Analysis Of Formation Mechanism And Motion Mechanism Of Ermanshan High-Speed Landslide Under Heavy Rain

In recent years, many large-scale high-speed landslide activitied happened in the world, which had brought great disasters to human because of their large size, occur suddenly, move fast and strong damage potential. Due to the high-speed landslide induced by rainfall intensity is more common, its formation mechanism and the motion mecha-nism has been concerned by the academic and engineering circles, and it is also a difficult problem in the geological disaster prevention.At wee hours of July 27,2010, a landslide-debris flow called Erman-shan landslide occurred on the backhill of Wangong town of Hanyuan city in Sichuan province because of heavy rainstorm.It started in high positon and moved at a high speed to long distance(about 1.4 km).The whole volume was about 48 million m3 when it started,entraining and scraping loose material accumulated in the gully bed and on both sides of slopes during its motion. When the landslide moved about 548m, it impacted the limestone rockmass on the left in the middle of the valley fiercely, and the lanslide was divided into about three parts.one part stopped and accumulated because of the strong collision, one part straightly threw over the channel and killed 20 villagers located on the left bank of Shuang-he village, the other part of debris flow deflected about 18° to the west and continued to flow down about 700m at high speed, filling the original bed and drainage facilities, and finally stopped to accumulate on the flat hillside behind the town.Half an hour later after the main slide, the secondary landslide of accumulation occurred in the large trench under the action of gravity, and moved farther to about 908m because of the elevation effects of the main landslide, eventually leading to part houses in the Wan-gong town destroyed. The Erman-shan landslide has double characteristics, named high-speed long-out and short- range. The source area to the collision(actually part of the whole landslide) can be studied as short-range motion stage, while the area between the collision and the accumulation can be considered as long-out stage. Therefore, The Erman-shan landslide was typical, and there is no doubt that further study to seek its law has great significance to enrich and improve the theory and practice of landslide dynamics.In this paper, The Erman-shan landslide induced by strong rainfall was taken as the research prototype, and comprehensive study on formation mechanism and motion mechanism of high-speed landslide under heavy rain was developed, based on detailed investigation and collection of the geological data of landslide. The main research results and innovations are as follows:1. The selected typical high-speed long-out and short-range landslides were depicted in detail, to summarize their common characteristics of the high-speed long-out landslides and short-range landslides separately. The study attempted to analyze the formation reason of the landslide caused by heavy rain from the geological background and formation conditions, and to seek the difference of the motion between the high-speed long-out landslide and short-range landslide.2. A perspective of "high-speed landslide induced by heavy rainfall is mainly due to the destructive effect chain after rainfall infiltration during the rain" was proposed, based on the analysis of initiation mechanism of the landslide under heavy rain.The" destructive effect chain " included about four effects,namely, high energy effect,starting effect, acceleration effect and low resistance effect on the sliding surface. The four effects complemented each other, together triggerring the high-speed landslide starting at a high speed. On this basis, the disaster model was put forward and the critical condition of landslide initiation was derived under the analysis of the stability of the landslide slope.3. In fact, the high-speed landslide experienced "start-acceleration-setout" activity during its departure stage, and starting velocity and set-out velocity was clearly distinguished in this paper. The relationship between the two velocity was also discussed and then their calculation formula were derived under the "bending beam" model.4. After accurately depict and analyze the long-out and short-range motion process of Erman-shan landslide, the specific motion effects of high-speed landslide during its motion process were discussed in detail, including mass ejection effect, the shoveling and scraping up effect, collision effect among the block bodies of debris flow, collision effect between debris flow and the moutain,and the base "roll" effect to long distance of debris flow.Then geological characteristics and stability of deposit under different motion were compared.5. Particle size analysis of the accumulation along the motion process of Erman-shan high-speed landslide in field were done. According to the statistic results of particles analysis at different positions in the longitudinal and transverse direction, the plane distribution of the accumulation and the variation law of each size particles at different motion stage were explored, to verify the theoretical kinematics mechanism of the high-speed landslide.6. This paper simulated the formation process and motion process of landslide using the advanced UDEC numerical simulation software to verify and enrich the theoretical results of formation mechanism and the motion mechanism of the high- speed landslide induced by strong rainfall.