Research on the characteristics of overlying rock fracturing in roadway maintained by roof cutting and connecting adjacent goaf and its application

To investigate the fracture evolution characteristics of the overlying strata in a roof cutting and roadway retaining (RCRR) mining face, physical similarity simulation was employed to analyze the development height of mining-induced fractures, bed separation, and strain distribution. The evolution characteristics of the roof structure and the asymmetric fracture pattern of the overlying strata in RCRR mining were systematically explored. Based on the virtual work principle and plastic hinge theory, calculation formulas for the broken block size of the roof at different stages of RCRR mining were derived. The results show that tIn the first mining face, fractures are more developed within the caved zone on the roof-cutting side, while fractures within the fractured zone are more prevalent on the non-roof-cutting side. After the non-first mining face of the roof-cut retaining roadway penetrates the adjacent goaf, the overlying rock fracture and the separation volume curve are asymmetrically distributed. The fracture on non-roof-cut side is compacted and closed, while fractures on the roof-cut side are stable. The bedding separation is reduced by 0.46~8.50 mm, along with the reduction of fracture development degree. The initial fracture morphology of lower-level rock strata in the first mining face of the roof cutting roadway is ''U + Y'', and the fracture line on the roof cutting side is parallel to the roof cutting direction. The lower-level rock strata of the non-first mining face is broken in a "rectangle" pattern, and the higher-level rock strata is broken in an asymmetric ''O + X'' shape for the first time. The periodic breaking is in a combined ''U + Y'' shape, and the fracture trace biased towards the non-cut side. In non-first mining faces, the breaking step of high-level rock strata increases, producing the larger sizes of fractured blocks with a decrease in the number of fractures per unit volume. Consequently, the upward migration of methane becomes more difficult. Based on this, it is proposed that for methane pressure relief management in non-first mining faces with top-cutting and gob-side entry retaining technology, the height of methane drainage boreholes should be appropriately reduced. By considering fracture characteristics, the position of key strata and air leakage effects, an efficient methane drainage area for working faces with roof cutting and roadway retaining is determined. Field tests on methane drainage from boreholes have verified the accuracy of the zoning.