Evaluation of Rockburst Hazard in Deep Coalmines with Large Protective Island Coal Pillars

Abstract

As the cover depth of coalmines in China increases, large protective coal pillars are widely used to support the overlying strata during mining. After the longwall panels on both sides of the protective pillar are mined, the protective pillar becomes an island pillar. The high static and dynamic stress accumulated in a protective island pillar may cause overall rockburst hazards in the gateway between the protective island pillar and the longwall panel being mined. Based on a rockburst accident that occurred in the Longyun coalmine, Shandong province, China, this study elucidates the occurrence mechanism of this type of rockburst. Firstly, a mathematical model is proposed to evaluate static stress distribution in the protective coal pillar. The calculation results indicate that large elastic energy is accumulated in the large protective pillar and overall rockburst hazards is very likely to be induced. To reduce rockburst risk, rockburst index is defined and long-borehole de-stress measures were adopted in this study. Then, dynamic stress evolution in the protective island pillar during mining the longwall panel was analyzed using PFC2D. The results show that the peak stress in the protective pillar increased continuously from 28 to 67 MPa with the working face advancing, indicating high rockburst risk in the protective pillar. The results of the mathematical model and numerical analysis agreed well with the rockburst accident that occurred in Longyun coalmine. This study paves the way for the design of large protective pillars based on overall rockburst prevention in deep coalmines.