CONTENTS
CHAPTER 1
· 1.1 INTRODUCTION
· 1.2
DEFINITION
· 1.3
WHAT IS LIQUEFACTION & WHY DOES IT OCCUR
?
· 1.4
CAUSE BEHIND LIQUEFACTION
CHAPTER 2
· LITERATURE
REVIEW
CHAPTER 3
· 3.1
SOIL PROPERTIES DURING
LIQUEFACTION
· 3.2
POREWATER PRESSURE DURING
LIQUEFACTION
· 3.3
EARTHQUAKE
LIQUEFACTION
· 3.4
FACTORS AFFECTING SOIL
LIQUEFACTION
· 3.5
CONSEQUENCE OF LIQUEFACTION
CHAPTER 4
· 4.1
SAND PHENOMENONS
CHAPTER 5
· 5.1
SOIL LIQUEFACTION TRAGEDIES
· 5.2
EFFECTS
· 5.3
MITIGATION METHODS
CHAPTER 6
· 6.1
SUMMARY
REFERENCES
REFERENCES
ABSTRACT
The presence of silt and clay particles has long been thought to affect the
behavior of a sand under cyclic loading. Unfortunately, a review of studies
published in the literature reveals that no clear conclusions can be drawn as
to how altering fines content and plasticity actually affects the liquefaction
resistance of a sand. In fact, the literature contains what appears to be
contradictory evidence. There is a need to clarify the effects of fines content
and plasticity on the liquefaction resistance of sandy soils, and to determine
methods for accounting for these effects in engineering practice.
In order to help answer these questions, a program of research in the form
of a laboratory parametric study intended to clarify the effects which varying
fines content and plasticity have upon the liquefaction resistance of sandy
sands was undertaken. The program of research consisted of a large number of
cyclic triaxial tests performed on two sands with varying quantities of plastic
and non-plastic fines. The program of research also examined the applicability
of plasticity based liquefaction criteria and the effects of fines content and
plasticity on pore pressure generation. Lastly, a review of how the findings of
this study may affect the manner in which simplified analyses are performed in
engineering practice was made. The results of the study performed are used to
clarify the effects of non-plastic fines content and resolve the majority of
the inconsistencies in the literature. The effects of plastic fines content and
fines plasticity are shown to be different than has been previously reported.
The validity of plasticity based liquefaction criteria is established, the
mechanism responsible for their validity is explained, and a new simplified
criteria proposed. The effects of fines content and plasticity on pore pressure
generation are discussed, and several recommendations are made for implementing
the findings of this study into engineering practice.
CHAPTER 1
1.1 INTRODUCTION
Liquefaction is the phenomena
when there is loss of strength in saturated and cohesion-less soils because of
increased pore water pressures and hence reduced effective stresses due to
dynamic loading. It is a phenomenon in which the strength and stiffness of a
soil is reduced by earthquake shaking or other rapid loading.
Liquefaction occurs in
saturated, saturated soils are the soils in which the space between individual
particles is completely filled with water. This water exerts a pressure on the
soil particles that. The water pressure is however relatively low before the
occurrence of earthquake. But earthquake shaking can cause the water pressure
to increase to the point at which the soil particles can readily move with
respect to one another.
Although earthquakes
often triggers this increase in water pressure, but activities such as blasting
can also cause an increase in water pressure. When liquefaction occurs, the
strength of the soil decreases and the ability of a soil deposit to support the
construction above it.
Soil liquefaction can
also exert higher pressure on retaining walls, which can cause them to slide or
tilt. This movement can cause destruction of structures on the ground surface
and settlement of the retained soil.
It is required to
recognize the conditions that exist in a soil deposit before an earthquake in
order to identify liquefaction. Soil is basically an assemblage of many soil
particles which stay in contact with many neighboring soil. The contact forces
produced by the weight of the overlying particles holds individual soil
particle in its place and provide strength.
1.2 DEFINITION
“A Phenomenon where by a
saturated or partially saturated soil substantially loses strength and
stiffness in response to an applied stress, usually earthquake Shaking or other
sudden change in stress condition, causing it to behave like a liquid” is
called Soil Liquefaction.
1.3 WHAT IS LIQUEFACTION & WHY DOES IT OCCUR ?
Liquefaction is the
process that leads to a soil suddenly losing strength, most commonly as a
result of ground shaking during a large earthquake. Not all soils however, will
liquefy in an earthquake.
