Liqufiction Phenomena

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The Masterbuilder - December 200976

Liquefactionthe PhenomenaLiquefactionthe Phenomena

Liquefaction is more likely to occur in silty sands,

gravel or in moderately saturated granular soils with

poor drainage. The space between individual sand

particles, in all these cases is completely filled with

water. The pressure of the water dictates the amount

of space available between the granular sand par-

ticles and how tightly they are pressed together. The

pressure of the water increases dramatically during

an earthquake and causes the soil particles to move

with respect to each other.

Apart from earthquake shaking, construction

related activities too could cause the increase in

the water pressure leading to liquefaction. The

M.K. Prabhakar

Special Correspondent

process reduces the strength of the soil and

consequently is not capable of supporting founda-

tions of structures. The liquefaction process can

also cause the retaining walls to tilt or slide,

because of the high pressure exerted on them.

This movement of the retaining walls can cause

the destruction of structures on the ground

surface. The sheer pressure exerted by water has

been responsible in many instances for the col-

lapse of structures such as dams and are also

known to be the causative factors for major

landslides.

Liquefaction describes the phenomena during

which the transition of soils from a solid state

to that of a liquefied state takes place. In this

state, the soils get a consistency of a heavy liquid.

Liquefaction usually occurs due to rapid loading or

by earthquake shaking. The strength and stiffness of

the soil is reduced by liquefaction, a phenomena

which has been responsible for great amounts of

destruction caused by historical earthquakes, in

various parts of the world.

There are some other phenomena too that can

have a similar effect to that of liquefaction. It is often

quite difficult to distinguish between these different

phenomena and liquefaction. The one major aspectthat needs to be looked at is the mechanism behind

the phenomena, which would be invariably different.

Based on the mechanism therefore, these phenom-

ena, which incur major changes taking place in the

earth's crust , as well as the area near the surface,

these phenomena can be broadly classified into (1)

flow liquefaction and (2) cyclic mobility.

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What Causes Flow Liquefaction?

Either static or dynamic loads leading to the flow

liquefaction destroy the static equilibrium of soil

deposits, with low residual strength sometimes. The

strength of the liquefied soil here is the residual

strength. There have been several instances of

buildings, particularly the ones that have been built

on slopes, exerting additional pressure on the soil

beneath, thereby destroying the static equilibrium

and triggering flow liquefaction. Pile driving,

blasting and earthquake shaking can also act as

triggers to flow liquefaction.

Flow liquefaction can lead to devastating destruc-

tion. One good example of flow liquefaction wreak-

ing havoc can be had from the example of theSheffield Dam area, which was destroyed by the

Santa Barbara Earthquake in 1925. An entire section

of the dam, measuring 300 ft, was found pushed to

as much as 100 ft downstream. A detailed study of

the dam area later on found that too much of silty

sand had been responsible for the flow liquefaction.

The famous Alaska Earthquake of 1964, which

triggered off the Tumangain Heights Landslide is

another example for the phenomenon.

Cyclic Mobility - Causative FactorsCyclic mobility, is a phenomenon that is triggered

by , as the name suggests, cyclic loading. It occurs in

soil deposits when the static shear stress is lower

than that of the soil strength. The deformations in

the case of cyclic mobility take place over a period of

time. Lateral spreading, which is a common effect of

this phenomenon can be found occurring in flat

ground close to water bodies or on grounds with a

gentle slope. A good example for lateral spreading

can be found along the Motagua River, which was a

result of the 1976 Guatemala earthquake.

High porewater pressure caused during the

process of liquefaction can result in the porewater

flowing quickly to the surface of the ground, either

during or after an earthquake. The high pressure

exerted by the porewater carries with it sand par-

ticles through the cracks on to the ground surface.

The sand particles are then deposited on the surface

in the form of sand boils or sand volcanoes. Almost

all the sites affected by liquefaction show this

characteristic feature.

Recent Instances

Historical references point out to liquefaction

having happened for thousands of years now. There

are a number of instances in the recent past, where

they have been associated with earthquakes. Let us

take a look at some of them here.

Kobe Earthquake, 1995 in Japan

Measuring 6.9 on the Richter Scale, the Kobe

Earthquake which occurred in Japan in 1995 isconsidered one of the most devastating earthquakes

the world has ever seen. Over 5,000 people were

killed in the quake with thousands of others injured.

The earthquake left a trail of death and destruction

and left the Japanese economy poorer by about US $

200 billion. The earthquake's fault line lay directly

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beneath a highly populated region and this explains

the large scale loss of lives. The severe liquefaction

damage that was caused by the earthquake shaking

lead to the spectacular collapse of the Hanshin

expressway. The high loads that was placed on the

soil beneath could not take the stress and this along

with the liquefaction wreaked havoc during theKobe Earthquake.

Northridge Earthquake, 1994 - Reseda, USA

A strong earthquake measuring 6.7 on the

Richter Scale jolted the Reseda neighborhood near

Los Angeles, California in the USA. Although the

tremors were felt only for 20 seconds, it left behind a

trail of death and destruction. Over 70 people lost

their lives and an estimated $20 billion worth of

damages took place, making it one of the most

costliest earthquake in the US history. The phenom-ena of liquefaction was clearly seen in many of the

areas adjoining the earthquake's epicenter.

