Characteristics of ground motion in liquefied deposits during earthquakes

Two well-documented case histories oil liquefaction of reclaimed fills during the 1995 Kobe earthquake are discussed in this paper. The two sites, Vertical Array site (VA-site) and Packing House site (PH-site) are in proximity to each other and have practically identical stratification with ail 18m thick fill layer overlying ail alluvial clay layer. At the VA-site, no ground improvement had been implemented and the reclaimed deposit of gravelly sand is loose with a low SPT blow count of about 5 to 10. The fill layer at the PH-site, oil the other hand, had been densified prior to the earthquake by means of the rod compaction technique resulting in ail increase in the SPT resistance to approximately 20 to 30 blow counts. To investigate the effects of ground treatment and evaluate the difference in the ground response between the densified fills and undensified loose fills during the Kobe earthquake, detailed studies were conducted for the two sites including field investigations, laboratory tests and effective stress analyses. It was found that both the extent of liquefaction and ground deformation were significantly different at the two sites. The undensified loose fills at the VA-site completely liquefied below the water table resulting in maximum shear strains of about 4% and settlement of the ground of about 25-30 cm. On the other hand, the analyses reveal that only the deep part of the densified fills at the PH-site liquefied where the maximum shear strains are estimated to have reached 2% and settlement of the ground was about 8 cm. Thus, the densification of the fills was found to be effective in preventing liquefaction from developing at shallow depths of the deposit and in limiting the overall deformation and settlement of the ground.