The following are
particular features of soils that potentially can liquefy:
· They
are sands and silts and quite loose in the ground. Such soils do not stick
together the way clay soils do.
· They
are below the watertable, so all the space between the grains of sand and silt
are filled with water. Dry soils above the watertable won’t liquefy.
When an earthquake
occurs the shaking is so rapid and violent that the sand and silt grains try to
compress the spaces filled with water, but the water pushes back and pressure
builds up until the grains ‘float’ in the water. Once that happens the soil
loses its strength – it has liquefied. Soil that was once solid now behaves
like a fluid.
Fig (1.30 & 1.31) Some examples of Soil Liquefaction.
WHAT HAPPENS NEXT ?
Liquefied soil, like
water, cannot support the weight of whatever is lying above it – be it the
surface layers of dry soil or the concrete floors of buildings.
The liquefied soil under
that weight is forced into any cracks and crevasses it can find, including
those in the dry soil above, or the cracks between concrete slabs. It flows out
onto the surface as boils, sand volcanoes and rivers of silt. In some cases the
liquefied soil flowing up a crack can erode and widen the crack to a size big
enough to accommodate a car.
Some other consequences
of the soil liquefying are:
· Settlement
of the ground surface due to the loss of soil from underground.
· Loss
of support to building foundations.
· Floating
of manholes, buried tanks and pipes in the liquefied soil - but only if the
tanks and pipes are mostly empty.
· Near
streams and rivers, the dry surface soil layers can slide sideways on the
liquefied soil towards the streams. This is called lateral spreading and can
severely damage a building.
It typically results in
long tears and rips in the ground surface that look like a classic fault line.
Not all of a building’s
foundations might be affected by liquefaction.
The affected part may
subside (settle) or be pulled sideways by lateral spreading, which can severely
damage the building. Buried services such as sewer pipes can be damaged as they
are warped by lateral spreading, ground settlement or flotation.
Fig (1.32 & 1.33) Some examples of Soil Liquefaction.
AFTER THE EARTHQUAKE
After the earthquake
shaking has ceased, and liquefaction effects have diminished (which may take
several hours).
The permanent effects
include:
• Lowering of ground
levels where liquefaction and soil ejection has occurred. Ground lowering may
be sufficient to make the surface close to or below the watertable, creating
ponds.
• Disruption of ground
due to lateral spreading.
The liquefied soil that
is not ejected onto the ground surface re-densifies and regains strength, in
some cases re-densified soil is stronger than before the earthquake.
Careful engineering
evaluation is required to determine whether ground that has suffered
liquefaction can be redeveloped.
Fig (1.34 & 1.35) some examples of Soil Liquefaction.
1.4 CAUSE BEHIND LIQUEFACTION
It is required to
recognize the conditions that exist in a soil deposit before an earthquake in order
to identify liquefaction. Soil is basically an assemblage of many soil
particles which stay in contact with many neighboring soil. The contact forces
produced by the weight of the overlying particles holds individual soil
particle in its place and provide strength.
· Soil
grains in a soil deposit. The height of the blue column to the right
represents the level of pore-water pressure in the soil.
· The
length of the arrows represents the size of the contact forces between
individual soil grains. The contact forces are large when the pore-water
pressure is low.
|
Occurrence of
liquefaction is the result of rapid load application and break down of the
loose and saturated sand and the loosely-packed individual soil particles tries
to move into a denser configuration. However, there is not enough time for the
pore-water of the soil to be squeezed out in case of earthquake. Instead, the
water is trapped and prevents the soil particles from moving closer together.
Thus, there is an increase in water pressure which reduces the contact forces
between the individual soil particles causing softening and weakening of soil
deposit. In extreme conditions, the soil particles may lose contact with each
other due to the increased pore-water pressure. In such cases, the soil will
have very little strength, and will behave more like a liquid than a solid -
hence, the name "liquefaction".
Fig (1.40) Nishinomia Bridge 1995 Kobe earthquake, Japan.
CHAPTER 2
LITERATURE REVIEW
Carmine Paul Polito (10 Dec 1999)
The published results of geotechnical studies were examined in order to
determine the state of knowledge on the effects of fines content and plasticity
on the liquefaction resistance and pore pressure generation characteristics of
sandy soils.
