Mechanics Of High-speed Landslide And Its Surge Simulation In Mountainous Area

Catastrophic high-speed long-distance landslide can extend unsafe area as far as the landslide area，this extension is realized indirectly by the high speed motion features (such as shock wave, surge anddam-break flood) or realized directly by long-distance sliding. In this paper geological background andengineering geological characteristics of Tangjiashan landslide are expounded comprehensively and thedynamic instability mechanism of Tangjiashan original slope is analyzed detailedly under Wenchuanearthquake. For the first time, it is systematicly studied for the landslide slope stability under earthquake,the dynamic process of high speed landslides, air shock wave caused by high-speed landslide andhigh-speed landslide surge characteristics. The main conclusions are as follows:1) The terrain slope, lithology, geological structure and the slope structure characteristics areexpounded comprehensively for Tangjiashan before the earthquake, and it formation conditions ofTangjiashan landslide respectively are high and steep slope, near the seismogenic fault location, the typicalsteep slop and Wenchuan8seismic intensity.2) Dynamic instability mechanism of Tangjiashan original slope under seismic can be explained asfour apects:①amplification effect of seismic forces makes slope top in tensile shear stress state, when thecombined effects of stress is more than the original weak surface strength, micro fracture in the rock beginsto expand, and the emergence of macroscopic destruction forms the tension crack, then the tension crackand the original weak surface constitute the landslide cracking surface.②the broken rock entries into thecrack surface, which produces a "wedge" effect, tension stress concentration zone produces in front of thecrack and the crack surface expands.③the broken rock and debris broken by seismic tension expansionentry into the level, clastic bodies rotation and flow mechanism of debris make the rock level shear slipincrease.④locked deep rock mass of the slope (not weak surface) burst cut off, the deformation stored inlocked section can instantly released, and most of the deformation translates into tangential kinetic energyfor rock layers to slide flash boot.3) Tangjiashan landslide is series landslide under earthquake. The slope stability is simulatednumerically under the natural state (only with the gravity action), and the calculated safety factor is1.46,which indicates that the slope is relatively stable under the natural state. Secondly in Wenchuan earthquake, Tangjiashan landslide stability is numerically simulated with large amplitude seismic wave of initial30sand the free field boundary condition for the slope. The results show that the piercing plastic zone in slopeis produced in Wenchuan earthquake, and the simulation plastic zone is good agreement with the reallandslide slope body. Because of the earthquake acceleration, the slope will generate larger displacementand velocity, which produce tremendous energy in the landslide and cause serious disaster and loss.4) In order to ensure the integrity of the landslide, for the first time the dynamic process of TangJiashan landslide is simulated by finite element method. The horizontal speed of landslide along the slopesliding down can reach50m/s, vertical speed can reach30m/s, and maximum speed is at about20s afterthe earthquake. Landslide slope slides to the bottom, the horizontal sliding distance is900m, verticalsliding distance is300m, which are agreement with the measurement results (horizontal slip900m,vertical slip350m) of TangJiashan landslide after the earthquake.5) Tangjiashan high-speed landslide leads air shock wave producing more than12wind in front of the100m range, strong airblast exists to tear down the house, broken trees. At the same time landslide unsafearea calculation results show that air shock wave could cause damage in the range of kilometer for livingenvironment.6) For the first time high-speed landslide surge is studied theoretically. Because speed of the slidingbody is bigger than the wave propagation speed, only high-speed landslide surge wave appears, which isthe water rising under the pressure caused sliding body. The maximum height of theoretical simulation ofTangjiashan high-speed landslide surge is about60m, which is higher50%than the wave height40m ofthe estimation according to leave surge in river across the traces.7) In order to effectively analyze characteristics of high-speed landslide surge, high-speed landslidephysical model is established for the influence factors of landslide volume, landslide velocity and waterdepth, calculation results show that:①landslide volume is the biggest impact on high-speed landslidesurge height.②In the process of the sliding body in the water movement, water can be extruded by thehigh-speed landslide body and local water pressure increases rapidly, which is the fundamental cause of thesurge.Through this research of high-speed landslide and its surge, the timely and appropriate engineeringprevention and control measures can adopt to minimize the landslide and surge disasters, which canprovide a scientific basis for disaster prevention and mitigation decisions, so the safety of people’s livesand property can be ensured.