The lecture explores why the liquefaction charts based on Seed and Idriss Simplified Procedure work so well. These are state-of-practice (SOP) empirical charts based on case histories of liquefaction and no liquefaction during actual earthquakes. The charts use either the Standard Penetration Test (SPT), the Cone Penetration Test (CPT), or shear wave velocity measured in the field to characterize the soil. There is a disconnect between the State-of-the-Art (SOA), which relies mostly on laboratory measurements and correlations with void ratio and relative density of the sand, and the SOP based on field measurements of penetration resistance or shear wave velocity coupled with empirical correlations. This gap hinders further progress in both SOP and SOA, and understanding why the liquefaction charts work is an important step to close the gap.
This exploration of why the charts work is accomplished through: a) a literature review of the SOA including factors influencing threshold shear strain and pore pressure buildup during cyclic strain-controlled tests; b) a discussion of factors influencing field penetration resistance and shear wave velocity; and c) a discussion of the meaning of the curves in the liquefaction charts separating liquefaction from no liquefaction, helped by recent full-scale and centrifuge results. It is concluded that the charts are curves of constant cyclic strain at the lower end (Vs1 < 160 m/s), with this strain being about 0.03 to 0.05% for earthquake magnitude, Mw ≈ 7. The curves at the upper end probably correspond to a variable increasing cyclic strain and Ko, with this upper end controlled by overconsolidated and preshaken sands, and with cyclic strains needed to cause liquefaction being as high as 0.1 to 0.3%. These conclusions are validated by application to case histories corresponding to Mw ≈ 7, mostly in the San Francisco Bay Area of California during the 1989 Loma Prieta earthquake.