Kinematic Process Simulation Of High-Speed Long Runout Landslide In Jiweishan

A massive landslide occurred in the Jiweishan Mountain of Tiekuang Township, Wulong County, Chongqing on June5,2009. About500million m3limestone which has been cut into building blocks by the discontinuity slid along the gently dipping weak shale interlayer. After blocking the front part of Tiejiang Gully which was approximately200m wide and50m deep, the landslide materials with a high-speed movement formed a deposition area with the average thickness of about30m and the longitudinal length of about2200m. And it has become a rare catastrophic landslide event in recent years for this accident has buried12houses and the entrance to a working mine and resulted in10deaths,64missing and8injured.Soon after, in the afternoon of June28,2010, a disastrous landslide caused by a sudden heavy rainfall happened in Dazhai Village of Gangwu Town, Guanling County, Guizhou Province, putting the death toll at99people. And this geological disaster also presented the characteristic of high-speed long runout sliding. The occurrence of these two high-speed long-distance landslide events has been the prelude of the research on western region’s complicated landslide disaster mechanism and high-speed long-distance landslide formation mechanism.On the basis of summing up the issues related to movement characteristics and mechanism of high-speed long runout landslide. And starting from the regional geological background and engineering geological conditions of Jiweishan landslides, apply the basic principles of the discrete element method and the ITASCA geotechnical engineering numerical software PFC3D to investigate the controlling factors such as topography of the high-speed long runout landslide and volume effects. Finally the high-speed long runout landslide’s movement process was discussed in-depth which occurred in Jiweishan, Draw the following seven conclusions:1, The investiment of the engineering geology and the process of deformation and failure reveals:The unstabilization of key block was induced by multiple factors, such as dissolution, creep of weak page rock, incision of two clusters of steep discontinuities and mining. That consequently led to the landslide.2, The principles and features of Discrete Element Method (DEM) and the software PFC3D are presented here. For instance:1it does not limit the displacement.2take Newton’s Second Law and Law of Force and Displacement as its basic equations. Owing to these features, DEM can suit very well for the high-speed long-distance landslide with great displacement. Furthermore, the methodology of both macro and micro parameters matchs in PFC is studied.3, From the comparison between the landslide processes with V valleys of different slope angle (i.e.0°,30°,45°and60°), it is concluded that:(1) in the flat state, the materials of the landslide will be spread due to no lateral limit, and with strengthening of the lateral limit (both sides slope angle become larger), slope material is limited to a narrow range;(2) can be seen increasing slope, the slope material and the center of gravity movement distance will continue to be less small,(black part presents the bond which do not rupture, which is the larger then the block greater is), the chunk of the body also accumulate in the rear;(3) named the ratio of collision energy (collision energy) and work the total body force(which actually is the gravity) done λ, on both sides of the slope angle of0°,30°,45°,60°, the ratio is as follows0.3239,0.2974,0.2817,0.1987. With slope angle increasing, the collision energy consumption reduced, actually, collision chances are reduced. By contrast the friction energy consumption part shared become larger and larger. Apparently a reality under the same conditions (non-scrolling of the main movement), the greater the friction energy consumption and then its movement (sliding) distance will be farther.4, Test of the volume can make the following conclusions:(1) with the increase in the number of blocks, the speed is more regularity, on the maximum speed of the front of the block as a whole were greater than the rear blocks. In addition, the blocks in front as a whole were more than10m, rather than the rear block were generally less than10m. Calculate energy consumption ratio of3x3x3,5x5x5model the values of λ was respectively0.187,0.212.(2) Can be seen that with the increase in the number of blocks, the aggregation effect of blocks become gradually realization. Proved that as the volume increases, the distal displacement of deposition body become greater, but the center of gravity of deposition materials (by computing the center of gravity position of the Y direction followed by13.6m>6.94m>5.38m) move shorter distance, to some extent confirmed the energy transfer model which proposed by G.H.Eisbacher(1980).5, By the discussion of topography on the different energy consumption can be summarized as followings:(1) low energy consumption terrain landslide materials were distributed along the valley, can be see that for the different blocks (a, low bond strength, block is small; b, bond strength, the larger block) situations of the deposition were different, for the narrow valley, the bottom was only twenty or thirty meters width, resulting in the big block deposited on the landslide’s sliding zone, but when the rupture into a more small piece of the body, part of the material can move through. It can draw one conclusion:the sliding mass volume or the size of the block must be matched to the landslide movement channel then the long-distance movement can take place.(2) Medium energy consumption terrain goes down wider and wider, blocks will move up to blocking boundary in finally.(3)The terrain of the high-energy which was T-shaped, landslide material slide down from the left side then encounter the right side blocking boundary, and deposited spread front and rear.(4) Through inspected to see that the values of A for low energy consumption, medium energy consumption, high energy consumption terrain was respectively0.3163,0.39077,0.4326which was increasing, actually that the frictional energy consumption reduced in order, followed by increase in the number of collisions.6. For the high-speed long runout landslide in Jiweishan we can draw the following five conclusions:(1) For large long-distance landslides, if the morphology of sliding surface along the rockslide movement path is determined, the friction coefficient of sliding surface and strength of sliding mass just significantly affect the distribution of sliding mass in deposited areas; the micro-topography would have more influences on the maximum displacement of landslide.(2) The whole process of the high-speed long-distance rockslide at Jiweishan mountain can be divided into several phases. The first stage is mainly the fragmentation of rock masses in source area, the second stage is the movement and run-out of fragmental blocks.(3) It is found that the velocity of blocks in surrounding regions of sliding mass firstly reach the maximum by monitoring of velocity and displacement. The blocks have the characteristics of second acceleration. Combining the velocity and displacement, it is considered that the sliding body at the upper edge of sliding mass is more likely to move for a long distance.(4) For the Jiweishan landslide, when the friction coefficient of sliding surface is0.05and the bond strength is medium(250-200MPa), the results obtained by PFC3D is in accord with the actual ones in the maximum degree. For such high-speed long-distance landslides,PFC3D has a good applicability, especially its three-dimensional deposition state and influential extent initially identified could provide a useful reference for disaster prevention and mitigation works such as determination of safe avoidance range.(5) The simulation of the landslide’s kinematic process with discrete element particle flow software PFC3D can provide a useful reference for disaster prevention and mitigation work and decisions.7. In the mechanism of high-speed long-runout landslide, should recognize the role of high potential energy which provide the energy basis for high-speed, but high-speed is not means necessarily long-runout, landslide’s long-runout movement that must be depend on the favorable terrain conditions.