what is soil liquefaction

7/27/2019 What is Soil Liquefaction

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What is soil liquefaction?

Description:

Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by

earthquake shaking or other rapid loading. Liquefaction and related phenomena havebeen responsible for tremendous amounts of damage in historical earthquakes aroundthe world.

Liquefaction occurs in saturated soils, that is, soils in which the space betweenindividual particles is completely filled with water. This water exerts a pressure on thesoil particles that influences how tightly the particles themselves are pressed together.Prior to an earthquake, the water pressure is relatively low. However, earthquakeshaking can cause the water pressure to increase to the point where the soil particles canreadily move with respect to each other.

Schematic behavior of sand grains in a soil deposit during liquefaction. The blue column represents thepore water pressure.
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Earthquake shaking often triggers this increase in water pressure, but constructionrelated activities such as blasting can also cause an increase in water pressure.

When liquefaction occurs, the strength of the soildecreases and, the ability of a soil deposit to support

foundations for buildings and bridges is reduced asseen in the photo of the overturned apartmentcomplex buildings in Niigata in 1964.

Liquefied soil also exerts higher pressure on retaining walls, which can cause them totilt or slide. This movement can cause settlement of the retained soil and destruction of

structures on the ground surface (photo 1.(

Increased water pressure can also trigger landslides and cause the collapse of dams.

Lower San Fernando dam (photo 2) suffered an underwater slide during the SanFernando earthquake, 1971.

Fortunately, the dam barely avoided collapse, thereby preventing a potential disaster offlooding of the heavily populated areas below the dam.

Ph

oto2

Photo1

Fl

ow Liquefaction & Cyclic Mobility:

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 todistinguish between them. The mechanisms causing them, however, are different. Thesephenomena can be divided into two main categories: flow liquefaction and cyclicmobility.


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Cyclic Mobility:

Cyclic mobility is a liquefaction phenomenon, triggered by cyclic loading, occuring

in soil deposits with static shear stresses lower than the soil strength. Deformations dueto cyclic mobility develop incrementally because of static and dynamic stresses thatexist during an earthquake. Lateral spreading, a common result of cyclic mobility, canoccur on gently sloping and on flat ground close to rivers and lakes. The 1976Guatemala earthquake caused lateral spreading along the Motagua river. Observe thecracks parallel to the river (photo 3).

On level ground, the high porewater pressure caused by liquefaction can causeporewater to flow rapidly to the ground surface. This flow can occur both during andafter an earthquake. If the flowing porewater rises quickly enough, it can carry sandparticles 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 beenaffected by liquefaction, such as in the field along Hwy 98 during the 1979 El Centro

earthquake (photo4.(

Photo 4 Photo3

When has soil liquefaction occurred in thepast?

Earthquakes:

Liquefaction has been observed in earthquakes for many years. In fact, writtenrecords dating back hundreds and even thousands of years describe earthquake effects

that are now known to be associated with liquefaction. Nevertheless, liquefaction hasbeen so widespread in a number of recent earthquakes that it is often associated withthem. Some of those earthquakes are listed below.
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Alaska, USA, 1964 Niigata, Japan, 1964

Loma Prieta, USA, 1989

1964 Alaska earthquake, USA:

As a part of the Pacific Ring, the southern coast area ofAlaska experiences many earthquakes. On Good Friday,March 27, 1964, great earthquake of magnitude 9.2 struckPrince William Sound and caused severe damage in the formof landslides and liquefaction as seen in the pictures. Thisseismic event is not only the second largest ever to have been

recorded but it lasted for over 3 minutes (radioannouncement) and was felt over an area of 500,000 squaremiles. A tsunami, heavily increased the amount of damage towharf and waterfront facilities, and caused five deaths hoursafter the earthquake in Crescent City, California

Liquefaction:

Liquefaction in sand layers, and in sand and silt seamsin the clayey soils beneath Anchorage, caused many of thedestructive landslides that occurred during the earthquake

(Seed 1973). The liquefied seams and lenses disturbed thesensitive clays, and caused their strengths to drop belowthe levels needed for stability.

1964 Niigata earthquake, Japan:
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The Niigata earthquake of June 16, 1964 had a magnitude of7.5 and caused severe damage to many structures in Niigata.The destruction was observed to be largely limited tobuildings that were founded on top of loose, saturated soil

deposits. Even though about 2000 houses were totallydestroyed, only 28 lives were lost (General report on theNiigata earthquake 1964). A tsunami, triggered bymovement of the sea floor associated with the fault rupture,totally destroyed the port of Niigata.