2.1 The Effects of Fine Content and
Plasticity on Liquefaction Resistance
Both clean sands and sands containing fines have been shown to be
liquefiable in the field (Mogami and Kubo (1953); Robertson and Campenella
(1985); and Holzer et al. (1989)) and in the laboratory (Lee and Seed (1967a);
Chang et al. (1982); and Koester (1994)). Additionally, non-plastic silts, most
notably mine tailings, have also been found to be susceptible to liquefaction
(Dobry and Alvarez (1967); Okusa et al. (1980); and Garga and McKay (1984)). A
review of the literature, however, shows conflicting evidence as to the effect
which fines have on the liquefaction resistance or cyclic strength of a sand.
The main factors that are reviewed here are the effects of non-plastic fines
content and the effects of plastic fines content and plasticity on the
liquefaction resistance of sandy soils.
2.2 The Effects Of Non-Plastic Fine Content
There is no clear consensus in the literature as to the effect which increasing
non-plastic fines content has upon the liquefaction resistance of a sand. Both
field and laboratory studies have been performed, and the results of these
studies indicate that increasing the non-plastic fines content in a sand will
either increase the liquefaction resistance of the sand, decrease the
liquefaction resistance of the sand, or decreases the liquefaction resistance
until some limiting fines content is reached, and then increases its
resistance. To further complicate issues, some researchers have shown that the
liquefaction resistance of silty sands is not a function of the silt content of
the soil so much as it is a function of the soil’s sand skeleton void ratio.
2.3 The Effects of Plastic Fines Content and
Plasticity And Plasticity Based Liquefaction Criteria
There is general agreement in the literature as to the effect which the
quantity and plasticity of the fine-grained material has on the liquefaction
resistance of a sandy soil. There is agreement that whether the fine grained
material is silt or clay, or more importantly, whether it behaves plastically
or non-plastically, tends to make an important, consistent difference in the
cyclic strength of the soil. The majority of studies have shown that the
presence of plastic fines tend to increase the liquefaction resistance of a
soil.
2.4 Plasticity Based Liquefaction Criteria
Jennings (1980) presents a listing of the “thresholds to liquefaction” used
by engineers in the People’s Republic of China to separate soils which are
considered liquefiable from those considered non-liquefiable. Soils meeting
these criteria are considered to be nonliquefiable and include those with
plasticity indexes greater than 10, clay contents greater than 10 percent,
relative densities greater than 75 percent, and void ratios less than 0.80.
Other criteria presented are related to epicentral distance, intensity,
grain size and gradation, the depth of the sand layer, and the depth of the
water table.
Seed et al. (1973) in their review of the slides that occurred in the Lower
San Fernando Dam during the February 1971 San Fernando earthquake presented a
modified form of the Chinese criteria. As reported by Marcuson et al. (1990),
soils with greater than 15 percent material finer than 0.005 mm, liquid limits
greater than 35 percent, and water contents less than 90 percent of the liquid
limit should be safe from liquefaction.
2.5 The Effects Of Fines Content And
Plasticity On Pore Pressure Generation
The rate and magnitude
of pore pressure generation may have important effects on the shear strength, stability, and settlement
characteristics of a soil mass, even if the soil does not liquefy.
Similarly, the peak pore pressure generated may affect the stability
of structure founded on, or in the soil mass.
2.6 Rate And Magnitude Of Pore Pressure
Generation
There are two methods of examining the rate and magnitude of pore pressure
generation during cyclic loading which have been reported in the literature.
The first is to examine the pore pressures generated in relation to the ratio
of the number of cycles of loading applied to the number of cycles required to
cause liquefaction. This is the method used by Lee and Albaisa (1974). Pore
pressures may also be measured in terms of the strain required to generate them.
This is the approached taken by Dobry et al (1982).
CHAPTER 3
3.1 SOIL PROPERTIES DURING LIQUEFACTION
· SHRINKAGE
LIMIT
The shrinkage limit (SL)
is the water content where further loss of moisture will not result in any more
volume reduction.
· PLASTIC
LIMIT
The plastic limit (PL)
is determined by rolling out a thread of the fine portion of a soil on a flat,
non-porous surface.
· LIQUID
LIMIT
The liquid limit (LL) is
often conceptually defined as the water content at which the behavior of a
clayey soil changes from plastic to liquid . Actually, clayey soil
does have a very small shear strength at the liquid limit and the strength decreases
as water content increases; the transition from plastic to liquid behavior
occurs over a range of water contents.
