Experimental study on compound dynamic disaster and drilling pressure relief of gas-bearing coal under true triaxial loading

Coal and gas outburst,impact ground pressure and other dynamic disasters seriously threaten the safe and efficient production of coal mines. After the coal mine enters deep mining stage,the impact and outburst compound dy- namic disasters are gradually increasing due to the increase of original rock stress and the change of geological conditions. The causes and mechanisms are more complicated than that of single dynamic disaster. Therefore,studying the mechanism of compound dynamic disasters in deep coal mines is crucial for the effective prevention and control of dy- namic disasters. Based on the self-developed “multi-functional true triaxial fluid-solid coupling test system”,the tests were carried out on the intact coal sample and the coal sample with pressure relief borehole considering the influence of gas under the condition of five faces loading and one face free. Results show that the compound disaster is a dynamic process of nonlinear transient damage of coal and rock under the action of strain energy and gas internal energy. The occurrence process has obvious stages,mainly undergoing particle ejection,fragment ejection,local coal body damage, overall instability after coal body throwing, and rebalancing state. The intermediate principal stress enhances the strength of coal sample within a certain range,the strength of the sample gradually increases with the increase of inter- mediate principal stress,and the main fracture surface parallel to the intermediate principal stress after the sample is destroyed. Drilling pressure relief measures can change the mechanical properties of coal rock,soften the structure,re- duce the strength,and enhance its plastic deformation characteristics,which cause the elastic energy released slowly reducing the possibility of the dynamic disaster. The comparison test results show that the plastic zone of the coal sam- ple is larger and the plasticity degree is stronger when the pressure relief hole is parallel to the intermediate principal stress. After the sample containing the pressure relief borehole is broken, a distinct stepped spallation structure is formed. Compared with the un-drilled and other drilling orientations,the degree of disaster is weakened,there is no ob- vious dynamic display characteristics,and the pressure relief effect is better. The pressure relief borehole leads to the static slow failure of the coal rock by optimizing the energy release structure and promoting the progressive slow dam- age. According to the specific engineering conditions,the technical solutions and feasible control measures were pro- posed for the relief drilling borehole according to the actual in-situ stress and geological conditions.