Lecture32 Liquefaction Hazards

8/12/2019 Lecture32 Liquefaction Hazards

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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,

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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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Sand Boil near Loma Prieta, California, Earthquake of October 17, 1989

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Source: wikipedia


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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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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

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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/

Lecture32 Liquefaction Hazards

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