Mechanism and prevention of coal seam rib spalling in remote protected layer across coal group

Coal wall spalling is one of the main factors restricting the safety and high efficiency of fully mechanized coal face. Based on the characteristics of the coal-bearing strata of Group A and Group B coal in Huainan Mining Area and the production conditions of the lower protective layer,the field test,physical simulation,theoretical analysis,in- dustrial test,etc. are adopted for studying the damage characteristics of the coal wall spalling at the 11224 working face in Pan’er Coal Mine. The development law of overburden mining fissures in the remote protective layer of multi-critical layer structure and the coal wall instability in the fully mechanized mining face of the large mining height soft coal are studied,the mechanism of coal wall spalling affected by mining before mining is revealed and the control measures are put forward. The results show that:① The coal wall fracture of the protected layer is mainly composed of irregular sliding motion along the fracture surface,accompanied by regional multiple and prone sprawl. ② Physical simulation shows that affected by the mining of the lower protective layer,the working face of the remote protected layer 11224 is located in the “three belts” of overlying rock in the bending subsidence zone,and a large number of randomly distrib- uted normal joints and fractures occur. ③ The ultimate deflection analysis of thin plate model shows that mining height,the spatial structure of key strata,the collapse and expansion coefficient of rock strata in caving zone and frac- ture zone are the main controlling factors to determine the development height of “three zones”,and the development characteristics of “three zones” obtained by theoretical calculation are consistent with physical simulation. ④ Based on the form of the coal wall panel and the characteristics affected by the disturbance,the simplified model of the coal wall unit of the protected layer is established,and the shear failure criterion of any triangular prism body with confining pressure is given. ⑤ The slip body is formed after the fracture of the triangular prism. The main control weak surface controls the friction and sliding of the slip body,and the criterion of friction and sliding instability of the slip body a- long the structural surface is proposed. ⑥ The stability of coal wall unit can be significantly improved by increasing the support resistance and retaining force,adopting the “ brown rope+grouting” flexible support and the i-steel auxiliary paving network to strengthen the support.