Liquefaction of an Artificial Island in Kobe, Japan

Ahmed-W. Elgamal*, Member, ASCE, Mourad Zeghal*, Associate Member, ASCE,
and Ender Parra*, Student Member, ASCE

Abstract:

Widespread liquefaction occurred during the recent Hyogoken Nanbu 1995 earthquake, in Japan. At the reclaimed artificial Port Island in Kobe, the observed large-scale liquefaction was documented by acceleration records from a downhole seismic array. In this array, four accelerometers recorded the soil stratum response, from the ground surface down to a depth of 83 m. These recorded accelerations are used herein to obtain direct estimates of the corresponding seismic shear stress and strain histories within the soil layers, which shed light on: (1) the site seismic response during liquefaction and associated loss of soil stiffness at shallow depths near the ground surface, and (2) the virtually linear site response at deeper elevations. Response of the liquefied upper layer is characterized by cycles of large shear strain and very small shear stress. Conversely, the lower strata exhibited no sign of stiffness degradation throughout the earthquake. A computational simulation of this case history is performed in order to assess the mechanisms of site amplification and excess pore-pressure buildup. The results of this study demonstrate that acceleration histories recorded by downhole arrays represent a valuable direct source of information on site response during seismic excitation.