Study on fissure evolution of overlying rock in lower protective mining.

Protective layer mining is the most effective and economical technical measure to prevent coal and gas outburst accidents. At present, in the study of multiple concentrated coal layer stress, plastic damage and mining pressure discharge range, there are problems such as less research on the protection effect of protective layer and less measured measurement points resulting in large experimental error. In view of the above problems, taking the 2305 working face of a Shanxi mine as the test mine, the crack evolution characteristics of downhole drilling were detected by this method, by means of the deep base point technology, and study the plastic damage, pressure relief characteristics and expansion characteristics in the mining process of protective layer, the findings suggest that, the crack development of the protective layer is mainly characterized by small width and quantity, after protective exploitation, the number of width and number of cracks were significantly increased, the injection is three times higher than before the protective mining, the degree of fissure development is greatly improved. The settlement change of each rock layer on the basic top presents periodic and nonlinear changes, divided into the initial deformation, violent deformation, deformation and decline of the three periods; the plastic damage area, pressure relief curve and expansion curve of protective layer are "saddle type", the overall pressure discharge rate of the protected layer has decreased by 60%, to meet the requirements of the critical value of expansion rate in the detailed rules for preventing coal and gas outburst, the protective layer mining technology applies to other working faces of the coal mine.