Santa Cruz region, north of San Francisco. The

earthquake measuring 7.1 on the Richter Scale

resulted in 63 deaths and left scores of people

injured. It also destroyed property worth billions ofdollars and some 12,000 people homeless. A slip

along the San Andreas fault caused this devastating

earthquake. The structural damage suffered by many

buildings in the area clearly showed the effect of

liquefaction on the reduction of soil strength.

Niigata Earthquake 1964 - Japan

Lom Preita Earthquake, 1989 - Santa Cruz, USA

Yet another instance of the widespread damage

that the process of liquefaction can cause was evident

in the 1989 Loma Preita Earthquake, that shook the

A strong earthquake measuring 7.5 on the Richter

scale severely damaged several buildings in Niigata in

Japan on June 16th , 1964. Close study of the dam-aged buildings revealed that the buildings were built

on loose soil. The porewater pressure in the area was

recorded to be substantially more and this had

resulted in loose, saturated soil deposits. A combina-

tion of the earthquake and a tsunami which was

triggered by the seismic activity caused widespread

destruction of structures in the Niigata Earthquake.

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Alarming Decrease in Soil Strength

In order to understand the phenomenon of

liquefaction, it is important that to get an insight

into the conditions that exist on the surface of the

earth, particularly with relation to the soil deposit

before any seismic activity. An assemblage of soil

particles is what makes up the soil deposit in a place.

When analyzed closely , the structure of the soil

deposits is such that each particle is in contact with a

number of other neighboring soil particles. This

strong bond is caused by certain contact forces and

this is what gives the soil its strength.

Whenever some rapidly applied loading takes

place , the structure of the loose, saturated soil

deposit breaks down, leading to the occurrence ofliquefaction. During an earthquake the individual

soil particles try and move into an area of denser

configuration. The water in the pores of the soil does

not get sufficient time to get out during the course

of an earthquake and is therefore trapped. This

trapped water is what prevents the soil particles

from moving closer to one another. The increase in

water pressure severely decreases the contact forces

between the loose soil particles. This directly leads to

the weakening of the soil strength in the particular

geographical location.

In many instances, the contact forces become so

weakened by the porewater pressure that the soil

particles lose contact with each other. In such cases,

the soil will have very little strength and may end up

behaving more like a liquid and thus , the term

'liquefaction' is used to describe the phenomenon.

Liquefaction Hazard Reduction Methods

There are basically three ways to avoid structural

damage to buildings and other structures such asroads, bridges and tunnels, in order to reduce

liquefaction hazards.

1. Avoid Soils Susceptible to Liquefaction

This is perhaps the easiest way to avoid liquefac-

tion hazard. A detailed scientific study of the soil

content in a particular geographical area can help in

the determination of liquefaction hazard of the soil.

The results of such a study based on certain standard

parameters will help in determining whether the soil

at the site is susceptible to liquefaction or not.

2. Design and construction of liquefaction

resistant structures

Rapid advancements in technology has meant that

today it is possible to make structures liquefaction

resistant. In instances when space restrictions force

the construction of structures on liquefaction suscep-

tible soils, the foundation elements are designed in

such a way, so as to resist the effects of liquefaction.

There are several key aspects that are considered

when designing and constructing liquefaction

resistant structures. The foundation design should be

such that it can span several soft spots and the

structure should posses ductility, which will help it

accommodate large deformations. All the foundation

elements in the case of a shallow foundation should

settle or move uniformly. This will in turn decrease

the amount of shear forces on the structural ele-

ments which are sitting upon the foundation.

Large lateral loads caused by liquefaction can cause

extensive damages to structures having pile founda-

tions. Piles , particularly those in the case of those driven

through liquefiable soil layer, not only have to carry

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vertical loads of the structure but also horizontal loads,

that are a direct result of the liquefaction process.

Additional reinforcement and larger dimensions are

necessary for piles to achieve sufficient resistance.

3.Improving Soil Quality

It is also possible to reduce liquefaction hazards by

improving the quality of the soil. This is done using

certain techniques which results in improvement in

the strength, drainage characteristics and density of

the soil. A variety of soil improvement techniques

are available for this very purpose nowadays.

The major aim of soil improvement techniques is

to reduce the pore water pressure which typically

increases during earthquake shaking. This isachieved either by improving the drainage capacity

of the soil or by densification of the soil.

Vibroflotation is one of the techniques used for

soil densification. This method involves the use of a

vibrating probe that is sent to depths of over 100

feet , penetrating granular soil along the way. The

grain structure of the soil collapses due to the

vibrations of the probe and this results in the densifi-

cation of the soil surrounding the probe. In many

instances, along with vibrofloatation , gravel backfill

is also used to in order to increase the amount ofdensification. This method known as Vibro Replace-

ment provides additional degree of reinforcement

and also helps in improving drainage.

Dynamic compaction is another method used for

densification. This method involves dropping of

heavy weights on a grid pattern. This method is

considered economical and may sometimes require

granular fill surrounding the drop point.

Compaction grouting is another technique which

is used extensively. In this method a slow flowing

mix of cement, sand and water is injected under a

particular pressure into the granular soil. The grout

gradually densifies the surrounding soil. This

method is particularly useful in the case of an

existing building requiring improvement, since it is

possible to inject the grout from an inclined angle or

a side to reach the areas below the building.

Liquefaction hazards can also be reduced by

improving the drainage ability of the soil. This is done

by techniques such as installation of drains of synthetic,

gravel or sand materials. Synthetic wick drains arethe most commonly used types since they can be

installed at various different angles, which is not

always possible in the case of sand or gravel drains.

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Liqufiction Phenomena