Study On Variation Law Of Floor Stress And Pressure Relief Effect In Mining Of Upper Protective Layer

The pressure relief effect and protection range of protective layer mining technology are important indicators of the safety of protective layer mining anti scour technical measures.Taking the mining of the upper protective layer of Hulusu coal mine as the research background and taking 2^-1coal and 2^-2coal of Hulusu coal mine as the research object,this paper firstly analyzes the stress on the floor,and calculates the vertical stress at different depths under the floor of the goaf and the maximum failure depth of the floor through theoretical analysis and empirical formula;secondly by exploring the pressure relief index of protective layer mining,the author puts forward the pressure relief critical value of protective layer mining in Hulusu coal mine with 10%stress release rate as the pressure relief critical value;thirdly,numerical calculation and similar simulation test are used to verify the variation law of floor stress after mining of protective layer;finally the paper establishes the pressure relief optical fiber monitoring system of floor coal and rock mass,in which three boreholes are designed in Hulusu coal mine,and the optical fiber is implanted into the floor of the working face to realize the real-time monitoring of the mining pressure relief effect of floor coal and rock mass in the space of 90.36m,128.47m and 36.45m.The detailed research steps of this study are as follows:（1）Through the calculation of theoretical and empirical formula,the maximum failure depth of protective layer mining is about 20.34-40.88m,indicating that protective layer mining will relieve the pressure of underlying 2-2 medium coal.（2）According to the on-site measured optical fiber monitoring data,the pressure relief monitoring data of the protective layer in the range of-300-+354m is obtained.The results show that the stress change of the floor is manifested in four stages:"not affected by the mining of the working face-stress concentration-stress release-stress stability".The pushing of the working face has an impact on the floor.The stress concentration stage is about50m in front of the working face,and the stress release stage is 5～10m behind the working face,the stress recovery stage is located in the area 52～77m behind the working face.The vertical upward floor stress change comes from the stress change in the goaf,and the stress transmission shows a decreasing trend from top to bottom.（3）The rock stress release rate is proposed as the pressure relief judgment index,and the pressure relief critical value of 10%stress release rate is determined.According to the monitoring data,the mathematical relationship between the critical value and peak value of stress release rate and the buried depth of rock stratum and the advancing distance of working face is gained.It is ascertained that the maximum relief depth of bottom plate is 29.95m,the relief angle of 2-2 medium coal strike direction is 63.5°,and the relief angle of dip direction is 53.7°.（4）Through the numerical calculation of the stress release rate of 2-2 medium coal after the mining of the protective layer,it is acquired that the pressure relief angle of the floor after the mining of the protective layer is 78°,and the pressure relief angles of 2-2 medium coal after the mining of the protective layer are 61.2°and 63.2°through the similarity model test.The pressure relief range of 2-2 medium coal in the mining of the protective layer obtained by these two methods verifies the accuracy of the optical fiber monitoring results.The research content of this paper provides a new means for the monitoring method of pressure relief range of protected layer mining,and realizes data acquisition and distributed and continuous monitoring of pressure relief effect of protected layer in spatial dimension.In engineering application,the fine discrimination and quantitative interpretation of deformation and failure of floor rock stratum are achieved.The application of the new technology is conducive to a comprehensive and full understanding of the deformation and failure of the stope floor and the characteristics of rock displacement,which is of important practical and guiding significance for the pressure relief effect of the mine floor and the spatial stability of the stope.