Experimental study of rock burst in coal samples under overstress and true-triaxial unloading through passive velocity tomography

Abstract

The influence of overstress state on rockbursts in deep underground coal mining was investigated. Laboratory experiments were conducted using a true-triaxial rockburst system. Passive velocity tomography, using acoustic emission (AE), was used to study the internal microfractures and energy evolution of coal samples during true-triaxial compression. AE activities were monitored using eight sensors and every 130 consecutive AE events were used for tomographic calculations. For each coal sample, three phases and between four and six typical tomography results were obtained which reflected significant variations in velocity redistribution. Experimental results show that there was a significant time-delay effect under different loading and unloading stress paths. Compared to triaxial loading, coal samples are more easy to deform and bring to failure under triaxial unloading. Under a constant confining pressure, bursting failure occurs when the axial stress is more than three times the uniaxial compressive strength of the coal sample. The wave velocity inversion results show that the macrofracture positions in coal samples after failure have a positive correlation with the region of wave velocity anomalies, and the low wave velocity zone corresponds to areas of microfracture evolution.