Lecture31-Evaluation of Liquefaction

8/12/2019 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 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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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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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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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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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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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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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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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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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.


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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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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:


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From SPT N value:


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From SPT N value:


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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

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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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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

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Lecture31-Evaluation of Liquefaction