Movement type of the first subordinate key stratum and its influence on strata behavior in the fully mechanized face with large mining height

The extraction height of a fully mechanized face with large mining height is large, so the mining-induced overlying strata failure area will be enlarged, which may result in various accidents such as water inrush accidents. In this paper, a combination of theoretical analysis, numerical simulations, and field observations was applied to investigate the movement types of the first subordinate key stratum and its influence on strata behavior in the fully mechanized face with large mining height. The results showed that there were two structural forms and six movement types of the first subordinate key stratum in the fully mechanized face with large mining height. Additionally, conditions favoring the formation of these movement types were presented. The two structural forms of the first subordinate key stratum were the cantilever structure and the stable hinged structure. The cantilever structure of the first subordinate key stratum was located in the caved zone and could present four movement types: direct caving movement of the cantilever structure, double-sided rotation movement of the cantilever structure, quadratic rotation movement of the cantilever structure, and alternative movement of the cantilever structure-hinged structure. The stable hinged structure of the first subordinate key stratum was located in the fractured zone and could present two movement types, namely, voussoir beam structure movement and short voussoir beam structure movement. Additionally, the movement law of each movement type of the first subordinate key stratum and its influence on the strata behavior of the fully mechanized face with large mining height were revealed. Finally, the correctness of the six movement types of the first subordinate key stratum and the corresponding formation conditions were verified using measurement data from four fully mechanized faces with large mining height in China. The outcomes may contribute to reasonable selection of supports and ensuring safe mining of the fully mechanized faces with large mining height.