Liquefaction: Behavior Evidence, Prediction, and Prevention

1. Liquefaction: Behavior Evidence, Prediction, and Prevention Richard P. Ray, Ph.D, P.E.

5. Today’s Menu • What is Liquefaction ? • How is it expressed ? • Can we estimate liquefaction potential ? • Can we prevent it from happening or reduce its impact ?

6. What is it • Worm’s eye view - Physical Behavior • Professor’s eye view - Laboratory and Analytical Approach

7. a b c Worm’s eye view

8. Triaxial Test on Sand

9. Triaxial Test on Saturated Sand

12. Expression of Liquefaction • Sand boils and flows – free-field • Landslides – static lateral stress • Foundation Failures – surface loadings

23. Evaluating Liquefaction Potential • Empirical Methods SPT, CPT • Stress or Strain-Based Simplified • Effective Stress Time History Models • Probabilistic Methods

24. Evaluating Liquefaction Potential Compare Earthquake Load to Soil Resistance Stress Based Strain Based • Seed & Idriss • One Dimensional Response Analysis • Laboratory Stress Cyclic Testing • Dobry et al • One Dimensional Response Analysis • Laboratory Strain Cyclic Testing

25. Seed & Idriss Simplified Method FS=Resistance/Load Resistance = res = fn( SPT Blow Count, Earthquake Mag, Percent Fines) Load = cyc = fn( Earthquake Accel, Vertical Stress)

26. Load Equation, Seed & Idriss Simplified Method • Divide profile into segments (~2ft.) • Compute  v , rd for each segment • Compute  cyc for each segment

27. Stress Reduction Factor (rd)

28. Seed & Idriss Load Profile,  cyc

29. Resistance Equation, Seed & Idriss Simplified Method • Use same depth segments (~2ft.) • Compute ’vfor each segment • Compute CSR for each segment • Compute res for each segment

30. Resistance Equation, Seed & Idriss Simplified Method • CSR = Cyclic Stress Ratio res/’v • CSR based on SPT N160 values • CSR influenced by Percent Fines Therefore • Must correct N blow counts • Must account for Percent Fines

31. Empirical CSR Based on Blow Count, N160

32. Load-Resistance and F.S. Profiles

33. Resistance Equation, Seed & Idriss Simplified Method (Alternatives) • CSR = Cyclic Stress Ratio res/’v • CSR based on CPT =>SPT N160 , or • CSR based on Vs => SPT N160

34. Loading Conditions, Dobry et al Simplified Strain Method • Divide profile into segments (~2ft.) • Compute  v , rd for each segment • Compute cyc for each segment (iterative procedure)

35. Liquefaction Resistance, Dobry et al. Simplified Strain Method • Perform strain-controlled laboratory tests at several stages of cyc-lab to measure number of cycles to liquefaction N(l,) • At cyc = cyc-lab compare Neqto N(l,) • If Neq >N(l,) liquefaction possible • If Neq <N(l,) liquefaction not possible

36. Settlement During Liquefaction, Dobry et al-Castro Method • Uses correlation of cyc to vol for dry and saturated sands (SP, SP-SC, SC) • For every profile segment use cyc to find vol • Multiply vol times segment height to get segment settlement contribution • Add all segment contributions for settlement estimate at surface.

37. Settlement Correlation - Castro

38. Alternative Methods for Liquefaction Potential Estimation • Empirical Approach, Cone Resistance • Empirical Approach, Shear Wave Velocity • MOC 1-D Analysis

39. Empirical Approach, Cone Resistance

40. Empirical Approach, Shear Wave Velocity

41. MOC 1-D Analysis

42. MOC 1-D Analysis Output

44. Prevention/Control • Soil modification • Reduced Loads • Drainage (pore pressure relief)