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BY: Mahesh Raj Bhatt
ME in Structural Engineering , Kathmandu University
What is liquefaction
When does it occur
Liquefaction of soil
How it works
Shear strength of soil
Types of liquefaction
How to identify
In soil mechanics term “liquefied” first used by
Allen Hazen (1918), In reference to
Failure of Calaveras Dam in California.
Attention of engineer after
-1964 Alaska Earthquake(Mw=9.2)
good Friday earthquake
- 1964 Niigata Earthquake( Ms=7.5) Japan
-1989 loma-Prieta Earthquake and others.
What is Liquefaction?
A phenomenon whereby 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.
When does it occurs??
-when the effective stress of soil is reduced to
essentially zero, which corresponds to a
complete loss of shear strength
May be initiated by
– Monotonic Loading
– Cyclic loading
Liquefaction of soil:
Soils behave like a liquid. How and why?
To understand the above phenomenon:
• some basics required regarding:
Total stress, (σt)
Pore water pressure (u)
Effective stress (σeff )
σt= σeff + u σeff = σt- u
How It Works??
• When the seismic waves pass through the soil, the
vibrations cause the individual grains in the soil to
– move around and
– re-adjust their positions
• This ultimately results in a decrease in volume of
the soil mass as
– the grains pack more tightly together
– a reduction in porosity
Soil loose its strength because of loss of effective
Saturated sand in ground vibration,
-it tends to compact and decrease in volume ;
-if no drainage, decrease in volume results
-increase in pore water pressure
Pore water pressure=overburden pressure
effective stress = zero,
-sand looses strength completely and
develops a liquefied state.
Shear strength of soil
Shear strength, τ = c + σt tanø
Effective stress gives more realistic behaviour of
soil, Shear strength can be expressed as
τ = c1 + (σt –u)tanø1
During the ground motion due to an earthquake,
static pore pressure may by an amount udyn, then
τ = c1 + (σn –u + udyn)tanø1
Let us consider a situation when u + udyn= σn,
then τ = c1
In cohesion less soil, c1= 0, hence τ = 0 (sand)
Influence of soil condition on liquefaction potential
Types of liquefaction:
1. Flow liquefaction 2. Cyclic Liquefaction:
τ static> τliquid state τ static< τliquid state
-flow of soil mass -spreading of mass
-slope ϴ >3 degree - slope ϴ <3 degree
-steep area -level area
-flow/often movement -lateral spreading
- ground oscillation
Liquefaction potential/evaluation of
Is the soil susceptible to liquefaction?
If so , will liquefaction be triggered?
If so , will damage occur?
We Should evaluate potential liquefaction hazards
How to identify??
a. Historical Criteria
- earlier earthquake data/Maps/documents availale
b. Geological Criteria
• Saturated soil deposits that have been created by
– sedimentation in rivers and lakes (fluvial or alluvial
– deposition of debris or eroded material (collegial deposits),
– or deposits formed by wind action (Aeolian deposits)
can be very liquefaction susceptible.
-soil types clay/sand/silt
-composition of soils
Critical aspects of hazard evaluation
susceptibility initiation effects
yes no -monotonic
-alteration of Ground
Hazard do not
-cyclic loading -sand boiling
-geological criteria -development
• Objective: does soil liquefy During Earthquake?
• Assumption: Soil act as rigid body
- moves horizontal with amax exerted by EQ
At force equilibrium:
Horizontal seismic force = Max. shear force at the base
of column (τmax)
Horizontal seismic force = Mass x Accl.= [(γt .z)/g]amax =
σvo (amax/g) = τmax
Mass = W/g = (γt .z)/g = σvo /g
If effective vertical stress = σ’vo ,
Then (τmax / σ’vo ) = (σvo / σ’vo )(amax/g)
In reality, during an earthquake, soil column does not act
as a rigid body
(τmax / σ’vo ) = rd (σvo / σ’vo )(amax/g)
rd ~ 1- 0.012z , also depends upon the magnitude of the
Conversion of irregular earthquake record to an
equivalent series of uniform stress cycle by
τav = τcyc = 0.65τmax = 0.65 rd (σvo / σ’vo )(amax/g)
To felicitate liquefaction analysis, define a
CSR or SSR = τcyc / σ’vo = 0.65 rd (σvo / σ’vo )(amax/g)
CSR = Cyclic stress ratio, SSR = Seismic stress ratio
FS = Factor of safety against liquefaction = CRR/CSR
CRR= Cyclic resistance ratio
Time history of shear stress during earthquake for liquefaction analysis
resistance of soil
Data used: EQ ~ 7.5,
Three lines contain- 35,
15 or ≤ 5 % fine
Data to the left of each
line indicate field
Data to the right of
each line indicate no
FS = CRR/CSR
FS = Factor of safety
can liquefaction be predicted??
• NOT BUT
• Possible to identify areas giving detailed
information that have the potential of liquefaction
• Liquefaction susceptibility:
(Controlling factor: soil type, density and water)
• Liquefaction opportunity:
(Frequency of earthquake occurrence, intensity of
seismic ground shaking)
How to mitigate Liquefaction:
a. Improving soil properties BY:
b. Lowering ground water table:
c. Ground surface correction.
d. BE CAREFULL ABOUT HAZARD MAPING.
Liquefaction is most important earthquake
caused hazard all over the world.
Attention and researched should be increased
Hazard mapping are compulsory in Nepal.
(terai and valley regions)
- pokhara valley is most susceptible in Nepal.
Implementation of mapping should be kept in
mind by all.
• Geotechnical earthquake engineering Steven L . Kramer. Pearson 2007.
• Seismic analysis of structures T. K, Datta. John Wiley & Sons (Asia)
Pte Ltd, 2 Clementi Loop, 2010