Undrained monotonic and cyclic torsional simple shear behavior of the Aso pumiceous soil deposits

The Takanodai landslide was a mobile earth slide caused by the 2016 Kumamoto earthquakes. Field observations suggested that the soil to cause the slope failure could be a thin layer of Aso pumice soil sandwiched between two layers of clay volcanic ashes. However, there exist debates among researchers about the actual failure mechanisms. In an attempt to provide new insights into this problem, this paper reports on preliminary results of monotonic and cyclic undrained torsional simple shear tests carried out on reconstituted specimens of Aso pumice. The test results confirmed that Aso pumice has the tendency to suddenly lose its shear strength and develop flow-type large shear strain exceeding 50% when subjected to both monotonic and cyclic torsional simple shear loading conditions. Moreover, in addition to high inertial stresses, it is the presence of an initial driving shear stress that significantly contributes to the flow-type failure and associated large deformation.