Liquefaction:

The Niigata earthquake, together with the Alaska earthquakealso of 1964, brought liquefaction phenomena and theirdevastating effects to the attention of engineers andseismologists. A remarkable ground failure occurred near theShinano river bank where the Kawagishi-cho apartmentbuildings suffered bearing capacity failures and tiltedseverely (left). Despite the extreme tilting, the buildingsthemselves suffered remarkably little structural damage.

Sand boils (photo 5) and ground fissures were observed at various sites in Niigata.Lateral spreading caused the foundations of the Showa Bridge to move laterally somuch that the simply supported spans became unseated and collapsed (photo 6).

Photo 5 photo 6

1989 Loma Prieta earthquake, USA:

The October 17 1989 Loma Prieta earthquake (M=7.1) caused severe damage not onlyin the vicinity of the epicenter near Santa Cruz, but also in more distant areas to thenorth around San Francisco and Oakland.
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Liquefaction:

Soil liquefaction caused major damage to waterfrontfacilities, structures, and buried pipelines at locations inthe Bay Area where loose saturated, sandy soils were

susceptible to liquefaction. The numerous sandboils thatwere observed provided indisputable evidence of theoccurrence of liquefaction. Liquefaction was observed at anumber of sites, including the Oakland airport, sites alongthe Salinas River, and Moss Landing Marine Station(right, below,photo).

1995 Kobe earthquake, Japan:

The 1995 Great Hanshin Earthquake (M=6.9), commonly referred to as the Kobeearthquake, was one of the most devastating earthquakes ever to hit Japan; more than

5,500 were killed and over 26,000 injured. The economic loss has been estimated atabout $US 200 billion. The proximity of the epicenter, and the propagation of rupturedirectly beneath the highly populated region, help explain the great loss of life and thehigh level of destruction. (On line report of Kobe earthquake). The spectacular collapse of theHanshin expressway illustrates the effects of the high loads that were imposed onstructures in the area. The strong ground motions that led to collapse of the HanshinExpress way also caused severe liquefaction damage to port and wharf facilities.

Where does soil liquefaction commonly occur?

Locations:
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Because liquefaction only occurs in saturated soil, its effectsare most commonly observed in low-lying areas near bodies ofwater such as rivers, lakes, bays, and oceans. The effects of

liquefaction may include major sliding of soil toward the bodyslumping and of water, as in the case of the 1957 Lake Mercedslide shown above, or more modest movements that producetension cracks such as those on the banks of the Motagua Riverfollowing the 1976 Guatemala Earthquake.

Port and wharf facilities are often located in areas susceptible to liquefaction, andmany have been damaged by liquefaction in past earthquakes. Mostports and wharves have major retaining structures, or quay walls, toallow large ships to moor adjacent to flat cargo handling areas.

When the soil behind and/or beneath such a wall liquefies, thepressure it exerts on the wall can increase greatly - enough to causethe wall to slide and/or tilt toward the water. As illustrated below,liquefaction caused major damage to port facilities in Kobe, Japan inthe 1995 Hyogo-ken Nanbu earthquake.

Damaged quay walls and port facilities on Rokko Island. Quay wallshave been Island. Quay walls have been pushed outward by 2 to 3meters with 3 to 4 meters deep depressed areas called grabens formingbehind the walls, Kobe 1995.

Retaining wall damage and lateralspreading, Kobe 1995.

Lateral displacement of a quay wall onPort Island, Kobe 1995.
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Liquefaction also frequently causes damage to bridges that cross rivers and other bodiesof water. Such damage can have drastic consequences, impeding emergency responseand rescue operations in the short term and causing significant economic loss frombusiness disruption in the longer term. Liquefaction-induced soil movements can pushfoundations out of place to the point where bridge spans loose support (right photo) orare compressed to the point of buckling (left, photo).

Why does soil liquefaction occur?

Explanation:

To understand liquefaction, it is important to recognize the conditions that exist in a soildeposit before an earthquake. A soil deposit consists of an assemblage of individual soilparticles. If we look closely at these particles, we can see that each particle is in contact

with a number of neighboring particles. The weight of the overlying soil particlesproduce contact forces between the particles - these forces hold individual particles inplace and give the soil its strength.

