Mechanism for stress abnormality and rock burst in variation zone of roof-stratum thickness

The roof stratum structure is one of the main factors affecting coal burst, and the coal burst is also easily induced in the variation zone of roof-stratum thickness. This phenomenon is gradually severe in deep mining areas in Inner Mongolia. The stress distribution in the variation zone of roof-stratum thickness is analyzed based on the theory of elastic mechanics. The FLAC^3D numerical modeling is then performed to investigate the influences of the variation of stratum thickness on the stress distribution characteristics and energy evolution in the coal seam. The coal burst mechanism due to the variation of stratum thickness is finally released. The results show that the tectonic stress in the thick roof zone is larger than that in the thin roof zone, and the stress gradient increases with the increasing variation in the stratum thickness or the roof properties. In the variation zone of roof-stratum thickness, the superposition of the advanced abutment pressure and the increasing tectonic stress results in a high-stress concentration area. A higher coal burst risk might thus occur in the roadway near the longwall in the roof variation zone to the thicker roof zone, where more intensive elastic energy is released in the coal/rock mass. The comparative analysis of two field cases shows that more seismic activities occur in the variation zone of stratum thickness and from the variation zone to the thicker stratum zone, and the roadway damage is obvious, which is consistent with the theoretical analysis.