Experimental research on energy characteristics of anomalously low friction effect in deep coal and rock mass

Under a high in-situ stress and strong excavation disturbance,the rock mass around the stope is constantly shocked,deformed and destroyed,forming blocky rock mass. When the accumulation energy of deep coal rock reaches the critical value of instability,the blocky rock mass is prone to a sudden release of energy due to dynamic disturbance under confining load,resulting in sliding between rock strata. It is prone to anomalously low friction sliding instability for blocky rock mass,and then induces a new typical rock burst,that is anomalously low friction rock burst disaster. Its essence is the process of energy accumulation and unstable release. To reveal the energy characteristics of anomalously low friction effect of deep coal and rock,granite and coal rock mass were investigated using experimental research method. The self-developed test apparatus of anomalously low friction effect was used to simulate high in-situ stress by axial pressure and confining pressure loading and strong excavation disturbance by vertical vibration and horizontal impact loading. A complete rock block,a broken rock block and coal block were considered as the working block of anomalously low friction effect experiment. The experimental results were compared with the previous conclusions,which verify the rationality and feasibility of the experimental scheme. The total amount of kinetic energy per unit mass based on the horizontal displacement of working block was defined as the characteristic parameter of the energy variation of anomalously low friction effect. The influence of axial pressure,number of working blocks and lithology on the time and frequency domain curves of per unit mass kinetic energy of horizontal displacement of working blocks was analyzed. The relationships among mining depth,block fragmentation,lithology and block kinetic energy were presented. The results show that when the anomalously low friction effect occurs,the kinetic energy of the working block has the characteristics of accumulation effect,short-term peak value and intermittent. The degree of fragmentation and lithology are important factors that affect the total kinetic energy change of anomalously low friction rock burst.The experiment reproduces the process from the foreshock to the main shock of rock burst. It is found that when the energy of anomalously low friction rock burst is released,the block kinetic energy has a characteristic of frequency transferring from high frequency to low frequency. The main frequency is mainly in the low frequency range within 20 Hz. The results are basically consistent with the field observation of rock burst. The calculation method of the magnitude of rock burst retrieved from the energy release test was proposed. A case study at Hongyang No.3 Mine in China shows the result is very close to that in the field monitoring.