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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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