Stress and fracture evolution of surrounding rock during mining above mined out area in contiguous coal seams

The integrity and bearing capacity of rock strata in the coal seam floor heavily affect the mining safety and efficiency at the working face under the state of mining above mined out area. Taking No.8201 working face at Caoduogou coal mine as the research background,based on the structure and load analysis of the roadway goaf in the No.11 coal seam under No.8-2 coal seam,the mechanical model of the coal seam roof pillar system is established under the assumption of elastic foundation. Then,the bending descending deflection function of the coal seam roof beam is deduced and analyzed. By regarding the working face floor as a semi infinite plane,this study presents the model of mining additional stress applied to the working face floor caused by the static load in different sections along the direction of the working face. The law of stress distribution at the different depths of the floor and the analytical solution of the damage depth of the floor are obtained. Subsequently,a numerical simulation is applied to simulate the distribution characteristics of surrounding rock stress,overburden rock fracture and its space time evolution in the mining process of No.8-2 and No.11 coal seam. The minimum thickness of stable rock formations between No.8-2 and No.11 coal seam is determined. Finally,the special borehole detection scheme is developed to investigate the formation of overburden rock fracture in the No.11 coal seam roadway goaf. The results show that ① the deflection function of the coal seam roof beam at the adjacent media interface is continuous,and the maximum descending of the coal seam roof beam at the intersection of the roadway goaf is 2 cm;② the balanced structure formed by the coal seam roof pillar system in the roadway goaf under the influence of mining stress at the No.8201 working face is stable. The minimum thickness of existing stable rock strata between No.8-2 and No.11 coal seams (Dmin1) is 14 m;③ special drilling detection indicates the minimum thickness of stable rock strata (Dmin2) is 12.2 m. Therefore,the stable rock strata at No.8201 working face floor is complete,the bearing capacity of rock strata is competent under repeated mining,which indicates some potential safety hazards of No.11 coal seams during mining above mined out area can be-eliminated.