Geological and geophysical characterization of the near-surface at a liquefied site

Abstract

Combined geological and geophysical surveying was conducted at a liquefied site where sand boiling from fissures widely occurred just after the main shock of the 2011 off the Pacific coast Tohoku Earthquake. The site is an urban park about 150m wide by 150m long. The park area was a reclaimed land facing the Tokyo Bay. The surface evidences of liquefaction, observed by many visitors, indicated the whole area of the site had been being liquefied. To clarify the near-surface structure and its geophysical properties, we conducted detailed geological and geophysical surveying at the park. We adopted highresolution CDP S-wave reflection survey, and surface wave survey using the Land Streamer. All-core drilling was also carried out at 4 points in the park to identify the liquefied horizons. Accordingly, a low Swave velocity layer was identified in the reclaimed fills about 3 to 5 m in depths, where fine grained silts were predominant but the deposition structure was strongly disturbed, or fluidized. High-resolution Swave reflection profiles successfully delineated the boundary between artificial layers and natural sediments at about 8 m in depth. Liquefied layers were featured by negative pore water pressures and significantly low water injection pressures in CPT and HPT profiles respectively, indicating the existence of high permeability zone. It is capable to trace the liquefied horizons on the S-wave seismic reflection profiles.