Formation Mechanism And Recognition Of Gentlely Anti-dipped Rockfall With Rigid Upper Layer And Soft Lower Layer

Collapse geological hazards,mainly induced by extreme weather and human engineering construction,are closely related to structural characteristics of the slope. There are a large amount of gentlely anti-dipped high-steep rock slopes with rigid upper layer and soft lower layer in Yun-Gui Plateau of China. Coal seam developing in underlying soft rock layer of this kind of slope often suffers excavation,leading to collapse hazards,which we call the gentlely anti-dipped rockfall with rigid upper layer and soft lower layer.On April 16,2013,the mountain in Chahe group,Yudong villege,Longchang town,Kaili city collapsed with the accumulation volume over 600,000 m3.Study on formation mechanism and development conditions is of referential values for stability evaluation of similar high-steep slopes of western mountainous area and nearby the research area. First,based on field investigation results,the engineering geological condition and geological prototype features of research area were analyzed. Second,conceptual model of formation mechanism of Longchang rockfall 2 was established combined with collapse characteristics. Third,failure mechanism and physical and mechanical parameters of rock were analyzed based on rock mechanical test. Finally, formation mechanism and development conditions of the rockfall were quantitatively analyzed by numerical simulation.The main achievements of this paper are as follows:(1)Longchang rockfall 2 slope is a very steep high slope with rigid upper layer and soft lower layer.Generally,the slope whose strike direction of the cliff face of about 210 m in hight is NNE,has gradient between 85~86.The rock bedding in the slope body is single attitude and dips slightly inwards the slope.The hard rock formation whose lithology is limestone consists of Qixia formation,Maokou formation and Yaosuo formation.Rock stratum of Liangshan formation dominated by mud shale and coal seam is classified as soft rock formation.There are two to three structural planes in overlying hard strata,and one of them is parallel to the cliff face with high dip angle.Carst development leads to clear corrosion phenomenon to the jointed rock mass with large long fissure,which amplifies the fracture trace length and fissure aperture. The large long corrosion fissure about 95~126 in maxmum depth is located in the left side of cliff,and becomes increasingly short to the right cliff.(2)The falling accumulation with the outline of irregular oval shape is estimated about 600,000 m3,and its maximum thickness is at the foot of the high slope according to accumulation features.It is inferred from spatula mark that the accumulation on the left with poor integrity of rock block comes from left side of collapse cavity.On the contrary,the accumulated rock block on the right which comes from right side of collapse cavity presents the good integrity. Bedding plane of block stone in central accumulated zone is evident. The rest of rock bridge left in the rock wall is fresh after fracture. However,other parts in the wall is more or less covered with rust stain. Rock mass on the sides of corrosion fissure mainly is destroyed in the manner of tension and shearing,however,the rock mass in the bottom of the corrosion fissure shows compression and shear failure.(3)Formation mechanism of Longchang rockfall 2 was established. A majority of fissures transfixion of steep structural plane at trailing edge due to erosion and unloading leads to small portion of rock bridge connection at slope shoulder and the base supporting the rock wall. Coal seam mining in Liangshan formation causes deformation and failure of roof rocks in the vicinity of goaf and tows stratum around goaf to be out of shape,leading to further stress concentration both at the top and bottom of rock wall,which forms locked segment that sustains stability of the slope. After slope deformation entering accelerated creep stage,the slope begins to evolve progressively. Rock bridge at slope shoulder gradually becomes weakening and cracking. Local small pieces of stones fall towards the base of slope. As rock mass at the bottom of the rock wall has suffered vertical stress,it takes the lead in buckling and crushing out of the slope while the stress exceeding its ultimate bearing capacity. And then,the rock wall starts to topple outwards after falling some distance as the loss of support at the heel of it.(4)Through direct shear test of limestone specimens,the peak friction angle and cohesion which are equal to 50o and 21 MPa respectively were arrived. The shear strength parameters and failure forms show that the shear damage properties mainly hinges on the differences in the structures and properties of the specimen and normal stress. The effect of increasing loading rate on mechanical properties of limestone specimens was studied through routine uniaxial compression test. The results suggest that both elastic modulus and Poisson's ratio,in general,tend to increase with the increase of stress loading rate. Under low loading rate,the deformation modulus decreases with the increase of loading rate. Under high stress,the deformation modulus increases with the increase of loading rate,but the added value has upper limit. The uniaxial grading loading creep test on one limestone specimen was conducted to research the effect of instantaneous loading rate and stress levels on axial strain rate and rock microrupture law. In addition,long-term strength of 61.14 MPa and viscosity coefficient of 1.09 × 1017 Pa · s was obstained before establishing creep constitutive equation.(5)Seam excavation and vibration of shock from Longchang rockfall 1 result in accelerated creep deformation of the slope,and the deformation degree and deformation speed relate to fissure depth. The discrete element software UDEC was used to simulate the deformation evolution process of Longchang rockfall 2. The slope with a crack 126 m in depth was mainly analyzed appending crack of 96 m and 66 m in depth as the sensitivity analysis. Slope deformation and failure characteristics under the same external factors including coal seam excavation and vibration of shock were investigated. Of the three types of slope,the slope with a crack 126 m in depth shows the maximum deformation degree. The occurrence of large deformation in the slope with a crack 96 m in depth has been postponed,and the rock mass deformed is much more smaller in size. Dangerous rock mass in the slope with a crack 66 m in depth,however,has not developed to instability and failure.(6)Deformation and failure of high-steep slope is closely related to such engineering geological condition and geological environment as lithology,nature of structure,topography,geomorphology and hydrology,etc. The gradient of Longchang slope 2 is above 75o,and the elevation difference between the peak of cliff and river surface is about 210 m,which provides good empty face. The main lithology of slope is soluble carbonate rock,which leads to many beaded karst caves distribution at the top of the slope and large long corrosion fissure development at trailing edge. Excavation in soft rock of 0.7m in thickness and vibration of shock cause deformation and creep of the slope to acceleration. However,potential dangerous rock mass can still keep stable for a considerable time in the role of rock bridge. All these above can be a basis for identification of Longchang rockfall 2.