· THE
ATTERBERG LIMITS
The Atterberg Limits are a basic measure
of the critical water contents of a fine-grained soil, such
as its shrinkage limit, plastic limit, and liquid limit. As a dry, clayey soil
takes on increasing amounts of water, it undergoes dramatic and distinct
changes in behavior and consistency. Depending on the water content of
the soil, it may appear in four states: solid, semi-solid, plastic and liquid.
In each state, the consistency and behavior of a soil is different and
consequently so are its engineering properties. Thus, the boundary between each
state can be defined based on a change in the soil's behavior. The Atterberg
limits can be used to distinguish between silt and clay, and
it can distinguish between different types of silts and clays. These limits
were created by Albert Atterberg,
a Swedish chemist. They were later refined by Arthur
Casagrande. These distinctions in soil are used in assessing the soils that are to
have structures built on. Soils when wet retain water and some expand in
volume. The amount of expansion is related to the ability of the soil to take
in water and its structural make-up (the type of atoms present). These tests
are mainly used on clayey or silty soils since these are the soils that expand
and shrink due to moisture content. Clays and silts react with the water and
thus change sizes and have varying shear strengths. Thus these tests are used
widely in the preliminary stages of designing any structure to ensure that the soil
will have the correct amount of shear strength and
not too much change in volume as it expands and shrinks with different moisture
contents.
As a hard, rigid solid in the dry state, soil becomes a
crumbly (friable) semisolid when a certain moisture content, termed the
shrinkage limit, is reached. If it is an expansive soil, this soil will also
begin to swell in volume as this moisture content is exceeded. Increasing the
water content beyond the soil's plastic limit will transform it into a
malleable, plastic mass, which causes additional swelling. The soil will remain
in this plastic state until its liquid limit is exceeded, which causes it to
transform into a viscous liquid that flows when jarred.
3.2 POREWATER PRESSURE DURING LIQUEFACTION
A state of 'soil liquefaction' occurs when the effective
stress of soil is reduced to essentially zero, which corresponds to a
complete loss of shear strength. This may be initiated
by either monotonic loading (e.g. single sudden occurrence of a change in
stress – examples include an increase in load on an embankment or sudden loss
of toe support) or cyclic loading (e.g. repeated change in stress condition –
examples include wave loading or earthquake shaking)
. In both cases a soil in a saturated loose state, and
one which may generate significant pore water pressure on a change in load are
the most likely to liquefy.
This is because a loose soil has the tendency to
compress when sheared, generating large excess Porewater
Pressure as load is transferred from the soil skeleton to adjacent pore water
during undrained loading. As pore water pressure rises a progressive loss of
strength of the soil occurs as effective stress is reduced. It is
more likely to occur in sandy or non-plastic silty soils, but may in rare cases
occur in gravels and clays.
OCCURRENCE OF SOIL LIQUEFACTION
· Liquefaction
is more likely to occur in loose to moderately saturated granular soils with
poor drainage, such as silty sands or sands and gravels capped or
containing seams of impermeable sediments.
· During wave
loading, usually cyclic undrained loading, e.g. seismic
loading, loose sands tend to decrease in volume, which produces an
increase in their pore water pressures and consequently a
decrease in shear strength, i.e. reduction
in effective stress
· The
resistance of the cohesionless soil to liquefaction will depend on the density
of the soil, confining stresses, soil structure. The magnitude and duration of
the cyclic loading, and the extent to which shear stress reversal occurs.
· Depending
on the initial void ratio, the soil material can respond to loading
either strain-softening or strain-hardening. Strain-softened
soils, e.g. loose sands, can be triggered to collapse, either monotonically or
cyclically, if the static shear stress is greater than the ultimate or
steady-state shear strength of the soil. In this case flow
liquefaction occurs.
3.3 EARTHQUAKE LIQUEFACTION
Fig (3.30) Sand boils that erupted during the 2011 Christchurch
earthquake.
The pressures generated during large earthquakes with
many cycles of shaking can cause the liquefied sand and excess water to force
its way to the ground surface from several metres below the ground. This is
often observed as "sand boils"
also called "sand blows" or "sand volcanoes" (as they appear to form
small volcanic craters) at the ground surface. The phenomenon may incorporate
both flow of already liquefied sand from a layer below ground, and a quicksand effect whereby upward flow of
water initiates liquefaction in overlying non-liquefied sandy deposits due to
buoyancy.
