Study on roof weighting control at the end mining in large mining height working face with thick and hard roof by hydraulic fracturing

Taking the end mining of 5217 working face with thick layered roof in Zhangjiamao Coal Mine as the research background. Through physical simulation experiment, numerical simulation and theoretical calculation, the reasonable layer of roof structure and pressure control by roof hydraulic fracturing is determined. The support resistance, stope stress distribution and coal wall displacement before and after fracturing are analyzed, and the roof caving law and weighting law are revealed. The weakening roof structure models of different layers are established, and the calculation formula of support resistance is given. Three weakening horizon criteria of low, medium and high are proposed. The results show that the average periodic weighting step is shortened by 14.5% after the low-level weakening, and the maximum resistance of the support is reduced by 14.1%. After the middle position is weakened, the average periodic weighting step is shortened by 47.3%, and the maximum resistance of the support is reduced by 21.6%. After the high-level weakening, the average periodic weighting step increases by 11.8%, and the maximum resistance of the support increases by 24%. The numerical simulation method of 'hydraulic fracturing + support resistance monitoring' is used to weaken the middle position. The average periodic weighting step is shortened by 60%, and the maximum resistance of support is reduced by 13.2%. After weakening, the average vertical stress in front of the working face is always 21% lower than that before weakening, the average vertical stress on the right side of the main withdrawal channel is always 33% higher than that before weakening, and the horizontal displacement of the coal wall of the working face is 52.2% lower than that before weakening. The ''high short masonry beam'' structural model with middle-level weakening and the ''step rock beam'' structural model with high-level weakening are established. The resistance required to control the roof stability support is 15 450 kN and 9 437 kN. After hydraulic fracturing in the middle layer of the roof, the periodic weighting step is 6.38 m, and the maximum support resistance is 9 463 kN, which is 46% and 26% lower than without fracturing. Scientific and reasonable fracturing horizon and structure control ensure the safe withdrawal of working face. This study can provide theoretical basis and reference for the determination of roof hydraulic fracturing layer and the practice of structural regulation.