m of the ground surface; few
instances of liquefaction have occurred in areas with ground water deeper than 20 m.
Dense soils, including well-compacted fills, have low susceptibility to liquefaction.
The liquefaction phenomenon by itself may not be particularly damaging or hazardous.
Only when liquefaction is accompanied by some form of ground displacement or ground
failure is it destructive to the built environment. For engineering purposes, it is not the
occurrence of liquefaction that is of prime importance, but its severity or its capability to
cause damage. Adverse effects of liquefaction can take many forms. These include: flow
failures; lateral spreads; ground oscillation; loss of bearing strength; settlement; and
increased lateral pressure on retaining walls.(1)
Keywords: Liquefaction Potential – Hydrologic environments
Introduction:
One of the biggest devastating phenomenon and main factor damage to structures and
technical buildings during earthquake in areas that have been built on sandy soils is
reduction or loss of shear strength at the occurrence of soil liquefaction.(1) Thus, the
evaluation of liquefaction potential is important. There are two main methods for
evaluation of liquefaction potential. The goal of first method is that to model complex
interactions in a non-linear site response analysis and by using an appropriate structural
model explicitly, and it will be remembered as numerical methods. According to the
complexity and the cost and required time, this method can be used more in research
studies. The second method is based on the empirical solidarity between seismic loading
(the stimulus) and soil resistance against liquefaction (capacity factor), and it is
remembered as experimental methods. These methods are compared to numerical models
have broader usage due to the simpler use and less expensive. In this method, stimulating.
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Flow failures are the most catastrophic ground failures caused by liquefaction. These
failures commonly displace large masses of soil laterally tens of meters and in a few
instances, large masses of soil have traveled tens of kilometers down long slopes at
velocities ranging up to tens of kilo-meters per hour. Flows may be comprised of
completely liquefied soil or blocks of intact material riding on a layer of liquefied soil.
Flows develop in loose saturated sands or silts on relatively steep slopes, usually greater
Page | 36 than 3 degrees
Lateral spreads involve lateral displacement of large, surficial blocks of soil as a result of
liquefaction of a subsurface layer. Displacement occurs in response to the combination of
gravitational forces and inertial forces generated by an earthquake. Lateral spreads
generally develop on gentle slopes (most commonly less than 3 degrees) and move
toward a free face such as an incised river channel. Horizontal displacements commonly
range up to several meters.(2) The displaced ground usually breaks up internally, causing
fissures, scarps, horsts, to form on the failure surface. Lateral spreads commonly disrupt
foundations of buildings built on or across the failure, sever pipelines and other utilities in
the failure mass, and compress or buckle engineering structures, such as bridges, founded
on the toe of the failure.
View of studying about Ground oscillation:
Where the ground is flat or the slope is too gentle to allow lateral displacement, liquefaction at depth may decouple overlying soil layers from the underlying ground, allowing the
upper soil to oscillate back and forth and up and down in the form of ground waves.
These oscillations are usually accompanied by opening and closing of fissures and
fracture of rigid structures such as pavements and pipelines. The manifestations of ground
oscillation were apparent in San Francisco's Marina District due to the 1989 Loma Prieta
earthquake; sidewalks and driveways buckled and extensive pipeline breakage also
occurred.(3)
When the soil supporting a building or other structure liquefies and loses strength, large
de-formations can occur within the soil.
Which may allow the structure to settle and tip. Conversely, buried tanks and piles may
rise buoyantly through the liquefied soil. For example, many buildings settled and tipped
during the 1964 Niigata, Japan, earthquake. The most spectacular bearing failures during
that event were in the Kawangishicho apartment complex where several four-story buildings tipped as much as 60 degrees. Apparently, liquefaction first developed in a sand
layer several meters below ground surface and then propagated upward through overlying
sand layers. The rising wave of liquefaction weakened the soil supporting the buildings
and allowed the structures to slowly settle and tip.
