The Mechanisms for Initiation and Motion of Chinese Loess Landslides

Abstract

Loess is a wind-blown sediment with characteristic loose structure and low water content. Moisture content and confining stress are the key factors controlling the mode of its deformation and failure. CU tri-axial tests suggest that in low moisture content and low confining stress, loess fails by cracking with the development of vertical fissures while if fails by shearing in moderate moisture content and moderate confining stress, with a decline shear plane. Also, loess fails by slow creep in high moisture content and high confining stress. Therefore, the failure mode of the loess in a critical slope varies with the moisture content and confining stress which defines the initiation of a loess landslide. Loess landslides can be recognized as four types: rapid long run-out slide, rapid flow slide, slow creep slide and quick slump slide. The mechanism of the landslide movement is also controlled by the moisture state on the sliding path. The rapid long run-out slide is that the sliding mass slides on a saturated loose ground, where the pore water pressure caused by undrained loading and undrained shear inside the ground is much higher than that inside the sliding mass. Therefore, the shearing may take place in the ground because of liquefaction of the loose material, such as sand and gravel, which may cause a rapid and long run-out movement. The quick flow slide is that the saturated sliding mass slides on an unsaturated or dry ground, which produces a lower or even negative pore pressure, a film of fluidized soil is stuck on the ground, and the slide occurs in the sliding mass. It also causes a quick movement because liquefaction occurs at the base of the sliding mass. The slow creep slide is that the sliding mass slides on an impermeable sliding bed, such as red clay or bedrock, where the pore pressure at the slip path neither generate nor dissipate, so the driving force are almost equal to the resisting force along the slip path, and the movement could be very slow or off and on. The slump slide is that the unsaturated or dry sliding mass slides on an unsaturated or dry ground. Because there is a sharp reduction from peak strength to residual strength for the unsaturated sliding surface, the sliding mass has a quick fall and then stops at the toe of the slope where none or even a negative pore water pressure generated on the sliding surface because of water dissipation.