Abstract

Some case histories surveyed by the authors are reported on liquefaction occurrence along the Tone River basin in Japan, caused by the 2011 off the Pacific coast of Tohoku earthquake. At the liquefied sites along the mainstream Tone River basin located in areas between Abiko and Katori cities in Chiba prefecture and those on the opposite side in Ibaraki prefecture, thirteen samples were retrieved from sand boiling. Investigation of past geomorphology and land use history and analyses of gradation of boiled sands suggested that most of the liquefied sites were reclaimed lands, which used to be old river beds, wet lands or swamps before reclamation. The boiled sands consisted predominantly of fine sand particles with limited amounts of fines, which originated possibly from the reclamation materials that had been dredged in the Tone River. In confirming the past geomorphology and land use history at some of the investigated sites, survey results conducted in about 1880 had to be referred to, since the past-swamps disappeared as of 1928 due possibly to earlier reclamation. Some other case histories of liquefaction along the mainstream Tone River basin suggested that not only reclaimed low lands but also natural deposits in back marsh area and filled soil layers in hilly area liquefied by this earthquake. At the liquefied sites along the Kokai River basin in Ibaraki and Tochigi prefectures, which merges into the mainstream Tone River, both sandy soils and gravelly soils boiled out. The latter soils included coarse grains that reached 20 cm in longer axis dimension. Based on field observations and gradation of boiled soils, it was inferred that the boiling of the coarse grains was caused by high excess pore water pressure generated in the sandy soil layer, which was possibly located below the gravelly soil layer and liquefied by this earthquake.