Soil Structure in Volcanic Pumice Soil of Dozou-sawa River Evaluated from In Situ and Laboratory Tests

Abstract Volcanic pumice soils are widely distributed in many countries and sometimes cause severe geo-disasters including large-scale slope failures and long-distance debris flows triggered by seismic ground motion. However, the understanding of the mechanical properties of volcanic pumice soils is still limited because volcanic pumice soils generally have a sensitive structure and are typically distributed in mountain areas that are sometimes difficult to approach. In this study, a portable in situ direct shear test apparatus was developed to evaluate shear strengths of a volcanic pumice soil without disturbing the natural soil structure. A series of in situ direct shear tests was conducted for the volcanic pumice soil in Dozou-sawa river, where a large-scale slope failure followed by debris flow occurred in the 2008 Iwate-Miyagi Nairiku Earthquake. A series of laboratory direct shear tests and triaxial tests were also carried out with intact and reconstituted specimens. The in situ direct shear tests and laboratory direct shear tests using intact specimens showed consistent shear strength. Intact specimens had an extremely loose and sensitive structure because of highly crushable porous particles and a very loose soil skeleton sustained by weak cementation between particles. Such a fragile soil structure could never be reproduced in reconstituted specimens. It was confirmed that the soil structure significantly affected peak shear strengths. For the evaluation of mechanical behavior of such sensitive soils, it is important to use as few disturbed specimens as possible. The newly developed in situ direct shear test apparatus was confirmed to be an effective tool to simply evaluate the shear strength of sensitive volcanic pumice soils on site under low disturbed conditions.