lecture31-evaluation of liquefaction
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Evaluation of Liquefaction
Lecture-31
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Evaluation of Liquefaction Potential
Since the first widespread observations of liquefaction in the1964 Niigata and 1964 Alaska earthquakes, liquefaction hasbeen responsible for significant damage to buildings andbridges in numerous earthquakes.
The phenomenon of liquefaction has been studied
extensively over the past 40 years and substantial advancesin the understanding of its development and effects havebeen made.
These advances have led to a series of practical procedures
for evaluating the potential for liquefaction occurrence andfor estimating the effects of liquefaction.
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Evaluation of Liquefaction Potential
Evaluation of liquefaction hazards involves three primary steps.
1. The susceptibility of the soil to liquefaction must be evaluated. If the soil is
determined to be not susceptible to liquefaction, liquefaction hazards do not
exist and the liquefaction hazard evaluation is complete. If the soil is susceptible
to liquefaction, the evaluation moves to the second step.
2. Evaluation of the potential for initiation of liquefaction. This step involvescomparison of the level of loading produced by the earthquake with the
liquefaction resistance of the soil. If the resistance is greater than the loading,
liquefaction will not be initiated and the liquefaction hazard evaluation can be
considered complete. If the level of loading is greater than the liquefaction
resistance, however, liquefaction will be initiated. If liquefaction is initiated, the
evaluation moves to the third stage
3. evaluation of the effects of liquefaction. If the effects are sufficiently severe, the
engineer and owner may consider improvement of the site, or alternative sites
for the proposed development.
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Liquefaction Susceptibility
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Factors that govern liquefaction in field
Ground water table
Liquefaction only occurs for soils that are located below thegroundwater table. Unsaturated soil located above the
groundwater table will not liquefy.
At sites where the groundwater table significantly fluctuates, the
liquefaction potential will also fluctuate. Generally, the historic
high groundwater level should be used in the liquefaction analysis.
If it can be demonstrated that the soils are currently above the
groundwater table and are highly unlikely to become saturated for
given foreseeable changes in the hydrologic regime, then such
soils generally do not need to be evaluated for liquefaction
potential.
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Factors that govern liquefaction in field
Soil Type
The soil types susceptible to liquefaction are mostly nonplastic
(cohesionless) soils.
In order for a cohesive soil to liquefy, it must meet all the following
three criteria:1. The soil must have less than 15 percent of the particles, based on
dry weight, that are finer than 0.005 mm (i.e., % finer at 0.005 mm
0.9 (LL)].
If the cohesive soil does not meet all three criteria, then it is
generally considered to be not susceptible to liquefaction. 7
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Factors that govern liquefaction in field
Relative density of soil
Based on field studies, loose cohesionless soils will contract during
the seismic shaking which will cause the development of excess
pore water pressures leading to liquefaction. Upon reaching initial
liquefaction, there will be a sudden and dramatic increase in shear
displacement for loose sands
For dense sands, the state of initial liquefaction does not produce
large deformations because of the dilation tendency of the sand
upon reversal of the cyclic shear stress.
Dilative soils are not susceptible to liquefaction because their
undrained shear strength is greater than their drained shear
strength.
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Factors that govern liquefaction in field
Grain size distribution and particle shape
Uniformly graded nonplastic soils tend to form more unstable particlearrangements and are more susceptible to liquefaction than well-graded
soils.
Well-graded soils will also have small particles that fill in the void spaces
between the large particles. This tends to reduce the potential contraction
of the soil, resulting in less excess pore water pressures being generated
during the earthquake.
Field evidence indicates that most liquefaction failures have involved
uniformly graded granular soils
Soils having rounded particles tend to densify more easily than angular-shapesoil particles. Hence a soil containing rounded soil particles is more
susceptible to liquefaction than a soil containing angular soil particles
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Factors that govern liquefaction in field
Placement condition/ Depositional environment
Hydraulic fills (fill placed under water) tend to be more
susceptible to liquefaction because of the loose and segregated
soil structure created by the soil particles falling through water.
Natural soil deposits formed in lakes, rivers, or the ocean alsotend to form a loose and segregated soil structure and are
more susceptible to liquefaction.