Soil grains in a soil deposit. The height of the bluecolumn to the right represents the level of porewater

pressure in the soil.

The length of the arrows represents the size of thecontact forces between individual soil grains. Thecontact forces are large when the porewater

pressure is low.
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Liquefaction occurs when the structure of a loose, saturated sand breaks down due tosome rapidly applied loading. As the structure breaks down, the loosely-packedindividual soil particles attempt to move into a denser configuration. In an earthquake,however, there is not enough time for the water in the pores of the soil to be squeezedout. Instead, the water is "trapped" and prevents the soil

particles from moving closer together. This is accompanied byan increase in water pressure which reduces the contact forcesbetween the individual soil particles, thereby softening andweakening the soil deposit. Observe how small the contactforces are because of the high water pressure. In an extremecase, the porewater pressure may become so high that many ofthe soil particles lose contact with each other. In such cases, thesoil will have very little strength, and will behave more like aliquid than a solid - hence, the name "liquefaction".

How can soil liquefaction hazards be reduced?

There are basically three possibilities to reduce liquefaction hazards when designingand constructing new buildings or other structures as bridges, tunnels, and roads.

Avoid Liquefaction Susceptible Soils:

The first possibility, is to avoid construction on liquefaction susceptible soils. Thereare various criteria to determine the liquefaction susceptibility of a soil. Bycharacterizing the soil at a particular building site according to these criteria one candecide if the site is susceptible to liquefaction and therefore unsuitable for the desired

structure.

Build Liquefaction Resistant Structures:

If it is necessary to construct on liquefaction susceptible soil because of spacerestrictions, favorable location, or other reasons, it may be possible to make the structureliquefaction resistant by designing the foundation elements to resist the effects ofliquefaction.

Improve the Soil:

The third option involves mitigation of the liquefaction hazards by improving thestrength, density, and/or drainage characteristics of the soil. This can be done using avariety of soil improvement techniques.
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Abstract:

Soil liquefaction describes the behavior ofsoils that, when loaded, suddenly suffer a

transition from a solid state to a liquefied state, or having the consistency of a heavyliquid. Liquefaction[1] is more likely to occur in loose to moderately saturated granularsoils with poordrainage, such as silty sands or sands and gravels capped or containingseams of impermeable sediments[2]. During loading, usually cyclic undrained loading,e.g. earthquake loading, loose sands tend to decrease in volume, which produces anincrease in theirporewater 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 thelast 10,000 years) sands and silts of similar grain size (well-sorted), in beds at leastmetres thick, and saturated with water. Such deposits are often found along riverbeds,

beaches, dunes, and areas where windblown silt (loess) and sand have accumulated.Some examples of liquefaction include quicksand, quick clay, turbidity currents, andearthquake liquefaction.

Depending on the initial void ratio, the soil material can respond to loading eitherstrain-softeningorstrain-hardening. Strain-softened soils, e.g. loose sands, can betriggered to collapse, either monotonically or cyclically, if the static shear stress isgreater than the ultimate orsteady-state shear strength of the soil. In this caseflowliquefaction occurs, where the soil deforms at a low constant residual shear stress. If thesoil strain-hardens, e.g. moderately dense to dense sand, flow liquefaction will generallynot occur. However, cyclic softeningcan occur due to cyclic undrained loading, e.g.

earthquake loading. Deformation during cyclic loading will depend on the density of thesoil, the magnitude and duration of the cyclic loading, and amount of shear stressreversal. If stress reversal occurs, the effective shear stress could reach zero, then cyclicliquefaction can take place. If stress reversal does not occur, zero effective stress is notpossible to occur, then cyclic mobility takes place.

The resistance of the cohesionless soil to liquefaction will depend on the density of thesoil, confining stresses, soil structure (fabric, age and cementation), the magnitude andduration of the cyclic loading, and the extent to which shear stress reversal occurs [4].

Although the effects of liquefaction have been long understood, it was more thoroughly

brought to the attention ofengineers and seismologists in theNiigata, Japan and Alaskaearthquakes. It was also a major factor in the destruction in San Francisco's MarinaDistrict during the 1989 Loma Prieta earthquake.
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what is soil liquefaction

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