Scale-span evolution law and mechanical mechanism of surrounding rock cracks in repeated mining roadway

In order to explore the damage degree and deterioration characterization of surrounding rock affected by repeated mining in double roadway layout working face layout working face, the 22205 air return roadway of Buertai Colliery was taken as the engineering background, the continuum-discrete coupling numerical simulation, theoretical analysis, field observation and other research methods were comprehensively used to study the crack distribution state of roadway surrounding rock under superimposed stress path and the evolution law of micro-macro scale-span crack behavior of micro-crack initiation, propagation and penetration, and to clarify the mechanical mechanism of crack scale-span evolution. The results show that: ① There is a corresponding relationship between crack evolution and time stage. According to the numerical simulation, there are four stages of macro failure of surrounding rock: excavation influence stability stage, primary mining influence stage, stability stage after primary mining, and secondary mining influence stage. ② According to the distribution form of “point, line and surface” presented by crack initiation, propagation and penetration, It is obtained that the shear crack “point” initiates and expands to form a “line” type fracture zone, and the “surface” type fracture zone is formed through the closed area, through the closed area, the “surface” type fracture zone is formed. The development of shear cracks in the two sides is mainly in the vertical direction, the roof is inclined upward, and the floor is inclined downward. The tensile cracks exist in the shallow part of the roadway in the form of “point” crack and “line” fracture zone, and the distribution of tensile cracks in the two sides is mainly in the horizontal direction. ③ Under the superimposed mining stress path, the principal stress difference of roadway surrounding rock determines the crack propagation depth, the principal stress direction determines the crack propagation direction, and the principal stress difference and direction jointly cause the asymmetric distribution of cracks in roadway surrounding rock. Cracks are initiated from the stress concentration area, resulting in the transfer of stress concentration in this area to the new crack tip area, resulting in macroscopic cracks. The stress concentration area rotates counterclockwise with the direction of the minimum principal stress deflection. ④ Through on-site peeping, the surrounding rock fissures are divided into fissure fully developed area, micro-fissure initiation area and surrounding rock stable area. The fissure distribution in the fissure fully developed area is complex, which is caused by tensile and shear failure. The fissure development pattern in the micro-fissure initiation area presents circumferential fissure and axial fissure, which is mainly caused by shear failure, and there is no fissure in the stable area of surrounding rock. It is verified that the crack failure form is basically consistent with the drilling peep results.