Study on the Mechanism of Loess Landslides Induced by Earthquakes

Abstract

The Loess plateau located in the Northwestern China is an area of intense seismic activity and where many disastrous earthquake-induced landslides have occurred. For example, the Haiyuan earthquake (M=8. 5) of Dec. 16, 1920 occurred in Ninxia Province, China, triggered many large-scale landslides and resulted in more than 100, 000 casualties.
We sampled Loess soil from these landslide areas and performed laboratory tests to examine the mechanism of the landslides, using the cyclic loading ring shear apparatus and the cyclic triaxial compression apparatus. From these tests, the following results were obtained.
1) The cyclic triaxial shear tests could not reproduce the sliding surface, and it is unsuitable for the research of landslides triggered by earthquakes because the shear deformation after failure did not correspond with the landslide phenomenon.
2) In the undrained cyclic ring shear test of saturated Loess samples, pore pressure was accumulated cycle by cycle when the cyclic shear stress was added and a large excess pore pressure was generated instantaneously after the sample failed. The sample was sheared continually after failure. From this test, it could be concluded that the occurrence mechanism of Loess landslide with high mobility during earthquakes is the result of the increase of excess pore pressure induced by cyclic shear stress under the undrained state.
3) In the cuylic ring shear test with dry Loess soil, the shear displacement stopped at the end of the loading of cyclic shear stress because the shear strength did not decrease below the initial shear stress. It can explain why the slope consisted of layer with lower degree of saturation degree can not form a landslide with high mobility of long distance and its displacement just occurred while the seismic force worked on it.