123 / 2021-09-30 21:36:29

Rockburst induced by pillars and hard-thick roof in shallow multiple coal seam mining

underground coal mining,rockburst,pillar,hard thick roof,shawllow multiple seam mining

Rockbursts pose a serious threat to the safety of underground coal mines. Rockbursts of coal mines normally happen in deep mining with a cover depth mostly over 400 m. While in recent years, large numbers of rockburst occurred in shallow seam mining mainly due to certain geological conditions and unreasonable mining layout. To give a reference for better controlling such rockbursts, a case study was done on panel LW203 of a Chinese coal mine. Panel LW203 adopted top-coal caving method to extract the entire seam with a thickness of 12.95 m, and is characterised by shallow multiple seam mining, retained coal pillars and hard coal and roofs. Field investigation was firstly conducted to analyse the characteristics of rockburst occurrence, overburden fracture and its induced loading on the coal body by means of on-site observation, microseismic (MS) monitoring and support pressure monitoring. Numerical modelling analysis was then conducted to present the stress evolution and energy accumulation in the process of panel extraction. The underlying mechanism of such rockbursts was finally discussed in detail. Results show that all the four rockbursts occur in the gob-side headentry with damage characterised by floor heave and roof subsidence. MS energy corresponding to the rockbursts exceeds 5×10⁵ J. MS events are mostly located within the panel and are close to the headentry. In the vertical section, MS events are mostly distributed within the coal seam and 0 m to 50 m above the seam where two hard and thick sandstone layers are located. This implies that the dynamic disturbance induced by MS events are mainly caused by fracture of the hard-thick roof strata and the coal body and has a more impact on the headentry compared with the tailentry. Support pressure near the headentry is generally much higher than that near the tailentry with 7500~9500 kN vs 5000~6500 kN, which indicates that the lower part of the panel near the headentry bears more loading than the upper part of the panel near the tailentry. Numerical modelling results show that the 15 m wide section coal pillar has the highest stress concentration compared with other part around the panel. Also, the stress concentration and energy accumulation within the pillar varies with the pillar width and reaches a peak when the width is 15 m. The pillar is just next to the headentry, which indicates that the pillar size is unreasonable.The hard-thick roof can form a cantilever structure over the headentry and transfer overburden loading to the underneath rock surrounding. The 15 m wide section coal pillar has sufficient bearing capacity, making the headentry surroundings have the highest stress concentration. Meanwhile, the hard-thick roof fractures easily in the process of panel extraction and puts much dynamic loading on the headentry surroundings. The headentry surroundings are most likely to fail and release energy under a coupled effect of stress concentration and dynamic loading, and thus triggering a rockburst.