Research On The Pressure Relief Deformation Law And Protection Effect Of Lower Protective Layer Mining On The Overlying Coal Sea

Protective layer mining can enhance the permeability of coal seam,reduce the difficulty of extraction and improve the gas extraction rate,which can fundamentally reduce the occurrence of coal and gas outburst accidents,so as to provide guarantee for the safe production of coal mines.Based on the practical background of Pingqiaojing and Xinjing well in Jiaozishan Coal Mine,Anshun City,Guizhou Province,theoretical analysis,numerical simulation,field practice and other research methods are used to study the influence of lower protective layer mining on the overlying limestone strata,the change of coal seam pressure relief law and the protection effect of protected layer.FLAC^3Dnumerical simulation software is used to simulate the influence of No.9 as the protective layer mining of No.8 coal seam on the coal and rock mass in the pressure relief range during the mining face advancing.At the same time,the protection effect of protected area is tested according to the maximum residual gas content of No.8 coal seam.The main conclusions are as follows:（1）Through the analysis of the basic form of coal seam movement failure,the law of rock fracture development and the failure characteristics of the coal seam on the goaf,it is concluded that the rock longitudinal presents obvious’three zones’.The heights of caving zone and fracture zone of No.9 coal seam are 5.84 m and 25.30 m respectively by theoretical calculation of empirical formula.The continuity and integrity of No.8 coal seam will not be destroyed after mining of No.9 coal seam and it is within the range of theoretical pressure relief.Therefore,it is theoretically feasible to use No.9 coal seam as the lower protective layer.（2）FLAC^3Dwas used to simulate the influence of the mining of the lower protective layer（No.9 coal seam）on the mining of the upper protective layer（No.8coal seam）.The study shows that during the mining process of the lower protective layer of the working face,the stress distribution of the surrounding rock of the upper protective layer changes significantly with the mining face close to the monitoring point,and the limestone and the protected layer are in the range of stress relief,indicating that the mining of the lower protective layer has a relatively obvious pressure relief effect on the upper protective layer.（3）After the protective layer mining,through the analysis of gas extraction data,it is known that before the protective layer mining in Jiaozishan Coal Mine,the measured gas content range of borehole is 13.9～15.7 m³/t.After the protective layer mining,the measured gas content range of Pingqiao well is reduced to 4.2～5.4 m³/t,and the measured gas content range of new well is reduced to 4.5～5.4 m³/t.It can be seen that after the protective layer mining,the protected layer is relieved,resulting in the redistribution of ground stress.More gas is released through the fracture channel,indicating that the protected layer is effectively protected.