Discussion on Evaluation of Liquefaction Potential of Soils

1. Discussion on Evaluation of Liquefaction Potential of Soils Mahadevan Ilankatharan Term Project 281a 12-01-2003

2. Liquefaction is defined as the transformation of a granular material from a solid to a liquefied state as a consequence of increased pore-water pressure and reduced effective stress . s’ = s - u What is liquefaction ?

3. Consequence of Liquefaction : • Settlements • Lateral spreads • Lateral flows • Loss of lateral support • Loss of bearing support • Flotation of bearing supports

4. Examples from Past earthquakes: The Showa Bridge's pile foundations moved due to lateral spreading Niigata (1964) Retaining wall damage and lateral spreading, Kobe (1995) Damage due to differential settlement, Chi-Chi earthquake Taiwan (1999) Soil failure due to Loma Prieta earthquake (1989) The Lower San Fernando Dam suffered failure by lateral spreading in 1971 Tilting of apartment buildings, Niigata (1964)

5. Cyclic Stress Ratio (CSR) and Cyclic Resistance Ratio (CRR ) : Estimation of two variables is required for evaluation of 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)

6. Evaluation of CSR : • Seed and Idriss (1971) equation

7. 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)

8. Laboratory studies to simulate field conditions for soil liquefaction : • Idealized field loading conditions • Limitations • Selection of representative samples • Stress concentrations & maintenance of uniform stresses and strains • Sampling disturbances etc.

9. 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 = Nm CN CE CB CR CS SPT – based liquefaction assessment chart (modified from Seed et al.1985)

10. 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)

11. 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)

12. Shear wave velocity liquefaction assessment chart : • Shear wave velocity is corrected for overburden pressure • Vs measurements are possible in soils that are difficult to penetrate with CPT and SPT or to extract undisturbed samples, such as gravelly soils, 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)

13. Comparison of field tests : Comparisons of advantages and disadvantages of various filed tests for assessment of liquefaction resistance(from NCEER,1997)

14. General comments : • The SPT- and the CPT-based liquefaction assessment charts are the preferred means of evaluating liquefaction potential . • They are the most reliable because they are supported by the largest 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 the most reliable penetration test. Many design procedures are also based on CPT data . • If the CPT is run with a seismic cone, the shear wave velocities can be measured at the same time. The shear moduli can be readily obtained from the velocity data and can be used as input into dynamic and static analyses. • The BPT is the most uncertain of the tools for evaluating liquefaction potential but its use may be in gravels

15. 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 seismic liquefaction potential” ,Computers and Geotecnics 29 (2002), pp. 329-341. • H.B. Seed and I.M. Idriss (1971). “Simplified procedure for evaluating soil liquefaction potential”. J. Geotech. Engrg. Div., ASCE 97 9 (1971), pp. 1249–1273. • Seed, H. B., and Idriss, I. M. (1982). ‘‘Ground motions and soil liquefaction during earthquakes.’’ Earthquake Engineering Research Institute Monograph, Oakland, Calif. • T.L. Youd and I.M. Idriss , Liquefaction of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. Journal of Geotechnical and Geoenvironmental Engineering 2001 (1996), p. 127 (4): 297-313. • http://www.eng.cam.ac.uk/research/div-d/geotech/research.html

16. Thank you.