Study On Instability Mechanism Of Karst Mountain With Deep And Large Fissures Under The Influence Of Mining And Rainfall

Karst landforms are widely distributed in Yunnan-Guizhou Plateau and surrounding areas in China,with abundant underground coal resources and frequent human mining activities.Due to long-term tectonic movement and groundwater erosion,karstification develops in the area,and fissures develop in the mountains.In recent years,due to frequent human engineering activities,mining has a strong disturbance to the mountain,causing rock mass rupture and crack expansion,leading to the decline of mountain stability.The rainfall intensity and duration are high,which intensifies the groundwater activities and makes the rock mass more broken,and easily triggers the landslide disaster of disturbed karst mountain.Therefore,it is of great importance to study the deformation law and instability mechanism of karst mountain under the action of mining and rainfall.Combining the theoretical knowledge of mining,fracture mechanics,engineering geology,taking Pusa Coalfield in Nayong County,Guizhou Province as the research object,through theoretical research,field investigation,similar model test and discrete element numerical simulation,this paper makes a in-depth study on the classification of collapse and instability modes of karst mountain in Southwest China,the deformation response of karst mountain with deep and large fissures under the action of mining and rainfall,the law of fissure expansion,the characteristics of seepage field,the process of disaster preparation,the failure mechanism and the influence of deep and large fissures on the stability of karst mountain.The main research contents and achievements are as follows:(1)Combined with literature investigation,geological prospecting data and field investigation,the influencing factors and deformation and failure characteristics of typical karst mountains in Southwest China are summarized.Based on previous classification methods and case analysis of mining and rainfall landslide disasters in karst mountain areas,a classification system of southwest karst mountain instability mode combining slope structure、deformation mechanism and failure mechanics is established.(2)The model test of karst mountain failure with deep and large fissures under the mining and rainfall was carried out for Pusa landslide.Model material ratio is determined based on orthogonal test and mechanical test.the test scheme is determined according to the engineering situation and geological data,and the stress variation law,hydrological response,deformation characteristics and failure process are analyzed.The results show that the overlying rocks are distributed in a nearly trapezoidal fracture zone after mining,and the slope is sinking.After rainfall,the deep and large fissures are connected with the mining fissures,and the mountain collapses to the outside,forming a potential sliding surface.The deformation of slope experienced three stages: roof caving-sinking,cracking-toppling,and sliding-collapse.(3)Using UDEC software,the deformation law and failure mode of mountain with deep and large fissures under the action of mining and rainfall were studied.The results show that,with the mining of M10 and M14 coal seams,the overlying strata of the mountain move down towards the goaf,and the new cracks develop towards the slope surface.The height of fracture zone of overlying strata in working face increases with the increase of goaf,and the deep karst fracture expands to the lower slope.After rainfall,the overlying strata is connected with deep fissure.Under the action of water pressure,the deep fissure expands to the free face to form a potential sliding surface,and the karst slope collapses and slides.Through comparative study,it is found that mining plays a controlling role in mountain deformation and failure,and subsequent rainfall is the main inducing factor.Combined with the model test,the stress change,deformation law and failure state of the mountain are compared and analyzed,and it is concluded that the failure mode of Pusa landslide is tensile crack-toppling-sliding failure.(4)The models with different dip angles,lengths and numbers of deep and large fissures are established by using UDEC software,and the influence laws of various factors on the stability of karst mountain are analyzed.The results show that the smaller the fracture dip angle,the greater the segmentation effect on the upper deformation zone,the smaller the fracture zone height,the more serious the dumping into the slope,and the more obvious the dislocation at the bottom of fracture groove.The longer the length of deep and large fissures,the greater the lateral displacement of fissures,the more broken fissures,and the more obvious the mountain subsidence and slip.Different numbers of deep and large fissures cut the top of the slope into different rock masses.The more the number,the more rock bridges,the more broken and deformed the rock mass at the top of the slope,and the collapse and sliding failure of the mountain are easy to occur.