Delineation of densified sand at Treasure Island by SASW testing

Areas of improved and unimproved soil near berthing Pier 1 at Treasure Island, California, were investigated by the Spectral-Analysis-of-Surface-Waves (SASW) test. The upper 12 m of sand fill beneath the approach to the pier had been densified by a vibrating probe technique in 1985. The area of improved soil performed well during the 1989 Loma Prieta earthquake, while sinkholes, sand boils and cracks formed in the adjacent unimproved areas. The SASW tests were conducted on a 240 m-long alignment that extended across the area of improved soil. Average shear wave velocities determined for the densified and undensified sand fill below the water table were 192 m/s and 167 m/s, respectively. Two simplified liquefaction assessment procedures based on shear wave velocity correctly predicted no liquefaction for the densified sand, and marginal liquefaction for the undensified sand.