Title: Liquefaction characteristic of alluvial soil distributed at Sawara dry riverbed in Chiba prefecture
Abstract: Large-scale sand boiling has occurred at Sawara dry riverbed in Chiba prefecture by Tohoku Region Pacific Coast Earthquake. Alluvial clay and sand layer, which is considered natural soil, are distributed at this site. In this study, the liquefaction characteristic of alluvial natural soil has been examined. The alluvial soft clay layer is distributed from earth surface to 3m in depth. The alluvial sand layer distributed below the soft clay is in loose condition. The grain size distribution curbs of sand layers are very steep and are not vary, the sand layers are almost uniformity. The content of fine-grained fraction is 20% from 5%, and water content is 40% from 30%. In addition, N value is 25 from 5. The cyclic triaxial test has been carried out using the samples obtained by GS sampler. The liquefaction resistance ratio ,RL20, is in the range of 0.15 to 0.26, which is small value. The initial shear modulus, G0, obtained by cyclic triaxial test and the one calculated by the result of PS logging are almost same. The sample quality obtained by GS sampler can be estimated good condition because G0 calculated by PS logging is regarded the original in-situ initial shear modulus. It is considered that the thickness of non-liquefaction layer near surface ground is related whether the sand boiling will occur or not. According to ‘ Guidance of the Countermeasure for Liquefaction in City Area provided by Ministry of Land, Infrastructure, Transport and Tourism, as the thickness of non-liquefaction layer becomes thinner, the sand boiling is easily to occur. In this guideline, if the clay layer is very soft, whose N value is less than 2, and artificial soil, it is not able to count to the thickness of non-liquefaction layer. But, it is not commented that when the soft clay layer is natural soil, it is able to count to the thickness of non-liquefaction layer or not. It is very important issue to estimate the sand boiling phenomena of natural soil.