A review of mechanism and prevention technologies of coal bumps in China

Coal bump refers to a sudden catastrophic failure of coal seam and usually can cause serious damages to underground mining facilities and staff. In this circumstance, this paper focuses on the recent achievements in the mechanism and prevention techniques of coal bumps over the past five years in China.Based on theoretical analysis, laboratory experiment, numerical simulation and field test, the characteristics of coal bumps occurrence in China's coal mines were described, and the difference between coal bumps and rockbursts was also discussed. In addition, three categories of coal bumps induced by'material failure' were introduced, i.e. hard roof, floor strata and tectonic structures, in which the mechanism of coal bumps induced by geological structures was analyzed. This involves the bump liability and microstructure effects on bump-prone coal failure, the mechanism of coal bumps in response to fault reactivation, island face mining or hard roof failure. Next, the achievements in the monitoring and controlling methods of coal bumps were reviewed. These methods involve the incorporated prediction system of micro-seismicity and mining-induced pressure, the distributed micro-seismic monitoring system, energy absorption support system, bolts with constant resistance and large elongation,and the 'multi-stage' high-performance support. Finally, an optimal mining design is desirable for the purpose of coal bump mitigation.

Numerical investigation on the assessment and mitigation of coal bump in an island longwall panel

This study presents a numerical investigation to assess the risk of coal bumps and produces a stress–relief technology using boreholes to mitigate risk during the extraction of an island longwall panel.Based on the geological condition in an island longwall panel in the Tangshan Coal Mine,Tangshan,China,a numerical FLAC3D(Fast Lagrangian Analysis of Continua in 3 Dimensions) model was established to determine and to map the zones in the panel with a high risk for coal bumps.The results of the numerical modeling show that the roof deformation starts to occur at more than 30 m ahead of the longwall face and the deformation starts to accelerate after a distance of 10 m in front of the longwall face.Large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel.Based on the numerical results,a stress–relief technology using boreholes,which was employed to release abutment pressure,was investigated through numerical methods.The modeled results suggest that the peak stress concentration could be released by drilling boreholes in the zones prone to coal bumps.The effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing.

Change of the mode of failure by interface friction and width-to-height ratio of coal specimens

Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height（W/H） ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.

Unloading-slippage mechanism of coal bump in gateroad of longwall

Coal bump seriously threatens the safe and efficient mining of coal,and the research on the occurrence mechanism of coal bump is of great significance.The roadway coal bump accounts for 86.8%of the total.The occurrence of coal bump in gateroad is summarized.It is considered that hard roof and hard coal are the geological characteristics of coal bump,and the sliding instability of rib coal mass is the failure characteristics of coal bump.Based on the elastic foundation theory,the upward deflection characteristics of the front and lateral roof of the working face under the condition of hard roof are analyzed,and compared with the engineering practice of roof rebounding.Taking the roadway coal mass as the research object,the unloading sliding mechanical model of roof-coal-floor composite structure is established.By analyzing the relationship between horizontal ground stress of coal mass,frictional force of coal-roof and coal-floor and tensile resistance of coal mass,the critical equation of coal bump is established.It is proposed that the vertical pressure of coal seam is reduced due to the upward deflection of the roof,and the coal mass loses its clamping and moves into the roadway after overcoming the friction between roof and floor and the tensile strength of coal mass under the action of horizontal ground stress,that is,the unloading and slippage mechanism of coal bump in hard roof mining roadway.The model reasonably explains the causality of coal bump in hard roof mining roadway.Based on the unloading-slippage model,the principle of influencing factors of coal bump,includes the buried depth,roof strength,roof elastic modulus and roof thickness,coal mass strength and elastic modulus.Finally,two coal bump events,''8.2''coal bump in Tangshan coal mine and''11.11''coal bump in Hongyang mine are analyzed and the unloading-slippage mechanism are the reasoning of two events.