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Discussion on Evaluationof Liquefaction Potentialof Soils

Term Project 281a

12-01-2003

Mahadevan Ilankatharan

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What is liquefaction ?

Liquefaction is defined as the transformation of a granularmaterial from a solid to a liquefied state as a consequence ofincreased pore-water pressure and reduced effective stress .

s = s- u

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Consequence of Liquefaction :

Settlements

Lateral spreads

Lateral flowsLoss of lateral support

Loss of bearing support

Flotation of bearing supports

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Examples from Past earthquakes:

The Lower San Fernando Dam suffered failureby lateral spreading in 1971

The Showa Bridge's pile foundations moveddue to lateral spreading Niigata (1964)

Tilting of apartment buildings, Niigata (1964)Soil failure due to Loma Prieta earthquake (1989)

Damage due to differential settlement, Chi-Chi earthquake Taiwan (1999)

Retaining wall damage and lateral spreading,Kobe (1995)

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Cyclic Stress Ratio (CSR) and CyclicResistance Ratio (CRR) :

Estimation of two variables is required for evaluationof liquefaction resistance of soils.

1. The seismic demand on a soil layer, expressed in

terms of CSR (CSR induced by the earthquake)

2. The capacity of the soil to resist liquefaction,expressed in terms of CRR.(CSR required to cause

liquefaction)

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Evaluation of CSR :

Seed and Idriss (1971) equation

d'

vo

vomax

'

vo

av r

g

a0.65

CSR

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Evaluation of liquefaction resistance (CRR) :

Laboratory Tests

Cyclic simple shear test

Cyclic triaxial test

In situ tests Standard penetration test (SPT)

Cone penetration test (CPT)

Shear-wave velocity measurements (Vs)

Beaker penetration test (BPT)

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Laboratory studies to simulate field conditionsfor soil liquefaction :

Idealized field loading conditions

Limitations

Selection of representative samples

Stress concentrations & maintenance ofuniform stresses and strains

Sampling disturbances etc.

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SPT liquefaction assessment chart :

Correction for effective overburden stress (CN) Correction for hammer energy ratio (CE)

Correction for bore hole diameter (CB)

Correction for samplers (CS)

Correction for rod length (CR)

(N1)60 = NmCNCE CB CRCS

SPTbased liquefaction assessment chart (modified from Seed et al.1985)

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CPT liquefaction assessment chart :

The CPT is more consistent and repeatable

Continuous penetration records are available

Liquefaction assessment chart is based on normalized tip resistance

CPT- based liquefaction assessment chart (modified from Robertson PK and wride CE in 1998)

q

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Becker penetration test (BPT) :

Useful in gravels

Number of blows required to drive the casing 300 mm in the gravel

Very little data linking BPT directly to filed liquefaction events

Becker blow counts are to equivalent SPT blow counts

Use SPT chart to find CRR

Correlation between SPT and BPT blow counts in sand (modified from Harder and Seed in 1986)

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Shear wave velocity liquefaction assessment chart :

Shear wave velocity is corrected for overburden pressure

Vs measurements are possible in soils that are difficult to penetratewith CPT and SPT or to extract undisturbed samples, such as gravellysoils, and at sites where borings or soundings may not be permitted

Vs is a basic mechanical property of soil materials, directly related to

small-strain shear modulus

Shear wave velocity based liquefaction assessment chart (Reproduced from Andrus and Stokoe in 2000)

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Comparison of field tests :

Comparisons of advantages and disadvantages of various filed testsfor assessment of liquefaction resistance(from NCEER,1997)

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General comments :

The SPT- and the CPT-based liquefaction assessment charts arethe preferred means of evaluating liquefaction potential .

They are the most reliable because they are supported by thelargest databases on the occurrence of liquefaction .

The SPT test provides soil samples for identification of soil type

and many empirical design procedures are based on the SPT, N.The CPT provides the best picture of soil stratification and is themost reliable penetration test. Many design procedures are alsobased on CPT data .

If the CPT is run with a seismic cone, the shear wave velocitiescan be measured at the same time. The shear moduli can bereadily obtained from the velocity data and can be used as inputinto dynamic and static analyses.

The BPT is the most uncertain of the tools for evaluatingliquefaction potential but its use may be in gravels

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References:

Kramer S. L. (1996). Geotechnical earthquake engineering, Prentice-Hall, Englewood Cliffs, N. J., 653.

Finn W.D.L. (2002). State of the art for the evaluation of seismicliquefaction potential ,Computers and Geotecnics 29 (2002), pp. 329-

341.H.B. Seed and I.M. Idriss (1971). Simplifiedprocedure for evaluatingsoil liquefaction potential. J. Geotech. Engrg. Div., ASCE 979 (1971),pp. 12491273.

Seed, H. B., and Idriss, I. M. (1982). Ground motions and soilliquefaction during earthquakes. Earthquake Engineering ResearchInstitute Monograph, Oakland, Calif.

T.L. Youd and I.M. Idriss , Liquefaction of soils: summary report fromthe 1996 NCEER and 1998 NCEER/NSF workshops on evaluation ofliquefaction resistance of soils. Journal of Geotechnical andGeoenvironmental Engineering 2001(1996), p. 127 (4): 297-313.

http://www.eng.cam.ac.uk/research/div-d/geotech/research.html

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Thank you.