Recent findings on liquefaction triggering in clean and silty sands during earthquakes

This is the written version of the H. Bolton Seed Medal Award Lecture presented at the ASCE Geotechnical and Structural Engineering Congress in Phoenix, Arizona, February 14-17, 2016. It discusses five recent findings on liquefaction triggering of clean and silty sands during earthquakes. Tools ranging from case history analysis to centrifuge tests were used in the corresponding studies. The findings are (1) pore pressure ratio during earthquakes is more uniquely correlated to cyclic shear strain, γ_c, than to cyclic stress ratio (CSR); (2) current penetration and shear wave velocity (V_s) charts are associated with small to moderate cyclic strains triggering liquefaction, γ_cl, that range from γ_cl ≈ 0.03% to γ_cl ≈ 0.3% depending on soil characteristics and earthquake magnitude; (3) for recent uncompacted clean and silty sand fills that have not been significantly preshaken, such as those in the San Francisco Bay Area of California, and a Richter scale magnitude M_w = 7.5, triggering occurs at γ_cl ≈ 0.03%; (4) for the heavily preshaken, geologically recent natural silty sands in the Imperial Valley of California, γ_cl ≈ 0.1-0.2% with a liquefaction resistance that is twice as big despite the fact that some of these sands were deposited as recently as the uncompacted fills in San Francisco; and (5) the static cone penetration resistance (CPT) tip penetration resistance of a clean sand is more sensitive to preshaking than V_s, with the CPT capturing better the increased liquefaction resistance due to preshaking.