Study of the Apparent Friction Angle After Failure During Undrained Shear of Loess Soils

Abstract

Large-scale landslides were often reported in loess deposits during earthquakes. These landslides were usually rapid with high mobility. It is very important to study their mechanism of motion for landslide hazard mitigation.
Samples were taken from loess landslides which took place during the Haiyuan Earthquake (China, 1920) . These were tested in a ring shear apparatus, and the apparent friction angle during post-failure motion was investigated. Because slip surfaces of rapid landslides are supposed to be almost under the undrained loading condition, laboratory tests were also done under undrained condition and samples were sheared for several meters of displacement. The main conclusions of laboratory tests are as follows:
1) Under undrained condition, the sample finally approached to a steady state condition, where shear stress, pore pressure and volume were constant, and only shear displacement was increasing. The apparent friction angle mobilized at the steady state was only 6. 5 degrees. This test result can give a theoretical and experimental explanation to the high mobility of earthquake-induced-landslides in loess soils.
2) It was found that the apparent friction angle after failure has no relationship with the OCR (over consolidation ratio) . It is shown that the apparent friction angle after failure is the strength of a new soil structure which is formed at the shearing zone during the deformation process, and the former structure has no influence on it.
3) The apparent friction angle after failure of unsaturated loess (which in the natural state with the degree of saturation of 32. 6 %) did not decrease during a large shear displacement. The unsaturated loess layer, therefore, can not form a long run-out landslide.