Critical states observed in triaxial compression tests on volcanic pumice soil related to debris flow in the 2008 Iwate-Miyagi Nairiku Earthquake

The Iwate-Miyagi Nairiku Earthquake (M7.2) that occurred on June 14, 2008, caused a slope failure in Dozo-zawa River near the summit of Mt. Higashi-Kurikoma. The debris flow caused by the slope failure flowed for about 10 km and took the lives of 7 people. The mechanisms of the slope failure and long-distance debris flow have not yet been well understood. In this study, a series of triaxial compression tests was conducted using a highly crushable pumice soil collected from the collapsed area, and shear behavior and particle breakage were discussed. Both reconstituted specimens and intact specimens showed contractancy and reached critical states that implies high flow potential. However, intact specimens preserved looser structure during consolidation and showed stronger contractancy during shearing than the reconstituted specimens, due to their natural soil structure. Significant particle breakage was observed in the tests with reconstituted specimens.