Comprehensive measurement of characteristics of deformation and failure of extra-thick coal seam floor induced by fully mechanized top-coal mining

In order to study the characteristics of deformation and failure of fully mechanized top-coal seam floor under mining in Xinglongzhuang Coal Mine in Yanzhou Coal Field, comprehensive measurements are performed by means of the strain testing system and ultrasonic imaging technology in the fully mechanized top-coal working face, and the variation curves of strain increment and the ultrasonic imaging pictures at different depths are obtained in the monitoring boreholes with extending of the working face. Through the comprehensive comparison of monitoring data of five strain transducers and many hole wall pictures, the disturbance depth and underground pressure influence range are approximately ascertained. The results show that: (1) the disturbance depth of the coal seam floor is about 19 meters, and there are obvious belts, namely, the belt of mining disturbance failure and the belt of mining disturbance deformation, which are 16 meters and 3 meters in thickness respectively; (2) the belt of mining disturbance failure belongs to the whole plastic deformation, and the rock mass in the belt fails, but the belt of mining disturbance deformation still belongs to elastic deformation and has a good bearing capacity and strong anti-permeability strength; (3) the underground pressure of mining obviously emerges ahead and behind, which has the weakening characteristics to the coal seam floor from shallow to deep. The successful applications of the comprehensive measurements provide important information for roadway support for fully mechanized top-coal mining and for controlling of goaf water, and important reference value for safely mining lower coal seam in coal fields in North China above the confined aquifers.