Experimental study on rock burst of coal samples under overstress and triaxial unloading

When the coal mining depth is over 1 000 m,the surrounding rock stress around the excavation and mining coalface exceeds the uniaxial compressive strength of coal mass,which is defined as overstress effect. Based on the field investigation and analysis,the distribution characteristics of uniaxial compressive strength of coal samples,and the relationship between mining depth,vertical stress and the uniaxial compressive strength of coal samples were obtained and then the concept of the overstress coefficient was proposed. In this study,acoustic emission (AE) and CT imaging techniques were combined to perform the CT inversing calculation for coal samples under overstress and triaxial unloa- ding path in a triaxial rock burst system. The relations between velocity distribution and micro-and micro-fractures of coal samples were studied,revealing the overstress loading effect and model in deep underground mining. Experimental results show:① There exists significant time-delay effect under different unloading stress paths. The more changeable the stress paths are,the shorter the time-delay will be. ② The failure models under triaxial unoading are more complex and changeable,and the main failure model is shear and tensile coupling failure,which embodies as the main cracks a- long the axial direction expand firstly,then many small tensile cracks along the axial direction distribute on the surface of coal samples. ③ In initial loading phase,the velocity variation in coal smaples is relatively smaller and a few high and low velocity regions appear. When loading increases,the high velocity regions transfer and extend,and the velocity anomaly regions are enlarging continuously. When loading further increases,large areas of low velocity regions occur, while the minimun value of wave velocity constantly decreases,and the high velocity and velocity anomaly regions transfer and change quickly. ④ The wave velocity inversion results indicate that good correspondence were found be- tween the macroscopic fracture surface of specimen and affluent velocity anomaly regions,as well as between micro- scopic fracture evolution and low wave velocity zones.