Drainage conditions
If the excess pore water pressure can quickly dissipate, the soil
may not liquefy. Thus highly permeable sand/gravel drains or
gravel layers can reduce the liquefaction potential of adjacent
soil.10
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Factors that govern liquefaction in field
Confining pressures
The greater the confining pressure, the less susceptible the soil
is to liquefaction. Conditions that can create a higher
confining pressure are a deeper groundwater table, soil that
is located at a deeper depth below ground surface, and a
surcharge pressure applied at ground surface.
Case studies have shown that the possible zone of liquefaction
usually extends from the ground surface to a maximum depth
of 15 m. Deeper soils generally do not liquefy because of the
higher confining pressures.
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Factors that govern liquefaction in field
Ageing of the deposit
Newly deposited soils tend to be more susceptible to liquefaction
than older deposits of soil. It has been shown that the longer a
soil is subjected to a confining pressure, the greater will be the
liquefaction resistance
The increase in liquefaction resistance with time could be due to
the deformation or compression of soil particles into more
stable arrangements. With time, there may also be the
development of bonds due to cementation at particle contacts
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Factors that govern liquefaction in field
Previous earthquake history
Older soil deposits that have already been subjected to seismic shaking
have an increased liquefaction resistance compared to a newly
formed specimen of the same soil having an identical density.
Liquefaction resistance also increases with an increase in the
overconsolidation ratio (OCR) and the coefficient of lateral earth
pressure at rest k0.
An example would be the removal of an upper layer of soil due to
erosion. Because the underlying soil has been preloaded, it will
have a higher overconsolidation ratio and it will have a highercoefficient of lateral earth pressure at rest k0. Such a soil that has
been preloaded will be more resistant to liquefaction than the same
soil that has not been preloaded.
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Factors that govern liquefaction in field
Loads from superstructure
The construction of a heavy building on top of a sand deposit can
decrease the liquefaction resistance of the soil.
The reason for this is the soil underlying the building will already
be subjected to certain amount of shear stresses caused bythe building load. A smaller additional shear stress will be
required from the earthquake in order to cause contraction
and hence liquefaction of the soil.
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Steady State Line as boundary for liquefaction
SSL marks the boundary between contractive and dilative behaviour andseparates the states in which a particular soil is susceptible or notsusceptible to flow liquefaction.
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Liquefaction Potential
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Cyclic Stress Approach: In the cyclic stress approach, boththe loading imposed on the soil by the earthquake and theresistance of the soil to liquefaction are characterized interms of cyclic shear stresses. By characterizing both
loading and resistance in common terms, they can bedirectly compared to determine the potential forliquefaction.
Cyclic Strain Approach:In the cyclic stress approach, both
the loading imposed on the soil by the earthquake and theresistance of the soil to liquefaction are characterized interms of cyclic shear strains.
Evaluation of Liquefaction Potential
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Estimation of two variables is required for evaluation ofliquefaction potential of soils by cyclic stress approach.
1. The seismic demand on a soil layer, expressed in terms of
Cyclic Stress Ratio, CSR (CSR induced by the earthquake)
2. The capacity of the soil to resist liquefaction, expressed interms of Cyclic Resistance Ratio, CRR.(CSR required to
cause liquefaction)
Cyclic Stress Approach
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Estimation of two variables is required for evaluation ofliquefaction potential of soils by cyclic stress approach.
1. The seismic demand on a soil layer, expressed in terms of
Cyclic Stress Ratio, CSR (CSR induced by the earthquake)
2. The capacity of the soil to resist liquefaction, expressed interms of Cyclic Resistance Ratio, CRR.(CSR required to
cause liquefaction)
Characterization of Loading
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For the purposes of liquefaction evaluation, loading is typically
characterized in terms of the cyclic stress ratio, CSR, which is defined as
the ratio of the equivalent cyclic shear stress, cyc, to the initial vertical
effective stress, .
