| Tanggudong landslide lies at the right bank of upstream section where is between Liqiu River estuary and Menggushan. The whole large-scale slipping destruction occurred on June 8, 1967. Within 5 minutes, the landslide slides into the Yalong River at a high speed and rushes to the other side. In this process a giant landslide dam is formed. In the 17th, the distance of the backwater area reaches at 53km, and reservoir capacity is 6.8×108m3. After its break, it causes a very large flood and water level rises up to 48m near 10 km to the downstream of the dam, which causes incalculable loss. After the dam's break, residual deposits of the landslide are mainly distributed in the middle and the front, which amount is up to 4,000 m3. Meanwhile, horizontal length of affected zone (A zone) where lies in the upstream side of the landslide is about 300m, and its vertical length is about 1180m. Signs of deformation and failure of the landslide are obvious. Therefore, researching on the formation mechanism of Tanggudong landslide has an important influence on analyzing evolutionary stage of slopes of zone A. In addition, stability of deposits and zone A is also an urgent problem. On the basis of clarifying geological environment conditions of the landslide and engineering geological conditions in the region, this paper summarizes basic characteristics factors and structural characteristics of landslide and analyzes formation mechanism of the landslide in the help of physical modeling and numerical simulation. Finally, using of numerical simulation software and limit equilibrium theory analyzes and evaluates stability of various districts of the landslide. Specific contents and results are as follows:(1) Tanggudong landslide occurred in the Triassic zhuwo group (T3zh), which is a set of middle or thick layered metamorphic rock, and the stratigraphic occurrence changes to N27~39W/SW∠30~51°owing to slight metamorphism. Under the action of subjecting to strong tectonic movements and the intrusion of granitic-pegmatite veins, there are different scale and heterogeneity faults and joints formatted in the rock mass. In addition, with the continuous uplift of crust and continuous cutting of the Yalong River, slope becomes more and more steep and transformation of superficial rock mass becomes more significant, which leads to deterioration of quality of rock mass. So, they lay the structural foundation for evolution of slope deformation.(2) After occurring of the landslide, the residual slopes are steep and they subject to strongly epigenetic alteration, which always induces collapses and landslides. Main features are as follows:①there are many fractures and multiple preferred planes in the bedrock. So they can form potential instable blocks.②Collapses usually occur in these loose deposits because of their poor cementation.(3) This paper completes simulation of deformation process of Tanggudong landslide using the bottom friction tester. Simulation results show that:①Tanggudong landslide is a giant and rock landslide, which deformation and failure model is slipping-cracking. It cuts off layers and moves along the ladder-like sliding surface which is composed of the steeply inclined outside joints and gently inclined outside joints in the strongly weathered layers.②During the long process of natural history, instability and failure of these front slopes firstly occur. Deformation and crack of these front landslides provides deformation space for the upper landslide in the rear. In the end, with the continuous deformation of the whole upper slope its instability and failure ultimately occurs.(4) Meanwhile, this paper also uses the discrete element software (udec) which is used to simulate starting process of damage evolution of the landslide under natural conditions. Simulation results show: the formation mechanism of Tanggudong landslide is slipping and cracking which sliding surface is along combinational joints within the strongly weathered layers, which coincides to these results of on-site survey and physical simulation.(5) With the help of three-dimensional numerical simulation (FLAC3D), the author simulates stress and deformational characteristics of the landslide under natural and rainstorm conditions, results are as follows:①Deformational area and size under natural conditions are smaller than under rainstorm conditions. The maximum displacement appears at the landslide deposits of the zone C, which value is up to 16.5cm.②plastic yield areas are mainly located at residue deposits where are in the front of landslide and banks of gullies. The distribution of plastic zone is rather sporadic. Rock and soil damage is always cracking, and local damage shows shear or crack-shear failure characteristics.③Under natural and rainstorm conditions, there do not exist throughout increased area of shear strain increment and plastic yield areas in the landslide, which has good stability. But these rapidly changing sites of topography and poor physical and mechanical properties may causes local instability and failure.(6) On the base of the above analysis, this paper calculates the local and overall stability of various districts of the landslide using limit equilibrium theory. We can see:①After its occurrence, the overall stability of the deposit is better, but there mainly exist some small collapses and landslides.②Under natural conditions the deep residual deposits of its surface are in a stable state with better overhead conditions, but under rainstorm or seismic conditions loose residual deposits are easy to being destabilization, and the influence of stability caused by earthquake (Ⅷdegrees) is larger than by rainstorm.③The upper residual deposits of zone A and B where lie in upstream side of the landslide are prone to slide along interface between cover and bedrock under rainstorm conditions. These simulation results are consistent with the deformation signs of the landslide. In addition, under strong seismic action, the overall stability coefficient of cover is less than 1.0; its overall stability is poor.(7) Integrating site investigation, numerical simulation and theoretical calculation, the author evaluates the stability of various districts of landslide. The overall stability of slopes in the zone A ~D is better under natural conditions. Their stability reduces to some extent under rainstorm conditions, but they are still in stable state. While the local residual deposits on the surface is in a stable state with better overhead conditions. Under seismic conditions (Ⅷdegrees) the overall stability of slopes lows significantly, and they are at barely stable and less stable state. The overall stability of slopes which slip along the interface between cover and bedrock is poor, but they are unlikely to slip along combinational sliding surface in highly weathered layers.
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