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Liquefaction Hazards
Lecture-32
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Alteration of ground motion
The development of positive
excess pore pressures causessoil stiffness to decrease during
an earthquake.
A deposit of liquefiable soil that
is relatively stiff at thebeginning of the earthquake
may be much softer by the end
of the motion. As a result, the
amplitude and frequencycontent of the surface motion
may change considerably
throughout the earthquake.
At 7 sec after the ground motionstarted, liquefaction occurred, causing
reduction of the stiffness of the
underlying soil. Acceleration
amplitude and frequency content both
changed dramatically from that point.
Niigata Earthquake 1964) [loose sand]
Liquefaction
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Alteration of ground motion
The decrease in surface acceleration amplitudes when pore
pressures become large does not mean that damage potential is
necessarily reduced because low acceleration amplitudes at lowfrequencies can still produce large displacements. These
displacements may be of particular concern for buried structures,
utilities, and structures supported on pile foundations that extend
through liquefied soils
Cyclic mobility
Kushiro Earthquake 1993) [dense sand]
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Lateral Spreading
Lateral spreading occurs when earthquake-induced shear stresses temporarily
exceed the yield strength of a liquefiable soil that is not susceptible to flowliquefaction.
Lateral spreading is characterized by lateral deformations that occur during
earthquake shaking (and end when earthquake shaking has ended).
The displacements may be small or large, depending on the slope of the ground,
the density of the soil, and the characteristics of the ground motion.
Lateral spreading can occur in gently sloping areas or in flat areas adjacent to free
surfaces.
Because the residual strength exceeds the static shear stress, large flow
deformations that could continue after the end of earthquake shaking do not
develop.
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Lateral Spreading
Lateral spreading can have a severe impact on structures.
Because it occurs so frequently in waterfront areas, it has historically had a
profound effect on structures such as bridges and wharves and consequently a
strong economic impact on transportation systems and ports.
The lateral spreading phenomenon is a complex one, and it has proven to beextremely difficult to make accurate a priori predictions of permanent
deformations using analytical/numerical procedures alone.
As a result, currently available procedures to estimate the lateral deformations
are empirical.
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Lateral Deformation and Spreading
Fissures caused by lateral spreading at North Wharf, Haiti Earthquake, 2010
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Lateral Deformation and Spreading
Cracked Highway due to liquefaction induced lateral spreading, Puerto Limon,
1991 8
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Sloping ground model
Log Du= -16.713 + 1.532 M 1.406 log R* - 0.012 R + 0.592 log W
+ 0.540 log T15
+ 3.413 log (100F15
)0.795 log (D5015
+ 0.1 mm)
Free face model
Log Du= -16.213 + 1.532 M 1.406 log R* - 0.012 R + 0.338 log S
+ 0.540 log T15+ 3.413 log (100F15)0.795 log (D5015+ 0.1 mm)
Where Du= estimated lateral ground displacement, m
M = moment magnitude of earthquake
R = nearest horizontal or map distance from the site to the seismic energy source, km
R0= distance factor that is a function of magnitude, M; R0= 10(0.89M-5.64)
R* = modified source distance, R* = R + R0
T15= cumulative thickness of saturated granular layers with corrected blow counts (N1)60< 15, m
F15= average fines content (fraction passing no. 200 sieves), %, for granular materials within T15
D5015= average mean grain size for granular materials within T15
S = ground slope, %
W = free face ratio defined as the height (H) of the free face divided by the distance (L) from the
base of the free face to the point in question
Estimation of lateral deformation: Youdsapproach
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Estimation of lateral deformation: Youdsapproach
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Large ground oscillations
The occurrence of liquefaction at depth beneath a flat ground surface can
decouple the liquefied soils from the surficial soils and produce large,transient ground oscillations. The surficial soils are often broken into blocks
separated by fissures that can open and close during the earthquake. Ground
waves with amplitudes of up to several feet have been observed during
ground oscillation, but permanent displacements are usually small.
High bending moments in piles Large ground oscillations
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Development of sand boils
Liquefaction is often accompanied by the development of sand
boils.
Seismically induced excess pore pressures are dissipated
predominantly by the upward flow of pore water. If the hydraulic
gradient driving the flow reaches a critical value, the vertical
effective stress will drop to zero and the soil will be in a quickcondition. In such cases, the water velocities may be sufficient to
carry soil particles to the surface.
In the field, soil conditions are rarely uniform so the escaping
pore water tends to flow at high velocity through localized cracks
or channels. Sand particles can be carried through these channels
and ejected at the ground surface to form sand boils.
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Development of sand boils
Sand Boil near Loma Prieta, California, Earthquake of October 17, 1989
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Source: wikipedia
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Development of sand boils
Ishihara (1985) examined the soil conditions associated with various
liquefaction related damage reports from various earthquakes and produced
estimates of the thickness of the overlying layer required to prevent level
ground liquefaction related damage.
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Source: Kramer (1996)
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Development of sand boils
Shaking table and centrifuge tests have shown that porewater
draining from the voids of the loose layers can accumulate
beneath the less pervious layers and form water interlayers. Sand
boils can develop when the water interlayers break through to the
ground surface.
Shaking table tests of Liu and Qiao, 1984
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Non-seismic Sand Boils
It is observed that sand boils have occurred without the presence of an earthquake
under certain conditions. These conditions consist of high hydraulic gradients induced byactive flooding and sub-horizontal flow intercepted by pre-existing ground cracks.
