Mechanism and control technology of strong ground pressure behaviour induced by high-position hard roofs in extra-thick coal seam mining

This work aimed at revealing the mechanism of strong ground pressure behaviour (SGPB) induced by high-position hard roof (HHR). Based on the supporting structures model of HHR, a modified voussoir beam mechanical model for HHR was established by considering the gangue support coefficient, through which the modified expressions of limit breaking span and breaking energy of HHR were deduced. Combined with the relationship between the dynamic-static loading stress of supporting body (hydraulic support and coal wall) and its comprehensive supporting strength, the criteria of ground pressure behaviour (GPB) induced by HHR were discussed. The types of I1, I2, Ⅱ1, and Ⅱ2 of GPB were interpreted. Results showed that types I1 and I2 were the main forms of SGPB in extra-thick coal seam mining. The main manifestation of SGPB was static stress, which was mainly derived from the instability of HHR rather than fracture. Accordingly, an innovative control technology was proposed, which can weaken static load by vertical-well separated fracturing HHR. The research results have been successfully applied to the 8101 working face in Tashan coal mine, Shanxi Province, China. The results of a digital borehole camera observation and stress monitoring proved the rationality of the GPB criteria. The control technology was successful, paving the way for new possibilities to HHR control for safety mining.