Research on the mechanism of multiple key strata controlling overburden movement and their classification in bedrock gully.

The incomplete sequence of key strata in the bedrock valley mining area leads to a decrease in its carrying capacity, which often leads to rock bursts and mine water inrush accidents. Therefore, a set of controlled physical similarity model experiments was established based on the double key stratum conditions of the overburden in the BJG coal mine. The deformation and break laws of key strata with different occurrence states in the bedrock valley mining area and the characteristics of overburden movement during mining were analyzed, and the mechanism of controlling overburden movement by multiple key strata was focused on. Research has found that when the lower key stratum sequence is incomplete, the key stratum breaks 5 times during the mining stage of entering the mountain, with an average breaking distance of 15 cm. The overburden moves violently and exhibits the "polygonal block" rotation movement. During the mining stage of being away from the mountain, the key stratum broke 3 times, with an average breaking distance significantly increasing to 36 cm. The intensity of overburden movement weakened, resulting in layered subsidence. When the lower key stratum sequence is complete, a total of 6 breaks occurred in the key stratum, with an average breaking distance of 26.0 cm. The overburden settled in layers and did not form the "polygonal block" structure. Compared to the upper key stratum, the significant reduction in carrying capacity of incomplete key strata is due to the inability to transmit horizontal stress generated by the rotation of fractured rock blocks. When both key strata are incomplete, the lower key stratum has a higher carrying capacity than the upper key stratum. When the dual key strata are complete, the lower key stratum has more influence on the pressure of the working face, while the upper key stratum has more influence on the migration characteristics of the overburden. At the same time, the limitations of the traditional key strata classification criteria proposed against the background of plain areas in the application of valley terrain conditions are analyzed in this paper, and strong and weak key stratum definitions and classification criteria applicable to valley terrain conditions are proposed. Under valley terrain, the key strata of an incomplete sequence are weak key strata; otherwise, they are strong key strata. This study provides a reference and guarantee for mining design and safe and efficient mining under similar conditions.