LAUSR

The demand for gob-side entry retaining (GSER) technology is extensive in Chinese coal mines because of its outstanding advantages. However, due to the occurrence of long-term roof movement disturbances, maintaining the conventional GSER over long distances is difficult. The urgent demand for this technology and its difficult maintenance are prominent contradictions faced in its application. In this paper, two characteristics of strata movement after mining that have great influence on GSER are determined in a physical simulation experiment. One is that the roof layers, which are dominated by the key strata (KS), collapse in multiple groups to form two-directional periodic pressures and superposed disturbances. The other is that the movement of the main roof will experience three stages of deformation, as follows: bending subsidence before collapse, sinking deformation at collapse and compressive deformation after collapse. In particular, the gradual compression of the bulging gangue in the gob extends the disturbance cycle. Therefore, it is difficult to maintain the GSER over long distances because of the long-term and multiple breaking disturbances of the roof layers. Mechanical models of KS breaking and GSER are established, and a method to determine the timing and strength of KS disturbances are proposed. Making use of the characteristics of staged collapse and fluctuating weighting of the overlying strata, an innovative technology named short-staged GSER is proposed. This technology maintains the maximum length of GSER within the optimal length, which ensures that the entry avoids superposed disturbances, and reduces the maintenance difficulty. The method for determining the key technical parameters is discussed, and an engineering case in panel 24202 of the Shaqu Mine in China is presented. From a back-calculation of measurements, the engineering practice demonstrates that the surrounding rock mass is stable, and the deformed entry size is safe when the length of the short-staged GSER does not exceed 100 m.