A Study on the Apparent Friction Angle Mobilized during the Undrained Loading in Long Run-out Landslides

Abstract

Long run-out landslides have often caused catastrophic disasters throught the world. These landslides have low apparent friction angles during motion in comparison with the usual values of internal friction angle of soils and rocks. Using the ring shear apparatus, this paper experimentally investigated the behavior and the mechanism of shear resistance mobilized during rapid loading on soil layers, which is very likely to occur in long run-out landslides. After residual state was obtained by constant speed shearing at a low normal stress in dry soils, normal stress was rapidly increased while shearing speed was maintained. In some cases, the stress deviated from the failure line, and the shear state was changed from the plastic failure state to the pre-failure deformation state even though shear deformation was continued. It was found out that the ratio of the shear speed to the loading speed should be greater than a critical value for the plastic failure state to be maintained throughout rapid loading. The ring shear apparatus was improved so that the undrained loading tests could be implemented and the generated pore pressure could also be measured. The undrained loading ring shear tests for the saturated samples taken from the alluvial deposits of the 1983 Sale landslide in China, as well as for the torrent deposits of the 1984 Ontake debris avalanche, Japan were conducted. These tests produced a high excess pore water pressure during the undrained loading and reproduced low apparent friction angles mobilized in both long run-out landslides. In the undrained ring shear tests of the torrent deposits from the Ontake debris avalanche, a high excess pore water pressure was generated not only by normal stress loading, but also only by shearing keeping normal stress constant. It was estimated that those coarse and volcanic grains were crushed during shearing and it caused negative dilatancy to generate excess pore pressure. On the contrary, the sample from the alluvial deposits from the Sale landslide did not show the pore water pressure generation due to shearing probably because the long-transported fine silty grains were hardly crushed.