Study on zone failure characteristics of top-coal in fully-mechanized top-coal caving mining of thick coal seam

Top-coal failure mechanism is a basic theoretical problem in fully-mechanized top-coal caving mining,and the study of the progressive failure process of top-coal can lay a foundation for the accurate identification of caving. In order to improve the prediction accuracy of top-coal caving,this paper studied the fragmentation characteristics of top-coal in Shanxi Xinliu Coal mine by means of theoretical analysis,numerical simulation and in situ measurement. The hydraulic support was simplified as a constant stiffness spring to establish a top-coal mechanical model. With the help of the Ritz method and the principle of minimum potential energy,the displacement field and stress field of the top-coal in front of the working face and above the top control area were obtained,and the initial in-situ stress and excavation unloading were considered. According to the mining stress distribution characteristics,the top-coal failure risk coefficient was defined as the difference between the radius of the stress circle and the vertical distance of the strength curve,so as to realize the quantitative characterization of the risk of top-coal failure; according to the size of the normal stress on the failure surface,the shear failure was divided into two forms：compression shear and tension shear,and the transition conditions between the two shear failure modes were derived; Taking the top-coal failure risk coefficient k=0 contour line,the horizontal stress σh=σts contour line and the parabolic tensile shear failure boundary line above the top control area as the boundary,the top-coal was divided into four stages：elastic state → compression shear failure → tensile shear failure → tensile failure. The progressive failure process of the top-coal in fully-mechanized top-coal caving mining was simulated and analyzed,and the top-coal crushing zone was extracted with the help of FISH language. The numerical results were in good agreement with theoretical analysis except that a shear-tension mixed failure region appeared between extension shear and tension failure scopes. Finally,the development characteristics of top-coal cracks were measured. The front of the coal wall was a closed shear crack,and the back of the coal wall was a closed shear fracture. It was an open-type tensile fracture,which was consistent with the regional fracture characteristics of top-coal obtained by theoretical analysis,which verified the accuracy of the top-coal subregional failure model.