The first condition is caused by seepage resulting from water head differences along
artificial levees that can carry sand to the ground surface, forming conical piles that have
very similar appearances to sand boils induced by strong ground shaking.
The second condition of non-seismic sand boil formation is found in areas where
extensive modern ground failures (earth fissures) are present and caused mostly from
ground water withdrawal. The up-slope portions of the fissures intercept large volumes
of silt-laden surface water runoff to form a large gulley and subsurface tunnels running
parallel in the fissure. The material is then discharged to the ground surface along the
down-slope part of the fissure. It is likely that the runoff event picks up significantadditional sediment load from the erosion of the earth fissure, adding to the capability
to form sand boils.
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Liquefaction induced Settlements
The tendency for densification due to applied shear stresses, produces liquefaction
in saturated soils. The generation of excess porewater pressure, however, is atransient event.
Following strong earthquake shaking, the presence of excess porewater pressure
implies the presence of hydraulic gradients that will cause the porewater to flow
until hydrostatic porewater pressure conditions are once again reached.
This dissipation of excess porewater pressure occurs through the process of
consolidation and is accompanied by a reduction in the volume of the soil, which is
typically manifested in the form of settlement of the ground surface.
Ground surface settlement following liquefaction has been observed in numerousearthquakes. Large areas of settlement can produce regional subsidence, which can
lead to submergence of low-lying coastal areas
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Liquefaction induced Settlements
Initially, the element is in drained equilibrium (zero excess pore pressure) at pointA.
Earthquake shaking causes excess pore pressure to build up under undrained conditions,thereby reducing the effective stress to that shown at point B. The excess pore pressure
produces a hydraulic gradient that drives the porewater out of the voids. The flow of water
reduces the hydraulic gradient until the excess pore pressure has completely dissipated
(point C). As the water flows from the voids, the volume of the element decreases. As
Figure clearly illustrates, the magnitude of the volume change increases with the
magnitude of the seismically induced excess pore pressure. 19
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Settlements in dry and saturated sands
Settlement from earthquakes occurs in dry and loose sands.
The settlement of dry sands due to earthquake loading is a function
of the density of the sand, the amplitude of the cyclic shear strains
induced in the sand, and the number of shear strain cycles applied
during loading.
The post shaking densification of saturated sands is influenced by
the density of the sand, the maximum cyclic shear strain induced in
the sand, and amount of excess pore pressure generated during
shaking.
Procedures are well established to estimate post-earthquake
settlements for both cases.
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Flow Slides
Flow slides can be triggered during or after strong ground shaking.
If the ground motion produces high porewater pressure in an area of a slope that is
critical to the maintenance of stability, flow liquefaction may be triggered during the
earthquake.
In some cases, however, the highest porewater pressures are generated in zones that
are not critical for stability
for example, under the central portion of an earth dam.
Following earthquake shaking, redistribution of excess porewater pressure will cause
porewater pressure to decrease in some areas but to temporarily increase in others.
If excess porewater pressures migrate into areas that are critical for stability, a flowslide may be triggered at some period of time after earthquake shaking has ended.
The occurrence of delayed flow slides depends on hydraulic as well as dynamic soil
properties, and is likely to be strongly influenced by the presence and distribution of
layers and seams of fine-grained soils.21
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Foundation Failures
Liquefaction can cause the failure of foundation systems by a variety of
mechanisms.
Both shallow and deep foundations can be damaged by soil liquefaction.
Shallow foundation failure mechanisms is through the loss of bearing capacity
associated with loose, saturated soils with low residual strength.
By this mechanism, the earthquake shaking can trigger flow liquefaction and
dramatic bearing failures
Local failure of shallow foundations can occur through the mechanism of cyclic
mobility. The static stresses imposed in the soil beneath a shallow foundation cancause the accumulation of permanent strain in a particular direction, leading to
excessive settlement of the shallow foundation.
Liquefaction can also have a significant impact on pile foundations as observed in
many earthquakes. 22
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Foundation Failures
Figure: Overturning failure of the structure due to ground liquefaction
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Foundation Failures
Figure: Pile damage due to lateral spreading in Kobe, Japan
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Paleoliquefaction Studies
During the past two decades, prehistoric evidence has been used to
identify the sites and conditions under which liquefaction has occurred.
The study of these prehistoric features, termed Paleoliquefaction,
examines exposed soil stratigraphy in the field to identify liquefaction
features that have been subsequently buried by sedimentation.
Mapping paleoliquefaction features, coupled with back analysis, is
becoming an increasingly utilized technique for determining the strength
of prehistoric ground motions.
The method takes the interpreted soil conditions at the time thepaleoliquefaction was produced, and back calculates the maximum peak
acceleration and magnitude that would be required to produce
liquefaction.
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Kramer, S.L. (1996) Geotechnical Earthquake Engineering, Prentice Hall.
Day, R.W. (2001) Geotechnical Earthquake Engineering Handbook, McGraw-Hill.
Idriss, I.M. and Boulanger, R. (2006) Soil liquefaction during earthquakes, EERI.
Animation of Seattle harbor liquefaction failures:
http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-
liquefaction-failures/(Accessed on 12 April 2012)
Christchurch Earthquake liquefaction:
http://www.youtube.com/watch?v=Or2Ic2Z6zn8(Accessed on 12 April 2012)
References
http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://www.youtube.com/watch?v=Or2Ic2Z6zn8http://www.youtube.com/watch?v=Or2Ic2Z6zn8http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/http://blogs.agu.org/landslideblog/2009/10/27/animation-of-seattle-harbour-liquefaction-failures/