One positive aspect of soil liquefaction is the tendency
for the effects of earthquake shaking to be significantly damped (reduced) for the remainder of the
earthquake. This is because liquids do not support a shear stress and so once the soil
liquefies due to shaking, subsequent earthquake shaking (transferred through
ground by shear waves)
is not transferred to buildings at the ground surface.
Studies of liquefaction features left by prehistoric
earthquakes, called Paleoliquefaction or Paleoseismology, can reveal a great deal of
information about earthquakes that occurred before records were kept or
accurate measurements could be taken. Soil liquefaction induced by earthquake
shaking is also a major contributor to urban seismic risk.
TECHNICAL DEFINITION
A state of Soil
Liquefaction occurs when the effective stress of soil is reduced to
essentially zero, which corresponds to a complete loss of shear strength.
This may be initiated by either monotonic loading or cyclic loading .
TYPES OF FAILURES
1. Cyclic
Mobility
2. Over
Turning
3. Sand
Boiling
These are some of failures.
3.4 FACTORS AFFECTING SOIL LIQUEFACTION
1. Soil Type
2. Grain size and its
distribution
3. Initial relative
density
4. Vibration
characterstics
5. Location of drainage
and dimension of deposit
6. Surcharge load
7. Method of soil
formation
8. Period under
sustained load
9. Previous strain
history
10. Trapped Air
These are some factors
affecting Soil Liquefaction.
3.5 CONSEQUENCE OF LIQUEFACTION
ü Settlements
ü Lateral
spreads
ü Lateral
flows
ü Loss of
lateral support
ü Loss of
bearing support
ü Flotation
of bearing supports
These are some
consequences of Soil Liquefaction.
CHAPTER 4
4.1 SAND PHENOMENONS
· QUICK
SAND
QuickSand forms
when water saturates an area of loose sand and the ordinary sand is agitated.
When the water trapped in the batch of sand cannot escape, it creates liquefied
soil that can no longer support weight. Quicksand can be formed by standing or
(upwards) flowing underground water (as from an underground spring), or by
earthquakes. In the case of flowing underground water, the force of the water
flow opposes the force of gravity, causing the granules of sand to be more
buoyant. In the case of earthquakes, the shaking force can increase the
pressure of shallow groundwater, liquefying sand and silt deposits. In both
cases, the liquefied surface loses strength, causing buildings or other objects
on that surface to sink or fall over.
The saturated sediment may appear quite solid until
a change in pressure or shock initiates the liquefaction, causing the sand to
form a suspension with each grain surrounded by a thin film of water. This
cushioning gives quicksand, and other liquefied sediments, a spongy, fluidlike
texture. Objects in the liquefied sand sink to the level at which the weight of
the object is equal to the weight of the displaced sand/water mix and the
object floats due to its buoyancy.
Fig (4.10 & 4.11) Some examples for QuickSand Phenomenon.
· QUICK
CLAY
Quick clay,
also known as Leda Clay in Canada,
is a water-saturated gel, which in its solid form resemble a unique
form of highly sensitive clay. This clay has a
tendency to change from a relatively stiff condition to a liquid mass when it
is disturbed. This gradual change in appearance from solid to liquid is a
process known as spontaneous liquefaction. The clay retains a solid structure
despite the high water content (up to 80 volume-%), because surface tension holds water-coated flakes
of clay together in a delicate structure. When the structure is broken by a
shock or sufficient shear, it turns to a fluid state.Quick clay is only found
in the northern countries such as Russia, Canada, Alaska in
the U.S., Norway, Sweden,
and Finland, which were glaciated during the Pleistocene epoch.
Quick clay has been the underlying cause of many
deadly landslides. In Canada alone, it has been
associated with more than 250 mapped landslides.
Fig (4.12 & 4.13) Some examples for Quick Clay Phenomenon.
CHAPTER 5
5.1 SOIL LIQUEFACTION TRAGEDIES
Fig (5.10) 1964 Niigata earthquake.
Fig (5.11) 1964 Alaska earthquake.
Fig (5.12) 1989 Loma Prieta
earthquake.
Fig (5.13) 2010 Canterbury
earthquake
Fig (5.14) Liquefied soil exerts higher pressure on
retaining walls,which can cause them to tilt or slide.
