The Study Of Kinetic Mechanism Of Shawei Paleoseismic Landslide Located By The Jinsha River

Formed in about 40,000 years ago, the Shawei paleoseimic landslide has a enormous size of about 2.69×108m3 and possess the characteristics of high-speed and remote debris flow typically. According to the on-site field survey, the rock environment of the landslide is relatively stable because it is an counter-inclined rigid slope with a low-moderate dip, and there is no obvious weak interlayer found.Combinating today's landslide features, regional geological structure background and region strong-earthquake history to consider, we conclude it's a strong earthquake in the ancient time that triggered Shawei landslide. To determine the specific strength of that earthquake and the landslide's failure mechanism, we took the method of combining geological prototype with procedural mechanism analysis, followed the basic idea of “Geological Conditions Analysis ? Understanding the Deformation Phenomenon ? Numerical Simulation Verification ? Discussion on Genetic Mechanism”, carried out a detailed field survey and finally discussed the landslide's deformation-destruction mechanism under dynamic load through the discrete element software which is also known as UDEC.The main results are as follows:1. Shawei landslide locates in the southeast of Sichuan-Yunnan rhombic block, where the regionally tectonic evolution and seismic activities are controlled by several deep faults, that is, Jinsha River-Red River fault, Xianshui River fault, Anning River fault, Zemu River fault and Xiaojiang Fault. In 1833, a Ms=8.0 earthquake occurred near the study area. The landslide spot has the landform of canyon with middle mountain to alpine which is mainly caused by strong tectonic erosion, that kind of high and steep terrain has a significant topographic amplification effect, which would lead some naturally stabled mountains to become instable under strong seismic activities, especially for those mountains which have a good free surface condition.2. The rock environment of Shawei landslide is relatively stable because it is an counter-inclined rigid slope with a low-moderate dip and there is no obvious weak interlayer found. Three groups of joints esixted widely in the landslide, which dip towards mainly SW and SWW direction at relatively high angles(39°~87°), they act as the landslide's rear, bottom and side boundary if there is a potential mountain failure to occur. There is also another group of joints which dip towards mainly NE at very low angles(6°~14°),it may cause stability failure easily together with the widespreaded interlaminar shear phenomenon between ground levels which is mainly a result of folding in the basement formations.3. We conducted a series of trial calculations for the landslide's dynamic instability process by applying different magnitude seismic loadings, found that the Shawei landslide would maintain overall stability if the maximum earthquake acceleration is lower than 0.4g; The landslide would disintegrate at any earthquake acceleration higher than 0.4g; The landslide would have a final accumulation shape in line with today's landscape when the earthquake acceleration is 1.3g.4. The root cause of Shawei landslide's damage is the shear failure of locked patches at the front zone, it is rather hard for the rocks there to resist the pushing force from the rear of the huge mountain, especially under such a huge dynamic load. It is worth noticing that the cavity structure at the turning point of slip surface plays a very important role during the whole process. The cavity provides space for the collapse of the rock mass from rear, which makes them disintegrate and fall rapidly and cause violent collision. A huge amount of energy transferred from the falling rocks to the landslide front zone through the collision and thus keep the landslide's movement going on. To sum up, the Shawei paleoseimic landslide's failure process under 1.3g seismic load can be divided into five stages : vibration compaction- toe limit equilibrium stage; toe stability failure- traction movement of the leading edge stage; rock in the rear cleavage and expansion- rapid buckling of slider body stage; rock and soil accumulation climb- superficial mass ejection stage; high-speed debris flow- remote migration stage.