Deformation Characteristics And Failure Criterion Of Stope Surrounding Rock In Upward Strip Mining Of Gently Inclined Shale Deposit

Shale,as typical sedimentary rock,widely exists in natural strata.The mechanical properties of the the shale rock stratum vary greatly affected by bedding and fracture development.The underground engineering structure in shale rock mass is prone to deformation and failure,and the failure mode is complex and changeable,which seriously affects the stability of the engineering structure.The mining of shale deposit is a typical underground structure engineering.The deformation,stress distribution,and failure mode of the surrounding rock of the stope are the main factors affecting the safe mining of the deposit.Therefore,with the help of physical similarity simulation test,numerical simulation test,and theoretical analysis,the study of stope deformation characteristics,stress state,failure mode,and support form is the key to the safe mining of shale deposit.Rock mechanics test and similar material ratio test was carried out,and shale similar materials with high similarity ratio and low strength were developed.Taking a typical gently inclined shale deposit as the background,the physical similarity simulation model of shale deposit was poured to simulate the upward strip continuous mining of " mining one room at an interval",and the deformation and failure characteristics of surrounding rock in strip stope of shale deposit were studied.The deformation of the surrounding rock of the stope has four stages: stable deformation → accelerated deformation → debris ejection →complete failure.The deformation of the stope roof presents non-uniformity and asymmetry,and the deformation of the free strip pillar is inclined along the ore body.The research shows that the non-uniform deformation of the reverse dip position of the stope roof,the lower end face of the free strip pillar,and the side slope area along the dip are significant,and are the key points in controlling the stability in the surrounding rock of the stope.Based on the theory of elastic-plastic mechanics,the relationship between the stress state parameters of stope surrounding rock and the direction of compressive shear stress was analyzed,and the anisotropy of shale in different principal stress directions was revealed.The results show that the stress state parameters of the surrounding rock of the stope decrease with the increase of the angle between the compressive shear stress and the vertical stress surface,and the shale presents anisotropy in different principal stress directions.Using Mohr-Coulomb strength theory,and based on the relationship between the maximum and minimum principal stresses of surrounding rock under the influence of continuous mining,the maximum principal stress,minimum principal stress,and mixed principal controlled stress path of surrounding rock strength were clarified,which provides a theoretical basis for quantitative description of surrounding rock deformation and failure.Based on the theory of the complex variable function,the mapping function from the boundary of the rectangular roadway to the unit circle of the complex plane was constructed,and the Burgers model was introduced to establish the viscoelastic solution model of stress and deformation of the rectangular roadway.The results show that the deformation increment(theoretical solution)of roadway roof solved according to the viscoelastic solution model of roadway stress and deformation is similar to the results of the physical similarity simulation test(after removing the abnormal value).The standard deviations of the two in the first to fourth-time steps are 2.5864,1.9259,2.3768,and 7.2805,respectively.Based on the theory of fracture mechanics,according to the strength characteristics of rock I-II composite crack and considering the crack propagation along the action surface of maximum axial stress,the calculation model of crack initiation angle was established,and the crack initiation angle intervals were determined as [52°,70°],[-90°,-70°],respectively.Two main modes of "main crack propagation" and "secondary crack propagation caused by main crack propagation bifurcation" of surrounding rock cracks in shale roadway under the influence of continuous mining were analyzed.The stress component model of Ⅰ-Ⅱcomposite crack tip was established,and the theoretical buckling failure strength of roadway surrounding rock was determined to be 1.43 times of uniaxial compressive strength.Among them,the average buckling failure strength of roadway roof in physical similarity simulation test is 1.53 times(1.28-1.76)of rock strength,and the difference between the theoretical solution and test results is 7.01%.Finally,combined with numerical simulation,the deformation law of surrounding rock in strip stope is compared,and the deformation and failure characteristics and support scheme of surrounding rock in stope were analyzed.The research reveals the deformation characteristics of the surrounding rock of the upward strip mining stope of the gently inclined shale deposit,analyzes the stress state of the surrounding rock of the strip mining stope,establishes the composite failure criterion and bolt support scheme of the surrounding rock of the stope,enriches the research theory of the stability of the strip mining stope of the shale deposit,and provides support for the mining engineering technology.