Monotonic and cyclic undrained behavior of Kumamoto-Aso pumice soil by triaxial and torsional shear tests
The Kyushu Island of Japan was hit by a series of moment magnitude, Mw= 6-7 earthquakes during April 14th-16th 2016, followed by hundreds of aftershocks. The earthquakes caused a significant damage to the build and natural environment over a wide area, triggering a number of geo-disasters in the Mount Aso Caldera, comprising a large-scale flow-type slope failure known as the Takanodai landslide. The Authors conducted a series of detailed field investigations in the affected areas. Field observations indicated that the Takanodai landslide was a mobile earth slide that developed into a flow-type slide on a low angle of slope (10-15o). In an attempt to provide insights into the failure mechanism associated with the Aso pumice soil, undisturbed and disturbed samples of volcanic soils were retrieved to be characterized in the laboratory. This paper reports on preliminary results of monotonic and cyclic undrained triaxial and large-strain torsional simple shear tests. Undisturbed sample were tested using a triaxial apparatus, while in the hollow cylindrical torsional shear device the tests were performed on reconstituted samples prepared by a special wet tamping method. The test results revealed that Aso pumice soil has the tendency to show a flow-type failure behavior characterized by an abrupt development of large shear strains when subjected to monotonic shear loadings. On the contrary, under cyclic shear stress condition, a progressive build-up of excess pore water pressure is the precursor for a flow-type failure with a rapid development of large shear strains.