Genetic Mechanism And Movement Process Of Baisha Village Landslide Debris Flow In Baisha Village,Leibo County

On the morning of July 2,2017,a landslide of clastic flow suddenly happened in the upper slope?elevation 1200-1380m?in Baisha Village,Jincheng Town,Leibo County,Liangshan Prefecture,Sichuan Province,which is located at E103 34 55.79and N28 15 41.04.The landslide-debris flow destroyed one house and buried three highways under construction as well as some farmland at the foot of slopes,and caused certain property losses.On the study of landslide development characteristics and genesis mechanism,it is of great significance to the prevention and control of landslides in later period and the development of follow-up projects.Based on the existing data,this paper identifies the development characteristics of landslide-debris flow by means of on-site investigation,visiting and inquiry,UAV aerial photogrammetry and on-site test.The structural characteristics of original rock are analyzed by line-measuring method and self-made small window-measuring method.The strength of original rock and sliding zone is obtained by in-situ shear test,Hoek-Brown criterion and indoor direct shear test.On this basis of the above,the influencing factors and genesis mechanism of landslide are studied.The change of shear stress and displacement before and after slope excavation is analyzed by FLAC^3D software and the evolution process of landslide-debris flow is simulated by PFC^2D software.The following conclusions are obtained:?1?The width of landslide-debris flow is about 340-400 m,with the longitudinal length of about 710 m and the total area of about 22.6×10⁴ m².The total volume of the landslide-debris flow is 560.0×10⁴ m³,which belongs to large-scale rock landslide.The deposits of landslide-clastic flow show different characteristics.The deposits in the sliding source area are pseudo-bedrock,while those in the scraping area and the stopping area are angular-subangular gravel intercalated layers.?2?The surface debris of landslide-debris flow has good sorting property which equivalent particle size is between 4 and 6 cm.The change of equivalent particle size and the content of boulders are the embodiment of the change of debris flow energy.The larger the particle size of debris flow,the stronger the ability to carry boulders.The maximum velocity of landslide-debris flow is about 28 m s^-1,and the equivalent friction coefficient is 0.481,which belongs to super-high-speed landslide but shows no long-distance characteristics.Volume effect and topographic constraints are responsible for the low fluidity of the landslide-debris flow.?3?The landslide occurs in the gray-yellow,brown-yellow medium to strong weathering compact basalt beds.The average spacing between structural planes of original rocks is less than 5.0 cm,which belongs to fractured-mosaic or fractured rock mass and constitutes the structural condition for landslides to evolve into clastic flow.?4?The cohesion of the original rock is between 0.399-0.676 MPa,which internal friction angle is between 21.6-30.8 degree,as well as the tensile strength is between0.003-0.014 MPa.The overall mechanical properties show the transitional nature between soil and rock mass,which is an important reason for the seat sliding along the lithologic interface in the middle of the landslide.?5?The external factors of landslide-debris flow in Baisha Village,Jincheng Town,Leibo County are slope excavation,geological structure and topography.The internal conditions are lithologic structure,which the rainfall has a certain promoting effect.The formation mechanism of landslide-debris flow can be summarized as three stages:?1?formation of medium-strong weathering fragmentation-mosaic fragmented rock mass in the surface layer,?2?slope excavation causes progressive weakening of the leading edge anti-sliding force,?3?deformation of the whole lower seat of the landslide and shearing of some rock mass in the leading edge to form debris flow.?6?The FLAC^3D numerical simulation results of the mechanism of landslide formation shows that the natural state of landslide,excavation state and the state of first degree landslide after sliding undergo the process of displacement increment concentrating from the front to the whole sliding as well as the shear stress increment concentrating from the front to the gradual penetration along the sliding surface.?7?The contact force field of particles in PFC^2D software is used to analyze the change of contact force between particles in the process of landslide movement,which shows that the contact force above the shear outlet decreases after the start of landslide as well as the contact force restores after the landslide is stable.The contact force is almost zero after the shear of particles,while the contact force restores after the front movement is blocked.The grain accumulation in the slip source area is thicker in the middle,while the cutting sections in the front and rear edges are thinner.The grain accumulation thickness from the shear outlet down to the accumulation area increases first and then decreases.The front particles of the debris flow and the first degree landslide particles form a mixed accumulation.?8?The monitoring results of designated particles in PFC^2D software show that obvious abrupt changes can be observed in displacement and velocity after the start of the landslide and the closer the particles are to the surface and front,the greater the displacement and velocity of the particles are.The change curve of frontal particles monitored shows an obvious acceleration and deceleration stage,which is quite different from other particles.From the side,it reflects that the landslide is a process of leading edge instability traction sliding,sliding resistance gradually declining,landslide occurring overall downfall,landslide reaching equilibrium.