Study On Formation Mechanism Of High-speed Landslide Controlled By Intensively Developed Karst

At 3 pm, on 5 June 2009, a large scale landslide happened at Cocktail Mountain of iron ore town, in Wulong County, Chongqing Municipality. It has buried 12 houses and the entrance to a working mine, caused 74 deaths and 8 injured. And it has become a rare catastrophic landslide event in recent years.This paper based on the mechanism analysis of the geological process, the quantitative evaluation of academic thinking and the principle of system engineering geology and by investigating the engineering geological conditions, collecting the data of Cocktail Mountain landslide and combing with indoor data processing, first analyzing the characteristics of landslide structure, the characteristics of karst and the deformation and destruction, and establishing a " concept model " of Cocktail Mountain landslide; studying the mechanical properties of rock mass in the landslide area and the development pattern of the strong karst boundary by the test method; Then quantitatively analyzing landslide formation mechanism by the test and three-dimensional numerical simulation. Major awareness and results are as follows :(1)Through the analysis of the data of the Cocktail Mountain landslide and the supplementary investigation, Cocktail Mountain slope is a small angle oblique anti-dipped high steep slope with soft foundation. The main strata from top to bottom in sliding source area are the limestone of Maokou formation, the limestone of Qixia formation, the argillite of Liangshan formation and the argillaceous shale of Hanjiadian formation. Karstification of MaoKou formation limestone in the upper of the slope develop strongly, and slip zone is a carbonaceous weak intercalated layer which is a transition zone between upper and middle Qixia formation. The strata of LiangShan formation in the lower of slope contain iron ore. There are two groups of orthogonal and steep structure planes in the slope rock mass. And they respectively control the spatial distribution characteristics of the backward edge cracks and the karst cracks on the left boundary of slope. In the front of the sliding edge area strong karst zone, the vertical ditch of karst, karst fracture and dolina strongly develop, karst rate reached 60% ~ 70%.(2)Supergene reformation makes sliding mass along the sliding surface slip deformation, forming trailing edge crack and left side karst cracks. After the instability and failure of locked segment rock mass in the front side of the strong karst zone boundary, the landslide body in the front are sliding along the strong karst zone boundary shear sliding out at a high speed. Lose front block rock mass, the back of the main sliding body along the N25 ° E direction sliding out of the bed, will slide along the source area of the side are sparkling outstanding chipped away form the chipped area of rock mass. By analyzing the deformation and failure phenomena of the accumulation area and measuring the data of the sliding body movement,we can obtain the speed of the main sliding body sliding out cut exports is 27.02 m/s.(3)Based on analyzing the basic characteristics of the landslide, it is known that the unfavorable geotechnical structure provide a bad geological basis for landslide formation; Inside of the front portion of the limiting factors of strong karst belt speed slide landslide; the lower part of the mountain is a large range of seam mining landslide predisposing factor. The formation and evolution of landslide are divided into three stages: Supergene reformation stage: under the valley erosion and long-term infiltration of surface water, the slope along the two original structure planes form relaxed-Tension crack corrosion fracture zone, the rock mass in the front side along the catchment favorable terrain form karst zone, resulting in the sliding body along the lower part of the weak intercalation slip tension crack deformation; The formation of locked segment : the lower part of the mountain mined-out area gradually expanded, resulting in deformation of the main slider sharply decline, promoting the development of strong karst zone in the front area and formation of locked section in the strong karst zone boundary. With the long term deformation process section rock has accumulated a lot of strain energy; Locked section shear failure-landslide speed start-up phase: when the locked section rock slide bear move shear force exceeds the shear capacity of rock, rock instability failure, rock accumulated in the high strain energy released instantaneously, causing landslide along the inside of the front of the border strongly karst formation of high-speed slide landslide.(4)According to the research,we need to develop stress- corrosion test equipment to carry out stress tests coupled dissolution study the effect of stress on the role of carbonate karst erosion from both qualitative and quantitative aspects. The results are as follows: Maokou formation limestone and Qixia formation limestone karst degree in the slide source area of Cocktail Mountain landslide have difference, in the same stress condition, the Maokou formation limestone dissolution rate is three times as much as the Qixia formation limestone dissolution rate. Stress through increasing the activation energy of the solid reactant and solid-liquid contact area in reaction system to promote the dissolution. Through the analysis of test results, limestone solution tend to increase with stress which exerted on two ends of sample;the dissolution rate of rock specimen was raising with the increase of the stress. When the stresses exceed 7MPa the corrosion rate increment becomes larger. The dissolution of carbon dioxide bubbles also has positive effect on carbonate dissolution, mainly manifested in the increase of solid-liquid contact area and enhances the high stress area of stress concentration degree of these two aspects.(5)Three dimensional numerical simulation analysis results can be drawn:?.The initial stress field that is not developed karst slope and the lower part of the mountain mining activity did not occur, the slope is in a stable state. The development and evolution of the front strong karst zone changed stress state of the slope, and the unloading zone appeared in the position of the landslide boundary. The joint fissure in the unloading zone forms the trailing edge and the left karst boundary under the long-term rain water, which isolate the sliding body and the stable mountain. After the formation of karst belt, along the weak layer slider body to decline, showing a rear sliding body driven features. Slider appears small plastic zone, mainly in the inside of the front portion of the slide body strong karst belt near the slope is still in a stable state.?.Mining activities, under the formation of karst border, makes the upper slope rock mass stress state change, further speed up process of slope deformation and destruction. The deformation and failure of the upper part of the slope and the position of the lower mined out area are consistent in time and space, and the deformation and destruction of the upper rock mass over gob is the biggest. With the mining process the sliding source rock displacement vector direction gradually toward the free direction of deflection, especially the rock mass in the front of landslide body. When the mining to the front slide below, front sliding displacement vector direction is N35°E, the landslide slip direction is N25°E. With the increase of the gob area, the plastic deformation range of the rock mass in the upper part of the slope is gradually increased. And after the gob area take shape, the front locking section rock occurred shear failure, which leads to the instability of the landslide.(6) Combined stress-corrosion tests and three-dimensional numerical simulation analysis shows that the evolution of slope deformation, landslide along with weak downward slip deformation squeeze inside of the front portion of karst rock band, so that the formation of local high stress karst belt area, enhanced corrosion. Mining the lower part of the slope, resulting in the upper sliding body sliding down the weak layer deflection, and karst rock band stress concentration increased, thereby further enhancing the dissolution of rock, karst formation rate the strong karst belt reached 60% to 70%, and the mechanical properties of rock mass decreases accelerated slope deformation and failure process, eventually leading to the landslide instability. The lower part of the role of mining mountain slope deformation and failure were accelerated, resulting in the formation of landslides; landslide Karst border and inside of the front portion of the slider with strong karst direction of movement played a controlling role.