Study On Deformation And Failure Law And Stability Of Gently Inclined High Steep Limestone Slope Under Underground Mining Disturbance

Karst mountainous areas in Southwest China are characterized by strong karstification,vertical and horizontal gullies,steep terrain,deep valley and concentrated rainfall,which are easy to form large high and steep rock slopes.These slopes are often steep in the top and gentle in the bottom,hard in the top and soft in the bottom,covered with thick limestone stratum,karst fissures are developed,rock mass structure is broken,and large-scale collapse and sliding disasters are prone to occur under the disturbance of underground mining.Therefore,this paper takes a mining landslide in southwest karst mountain area as the research object.By means of field geological background investigation,indoor model test and numerical simulation,the deformation and failure mechanism of karst slope under underground mining disturbance is studied,and the influence of different mining conditions on the deformation and fracture expansion of slope underground mining is analyzed,the deformation and instability evolution process and failure mode of the slope under mining disturbance are revealed,and the stability evaluation model of slope is proposed.The critical research works are described as follows:(1)By means of field survey,data collection and UAV aerial photography,the typical collapse and landslide disasters affected by mining in the study area were carefully investigated,and the typical geological environment characteristics and mining activity characteristics of the study area were obtained.The slope is steep on the top and gentle on the bottom,and the lithology is hard on the top and soft on the bottom.The joints and fissures of the rock mass are developed,and the karstification is strong.There are several groups of deep and large structural planes developed at the back edge,which have the geological conditions for the occurrence of catastrophic collapse and sliding disasters.The large area goaf of underground coal seam provides the inducement for collapse and slide disaster.The process of collapse and sliding instability of the mountain has experienced three stages: Mountain subsidence-tensile deformation stage,slope cumulative deformation and fracture expansion stage,overall collapse and collapse and sliding stage.(2)The basic mechanical properties of limestone samples were studied.The results show that the uniaxial compressive strength,tensile strength,elastic modulus,Poisson's ratio,cohesion and internal friction angle of limestone are 75.6MPa,5.6MPa,36.7GPa,0.27,16.6MPa and 41.5° respectively.Under uniaxial compression and low confining pressure(5MPa)triaxial compression,the failure mode is brittle tensile splitting failure with the same axial loading direction,and the brittle failure characteristics are obvious.With the increase of confining pressure,the compressive failure of limestone changes from brittle tensile failure to ductile shear failure.(3)The experimental study on shear characteristics of limestone with parallel discontinuous discontinuities is carried out.Under the condition of direct shear test,the fracture surface of rock bridge is mainly tensile failure,while the fracture surface on both sides is mainly shear failure,and the tensile failure is mainly distributed in the middle of the fracture surface.The failure mode of the specimen changes with the change of structural plane inclination and normal stress.The shear failure characteristics are more obvious under the action of low inclination and high normal stress,and the tensile failure characteristics are more obvious under the action of high inclination and low normal stress.The shear strength of the specimen increases with the increase of normal stress,and decreases first and then increases with the increase of inclination angle of structural plane.Under the normal unloading shear test condition,except for the limited compression failure at the tip of the prefabricated crack,the rock bridges of all specimens are tensile failure.The failure of the two sides is usually tensile shear composite failure,and the tensile failure area is mainly concentrated in the middle of the failure surface on both sides.The failure normal stress first increases and then decreases with the increase of crack dip angle,increases with the increase of initial normal / shear stress,and decreases with the increase of unloading rate.The shear strength of the fracture specimen is obviously smaller than that of the intact rock,and its size is about0.3-0.4 times of that of the intact rock.It can be seen that the initial damage of the rock mass caused by intermittent fractures is very large.Compared with the direct shear condition,the shear strength under normal unloading shear condition is slightly improved.(4)Through the discrete element numerical simulation method,the deformation and failure mechanism of the slope under the condition of pillar retaining mining is studied.When mining downward,the toe of the slope first moves and subsides,and then the overlying slope undergoes cantilever tension fracture.The deep cracks expand downward,and the rock mass structure becomes loose.Then,the slope continues to sink and extrude,and the toe and middle of the slope extrudes.Finally,the rock bridge of the slope is sheared and the overall instability is caused.When mining upward,the overburden will bend and sink,the slope will move downward first,the fracture zone will pass upward continuously,and the slope will become loose.When mining exceeds the toe of the slope,the slope will reach full mining and continue to sink and squeeze,and the toe and middle of the slope will be squeezed under pressure,which will eventually lead to the shear of the slope rock bridge and the overall instability.When there is no coal pillar mining,because of the large influence range of mining,the mining cracks are directly transmitted to the surface,and the underground mining has a great influence on the slope deformation and failure.In terms of influence degree,the fourth working face of downward mining is the largest,the fourth working face of upward mining is the second,and the third working face of downward mining is the smallest.(5)Through the numerical simulation of discrete element,the deformation and failure mechanism of slope under the condition of coal pillar retaining mining is studied.When the coal pillar is retained,the mining fracture does not develop to the surface.Under the same mining sequence,the overlying rock fracture development degree and slope top settlement of long working face mining are significantly greater than that of short working face mining.Under the same working face layout,the overlying rock fracture development degree and slope top settlement of upward mining are significantly less than those of downward mining.When mining upward,the development height of mining induced fractures is smaller in the shallower part,while when mining downward,it is larger in the shallower part,which indicates that the influence of pillar retained mining on slope deformation and fracture expansion is much smaller than that of non pillar mining.Under the same coal recovery rate,the impact of upward mining on slope is less than that of downward mining,and the impact of short face mining on slope is less than that of long face mining.The larger the thickness of coal seam,the greater the deformation disturbance of slope caused by mining.Compared with short face mining,the change of coal seam thickness is more sensitive to the change of slope settlement in long face mining.When the dip angle of coal seam is 7°,the deformation degree of slope is the smallest,followed by 0° and the largest when the dip angle is 15°.(6)The evolution process and failure mode of mining instability of gently inclined karst slope are revealed.Under the disturbance of underground mining,the failure mode of mining slope is collapse tensile fracture shear(slip).It has experienced the evolution process of "goaf deformation,surface subsidence,slope top cracking and rock mass loosening,slope medium pressure shearing extrusion,rock bridge shearing,slope collapse and tension shear sliding instability",which can be divided into four stages: "mining disturbance stage,slope top cracking subsidence stage,deformation accumulation development stage,collapse sliding instability stage".