Pressure relief effect of protective layer mining and its optical fiber monitoring

The pressure relief effect evaluation and scope demarcation are the key to the application of antiscour technology in protective layer mining. With the theoretical analysis,the combination of optical fiber sensing field measurement method,based on Hulusu coal mine protective layer mining as the research background in the paper,the damage depth,and the stress distribution of the floor after the mined protective layer was analyzed,the stress release rate and its variation characteristics,protective layer parameters such as pressure relief angle and floor damage depth were obtained. To realize the realtime monitoring of the pressure relief effect of the floor coal and rock mass,the optical fiber sensors were implanted into the working face floor by three boreholes 90.36 m strike,128.47 m inclination,and 36.45 m vertical. The pressure relief monitoring data of the protective layer in the range of -300 m to +255 m are obtained. The results show that along the advancing direction,the floor coal and rock mass stress are divided into four stages: “original rock stress,stress concentration,stress release,stress recovery” in the process of mining the upper protective layer. The stress concentration stage is located within 50 m in front of the working face,the stress release stage is located 8-10 m behind the working face,and the stress recovery stage is 58-67 m behind the working face. The stress release rate of the floor after the mining of the protective layer has obvious spatial effect. The fluctuation range of the peak stress release rate in the vertical direction is 4.2%-71.6% and the peak stress release rate decreases in a negative logarithm with the increase of the floor depth. In the horizontal direction,there is a logarithmic relationship between the peak position of stress release rate and floor depth,and the greater the floor depth,the further the lag distance of peak stress release rate behind the working face. Taking the stress release rate of 10% as the pressure relief critical value,it is determined that the inclined pressure relief angle of 2-2 medium coal is 53.7°,and the strike pressure relief angle is 63.5°. The critical relief depths are 29.8 and 30.0 m respectively measured by fiber Bragg grating and distributed fiber monitoring system,and the deviation from the theoretical calculation results is 1.1%-1.3%. The maximum stress release rate obtained by distributed fiber and fiber Bragg grating are 24.5% and 63.3%-81.0%. Finally,the difference of results monitored by distributed optical fiber sensing technology and fiber Bragg grating in the pressure relief effect of protective layer mining is analyzed,and the floor failure depths obtained by the optical fiber and the empirical formula are compared. The optical fiber sensing technology provides a new means for monitoring the pressure relief range of protective layer mining,and realizes data acquisition and distributed and continuous monitoring of the pressure relief effect of protected layer in spatial dimension.