The pressure arching effect and distribution characteristics of fractured strata of single key layer under shallow buried condition

A large amount of thick coal seams with shallow depth and thin bedrock occur in Shendong mining area of China. The structure form and weighting appearance of roof strata of shallow longwall working face show some specific characteristics. In order to explore the load transferring characteristics and load bearing state of the fractured overburden strata in the mining area,in terms of the arching characteristics of transferred pressure in fractured roof under the condition of shallow thin bedrock,taking the fractured single key strata as research object,according to the typical compressed state of fractured strata of mining area during rotary subsidence,this paper discussed the transferring path and distribution characteristics of compressive stress in the key strata block and its contacting surface,and analyzed the forming mechanism of arching effect of concentrated compressive stress in key strata block. By building the mechanical models of hinged arch structure of fractured overlying strata and deducing the analytical solution formula for the arch axial force of corresponding arch structure of strata,the rotary contacting characteristics and stress transferring law of key blocks in periodic fractured strata were studied. Taking the typical mining condition of shallow coal seam with single key layer in Daliuta mine of Shendong mining area as engineering background,using the block discrete element software UDEC to build a numerical model of coal seam and strata,considering the condition that the working face advancing along the direction of major principal stress,the evolution characteristics of principal stress and structure form of fractured strata at different mining stages are simulated,and the distribution law of the load bearing area of initial and periodic fractured strata were obtained. The results show that the distribution of principal stress in the region of arch foot,arch shoulder and arch top conforms to the parabolic law of quadratic function,the principal stress of the arch foot decreases from the bottom to the top along the vertical direction,while the principal stress of the arch shoulder and the arch top increases along the vertical direction and the principal stress of the arch waist changes in a transitional way. By analyzing the fitting law of the major principal stress and the vertical stress distribution characteristics of the key parts of the concentrated principal stress zone,the load bearing conditions at the front and rear support area of hinged arch structure,step structure and voussior beam structure of fractured strata were identified under the arching effect of principal compressive stress.