Failures of Artificial Valley Fill by Volcanic Soil and Its Undrained Cyclic Shear Strength

概要

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As a result of the 2011 Great East Japan Earthquake, five slope failures occurred in a residential area on artificial valley fills in Taiyo New Town, Yamamoto, Miyagi Prefecture. The site was constructed by leveling the hilly area and using the cut materials as fills for the valleys to provide foundation ground for houses. The fill material is sandy, derived from the weathering of tuffaceous sandstone which formed the natural ground. Each of the five slope failures was observed to occur at the boundary between the filled-up valley and the fill or on the shoulder of the embankment. The sandy fill material has a fines content of Fc=20%. Even when compacted at 90% degree of compaction, the fill material showed a very low internal friction angle and low cyclic shear strength. Moreover, the cyclic shear strength decreased with application of initial shear stress. Pseudostatic slope stability analysis using conventional strength parameters could not explain the slope failure, however it could when the dynamic strength was used to represent the soil strength at the slip surface. Thus, the slope failure in Taiyo New Town can be attributed to cyclic shear failure of the fill material induced by intense shaking.