Fig (5.15) Foundation failure in Kerala during Tsunami
(December 26th, 2004)
5.2 EFFECTS
The effects of
lateral spreading (River Road in 2011 Christchurch earthquake)
Damage in
Brooklands from the 2010 Canterbury earthquake, where buoyancy caused by soil
liquefaction pushed up an underground service including this manhole
The effects of soil liquefaction on the built environment
can be extremely damaging. Buildings whose foundations bear directly on sand
which liquefies will experience a sudden loss of support, which will result in
drastic and irregular settlement of the building causing structural damage,
including cracking of foundations and damage to the building structure itself,
or may leave the structure unserviceable afterwards, even without structural
damage. Where a thin crust of non-liquefied soil exists between building
foundation and liquefied soil, a 'punching shear' type foundation failure may
occur. The irregular settlement of ground may also break underground utility
lines. The upward pressure applied by the movement of liquefied soil through
the crust layer can crack weak foundation slabs and enter buildings through
service ducts, and may allow water to damage the building contents and
electrical services.
Bridges and large buildings constructed on pile foundations may lose support from the
adjacent soil and buckle, or come to rest at
a tilt after shaking.
Sloping ground and ground next to rivers and lakes may
slide on a liquefied soil layer (termed 'lateral spreading'), opening
large cracks or fissures in the ground, and can cause significant damage to
buildings, bridges, roads and services such as water, natural gas, sewerage,
power and telecommunications installed in the affected ground. Buried tanks and
manholes may float in the liquefied soil due to buoyancy. Earth embankments such as
flood levees and earth dams may lose stability or collapse
if the material comprising the embankment or its foundation liquefies.
5.3 MITIGATION METHODS
Methods to mitigate the effects of soil
liquefaction have been devised by earthquake engineers and
include various soil compactiontechniques such
as :
These methods result in the densification of soil and
enable buildings to withstand soil liquefaction.
Existing buildings can be mitigated by injecting grout
into the soil to stabilize the layer of soil that is subject to liquefaction.
1. Vibro Compaction.
2. Dynamic Compaction.
These are some methods to mitigate the effects of
Soil Liquefaction.
CHAPTER 6
6.1 SUMMARY
This Promotes simple
criterion based on “key” soil parameters that help partition liquefiable and
non-liquefiable silty soils. A brief review of the physical characteristics of
silts and clays is
first given to help
clarify some misconceptions about silty soils. Clay content and liquid limit
are
then considered as two
“key” soil parameters that help partition liquefiable and non-liquefiable
silty soils. Several
case histories are presented that illustrate the applicability of using clay
content as a “key” soil parameter. Attention is drawn to an analogy between the
liquid limit and the shear strength of a soil.
This analogy is expanded
to show that the liquid limit can be regarded as a “key” soil parameter that
gives a relative measure of liquefaction susceptibility. Inadequacies of basing
criteria for liquefaction of silty soils on just one “key” parameter are
finally discussed, leading to the promotion of simple criteria for liquefaction
of silty soils, utilising together both the clay content and the liquid limit
soil parameters.
REFERENCES
1. Kenji Ishihara, Norio
Oyagi, Text Book of Soil And Foundations, Vol 30, No 4, 73-89, Dec 1990.
2. Carmine Paul Polito,
‘The Effects Of Non-Plastic and Plastic Fines On The Liquefaction Of Sandy
Soils’, 10 December 1999.
3. Hans F.Winterkorn and
Hsai-Yang Fang., ‘Foundation Engineering Handbook’.
4. T.G.Sitharam,
L.GovindaRaju and A. Sridharan (2004).,‘Dynamic properties and liquefaction
potential of soils’, Special Section: Geotechnics and Earthquake Hazards,
Current Science, Vol.87,No.10,25 November 2004.
5. Alisha Kaplan
(2004).,‘Soil Liquefaction’ Undergraduate Research, Mid-America Earthquake
Center and Georgia Institute of Technology, May 2004.
6. EN1998-5:2004
Eurocode 8 – Design of structures for earthquake resistance. Part 5:
Foundations, retaining structures and geotechnical aspects. Brussels: European
Committee for Standardisation. 2004.
7. http://en.wikipedia.org/wiki/soil_liquefaction
8. http://geology.com>Home>Geological Hazards
9. http://a4academics.com>Home>Seminar Topics>Liquefaction
10.http://slideshare.net/jagadanand/liquefaction_of_soil
