Stability and control of surrounding rock of narrow coal pillar in vertical stratification of extra-thick coal seam

Aiming at the problem of large deformation and instability control of narrow coal pillar in the vertical stratification section of extra-thick coal seam, the stability and control technology of the surrounding rock of narrow coal pillar in extra-thick coal seam are studied by combining theoretical analysis, numerical simulation and field test. Based on the calculation of the triaxial stress in the narrow coal pillar, the yield criterion of the plane strain of the narrow coal pillar is obtained based on the Mohr-Coulomb failure strength, the failure characteristics and the evolution law of the damage degree of the narrow coal pillar are qualitatively analyzed, and the instability mechanism and size effect of the narrow coal pillar are further analyzed. Finally, the key technologies of surrounding rock control of the narrow coal pillar are given. It is verified by numerical simulation and field test. The research results show that ① The damage degree of the middle part of the narrow coal pillar is greater than that of the two sides, and the damage degree of the middle part from the top to the bottom develops from serious to slight. With the increase of width-height ratio, the damage degree and the proportion of serious damage area in the narrow coal pillar gradually decrease. When the width-height ratio is greater than 1∶1, a large range of slight damage area begins to appear in the middle and lower part of the narrow coal pillar. When the width-height ratio is greater than 5∶3, the proportion of slight damage area exceeds 50%. When C≥3 MPa, or φ≥20°, the damage degree of the two sides of the narrow coal pillar becomes slight. ② The side of the mining roadway in the working face is a low-stress bearing area of the narrow coal pillar. Under the action of high stress in the upper part, the coal body in the large-scale low-stress bearing area moves to the roadway, resulting in a continuous large deformation of the two sides, which in turn affects the stability of the roof. The height of coal pillar is the main controlling factor for the stability of the narrow coal pillar in the middle and bottom layered sections. The increase of width-height ratio of the narrow coal pillar has little influence on the bearing strength of high strength bearing area, while the bearing strength of low strength bearing area increases obviously. Reasonable width-height ratio of the narrow coal pillar can balance the proportion of high and low strength bearing area in coal pillar. ③ The stability of surrounding rock should be considered in the design of layered narrow coal pillars when both sides of the working face are mined out, so as to ensure that the range of high-strength bearing area inside the coal pillar is greater than half of the width and height of the coal pillar. Through the joint strengthening support measures, multiple joint control areas from shallow to deep are formed in the coal pillar to jointly maintain the self-stability of the coal pillar.