Identification of the degree of stress release and unloading in the underlying coal rock of protected seam mining

Protective layer mining is one of the methods to effectively prevent and control the impact ground pressure disaster in coal mines, and the measurement of the unloading degree of its underlying coal rock is the key, and the method of discerning the unloading degree with the stress release rate as the index is proposed. The mechanical model of the bottom plate was established through the elasto-plastic mechanics theory, and the formula for calculating the stress release rate at any point of the bottom plate was given；Combined with the theory of slip line field, the mechanical state and failure form of the bottom plate during the advancing process of the working face are analyzed；Numerical simulation was used to obtain the distribution law of stress release rate along the working face and tendency of the bottom plate, and compared with the calculated formula of stress release rate, and its two distribution curves have good consistency；Using distributed optical fiber and optical fiber grating monitoring technology, the law of stress release is analyzed based on the stress monitoring results at different depths of the 21104 working face floor of the 2−1 coal mine in Cucurbitacin Coal Mine. The results show that: affected by coal seam mining and bottom drum, the stress disturbance within 5m of the bottom plate is relatively severe, and the stress release rate shows irregularity; Peak and smoothed values of stress release rate at different depths of the bottom plate were obtained. Based on the distribution curve of stress release rate, it reveals the change rule of the degree of pressure relief and the depth of the base plate and the advancement distance of the mining face, and concludes that the stress in the deep rock layer of the base plate recovers faster and takes shorter time than that in the shallow rock layer. In 10m, 20m and 30m of the bottom plate of the coal rock under the protective layer mining, the maximum degree of decompression can reach 44%, 64% and 85%, and with the gradual recovery of the bottom plate stress, it finally reaches 56%, 72% and 89%, and the degree of decompression is from the pre-decompression to the late process of decompression, the decompression angle and the range of decompression in the bottom plate is gradually increasing. The results of the study on the identification of decompression of the bottom plate for the protection of mining and the guidance of reasonable mining of the protective layer is of great theoretical and practical significance.