Engineering definitions of liquefaction:
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 (e.g. single sudden occurrence of a change in stress
- examples include an increase in load on an embankment or sudden loss of toe support)
or cyclic loading (e.g. repeated change in stress condition - examples include wave
loading or earthquake shaking). 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 pore water 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 (see quick clay). 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
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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
Page | 37 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.(2)
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 (Holocene-age, deposited within the
last 10,000 years) sands and silts of similar grain size (well-sorted), 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 (loess) 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 strainsoftening 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.(3)
The resistance of the cohensionless soil to liquefaction will depend on the density of the
soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and
duration of the cyclic loading, and the extent to which shear stress reversal occurs.
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
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foundation slabs and enter buildings through service ducts, and may allow water to
damage the building contents and electrical services.(3)
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 (termed 'lateral spreading'),
opening large cracks or fissures in the ground, and can cause significant damage to
Page | 38 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.(3)
Methods to mitigate the effects of soil liquefaction have been devised by earthquake
engineers and include various soil compaction techniques such as vibro compaction
(compaction of the soil by depth vibrators), 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.(4)
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
(upwards) flowing underground water (as from an underground spring), 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 liquefaction, causing the sand to form a suspension with each grain
surrounded by a thin film of water.(4) This cushioning gives quicksand, and other
liquefied sediments, a spongy, fluid like 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.
Discussion and conclusion:
Emergency response plans at the local jurisdictional level need to identify those areas
most vulnerable to liquefaction. Sites where liquefaction can cause major problems can
be identified using the liquefaction hazard or risk maps discussed above. Emergency
plans might want to specify a reconnaissance survey of these areas immediately after an
earthquake occurs(5). Problems resulting from liquefaction such as damage to
underground pipelines also need to be factored into any emergency response planning.
Emergency responders should expect interrupted water supply, and natural gas and
sewage leaks. Back-up sources of water need to be identified or developed. In addition,
the roadbeds in areas potentially subject to liquefaction may be seriously damaged,
greatly complicating the ability to evacuate residents or to bring in emergency response
equipment. These systems should have redundancy or alternatives.(5)
References
1.California
Division
of
Mines
and
Geology.
1992.
Draft
Guidelines: Liquefaction Hazard Zones. Sacramento, California: CDMG.
2. Leighton and Associates. 1993. Liquefaction o f Soils and Engineering Mitigation
Alternatives. Diamond Bar, California: Leighton ind Associates.
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3. Youd, T. Leslie. 1992. "Liquefaction, Ground Failure, and Con-sequent Damage
During the 22 April 1991 Costa Rica Earthquake," in Proceedings o f the NSF/UCR U.S.Costa Rica Workshop on the Costa Rica Earthquakes o f 1990-1991: Effects on Soils and
Structures. Oakland, Cali-fornia: Earthquake Engineering Research Institute.
4. Youd, T. L., and S. N. Hoose. 1978. Historic Ground Failures in Northern California
Triggered by Earthquakes. U.S. Geological Survey Professional Paper 993.
Page | 39 5. Seed, H.B., Idriss, I.M., and Arango, I. (1983)."Evaluation of Liquefaction Potential
(^Using Field Performance Data", J. of Geotech. Eng.,ASCE, 109(3), 458-482.
6. Tokimatsu, K. and Yoshimi, Y., 1983. Empirical correlation of soil liquefaction based
on SPT-N value and fines content ,SoiI Mech. Found., 23(4): 56-74.
7.Youd TL, Idriss IM, editors. Proc. NCEER Workshop on Evaluation of Liquefaction I
Resistance of Soils. State University of New York at Bu.alo. National Center for V
Earthquake Engineering. Research, 1997.
8. Hazen, A. (1920). Transactions of the American Society of Civil Engineers 83: 17171745. Missing or empty |title= (help)
9. "Geologists arrive to study liquefaction". One News. 10 September 2010. Retrieved 12
November 2011.
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