The equivalent cyclic shear stress is generally assumed to be equal to
65% of the peak cyclic shear stress, a value arrived at by comparing rates
of porewater pressure generation caused by transient earthquake shear
stress histories with rates caused by uniform harmonic shear stress
histories. The factor was intended to allow comparison of a transientshear stress history from an earthquake of magnitude, M, with that of N
cycles of harmonic motion of amplitude 0.65max ,where N is an
equivalent number of cycles of harmonic motion.
Characterization of Loading
'
vo
'
vo
cycCSR
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Evaluation of Liquefaction Potential
Cyclic Stress Approach
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Stress Reduction Factor rd
Fig: Variation of rd with depth below level or gently sloping ground surfaces
(Seed and Idriss, 1971)
22Source: Kramer (1996)
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Characterization of Resistance
Lab Based Approach:
(a) loose soil that reaches initial liquefaction after 9 cycles and
(b) dense sand with much higher loading amplitude that does not reach initial
liquefaction after 16 cycles. (Ishihara, 1985) 23
Source: Kramer (1996)
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(CRR)triaxial= dc/2 3c
(CRR)ss= cr (CRR)triaxial
CSR required to produce initial liquefaction in field is about 10% less
than that required in laboratory simple shear tests (Seed et al., 1975)
(CRR)field= cyclic/v0 = 0.9 (CRR)ss= 0.9 cr (CRR)triaxial
Cr= (1+k0)/2 Finn et al. (1971)
Lab Based Approach:
Characterization of Resistance
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From SPT N value:
Characterization of Resistance
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From SPT N value:
Characterization of Resistance
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Characterization of ResistanceFrom SPT N value:
Fig: Relationship between cyclic stress ratio and (N1)60 for Mw = 7.5
earthquakes27
Source: Kramer (1996)
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Correction factors for obtaining CSR for earthquake magnitudes other than
7.5 have been proposed by various researchers
Magnitude CSRM/CSRM=7.5
1.50
6 1.32
1.13
1.00
0.89
4
15
4
3
6
2
17
2
18
Characterization of Resistance
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Characterization of ResistanceFrom CPT value:
Fig: Relationship between cyclic stress ratio and normalized cone resistance
(Mitchell and Tseng, 1990)29
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Cyclic Stress Approach:
Zone of Liquefaction
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Evaluation of Liquefaction Potential: Cyclic Strain Approach
In the cyclic strain approach, earthquake-induced loading is expressed in terms of
cyclic strains.
The time history of cyclic strain in an actual earthquake is transient and irregular. To
compare the loading with laboratory measured liquefaction resistance, it must be
represented by an equivalent series of uniform strain cycles. The conversion procedure
is analogous to that used in the cyclic stress approach. . Cyclic strains are considerably
more difficult to predict accurately than cyclic stresses.
Dobry et al. ( 1982) proposed a simplified method for estimating the amplitude of the
uniform cyclic strain from the amplitude of the uniform cyclic stress using equation:
Where G(gcyc) = shear modulus of the soil at g= gcyc
If gcycis less than the threshold shear strain, then no pore pressure will be generated
and consequently liquefaction can not be initiated.
cy cG
r
g
a dvcyc
g
g
max65.0
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Characterization of Resistance
Cyclic Strain Approach:
Dobry and Ladd (1980) 32
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Zone of Liquefaction
Cyclic strain Approach:
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Factor of Safety Against Liquefaction
Factor of Safety against liquefaction FSL= CRR / CSR
CRR: Cyclic Resistance Ratio / Cyclic Shear stress required to
cause Liquefaction
CSR: Cyclic Stress Ration/ Cyclic shear stress induced by the
earthquake
For the soil to be safe against liquefaction, FSLshould be more
than 1.
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Kramer, S.L. (1996) Geotechnical Earthquake Engineering, Prentice Hall.
Jefferies, M. Been, K. (2006) Soil Liquefaction: A critical state approach, Taylor &
Francis.
Day, R.W. (2001) Geotechnical Earthquake Engineering Handbook, McGraw-Hill.
Braja M. Das, Ramana G.V. (2010) Principles of soil dynamics, C L Engineering.
Prakash, S. (1981) Soil Dynamics, McGraw-Hill.
Idriss, I.M. and Boulanger, R. (2006) Soil liquefaction during earthquakes, EERI.
References