Underground » Coal Burst
The challenges associated with designing and operating a safe and profitable mine increase as near surface deposits are depleted and the pursuit of mineral resources is pushed deeper underground. Mining induced coal bursts pose great threats to mine safety and production. Coal bursts may severely damage work equipment and affect coal production. This research investigates the mechanism of coal burst, studying the energy concept and fracturing characteristics at laboratory conditions.
At first, a simple yet effective and versatile modified thick-walled hollow cylinder (TWHC) testing system that removes the reliance on the expensive true-triaxial testing system is introduced for coal/rock bursts at the laboratory. The rock/coal ejection phenomena observed in the underground rock/coal burst are successfully reproduced, which can help understand the bursting mechanism of rock/coal under polyaxial stress conditions. Fracture initiation and propagation, leading to bursting on the wall of a cylindrical opening in coal, granite and bluestone, are characterised by real-time acoustic emission (AE) responses. In this project, a borescope was used synchronously to observe the dynamic failure processes of coal/rock bursts and verify AE results. The internal fracture morphologies of granite, bluestone and coal specimens were acquired using X-ray micro-computed Tomography (μCT). The intrinsic mechanism of strain energy evolution of bursting was analysed. It was found that the average energy consumed by the induced cracking and released kinetic energy during bursting is ~8, 25 and 18 percent of the total absorbed strain energy of granite, bluestone, and coal, respectively. The average mass of the ejected rock/coal fragments was found to be ~0.3-0.5g, while the rock/coal ejection velocity ranged from 10.5 to 19.8 m/s for the hollow coal, bluestone, and granite.
Secondly, coal burst tests under different confinement levels were carried out to characterise the failure mechanism during coal burst in a confining loss process, and AE responses were monitored during the confinement loss stages. This method has provided a good indication of coal's in-situ stress state and energy characteristics that can lead to bursting. The X-ray μCT has revealed the progressive damage and evolution of fracture networks, which involved the micro-crack initiation, propagation, branching, coalescence and bursting under confinement loss. A coal bust potential index has also been proposed to quantify the bursting scale. It can be concluded that the overall coal burst potential diminishes with the increase in the level of confinement. Thus, coal with a lower lateral confinement gradient is less likely to undergo coal burst failure, whereas structural coal failure becomes more remarkable. Higher fracture area creation reduces the stored strain energy component available to cause burst failure, consequently coal burst potential measured by X cb reduces.
At last, to relieve the concentration degree of the vertical stress, multiple stress relief holes in the hollow cylinder coal samples were introduced. The influence of pre-conditioning on coal burst severity and fracture characteristics has been investigated by stress-strain behaviour, AE responses and μCT scanning. Some suitable stress relief borehole layout parameters for coal burst prevention have also been determined. It can be concluded that when the borehole density is higher, the pressure relief effect is more pronounced. Compared with a hollow coal sample without drilling, when there are 12 holes, the energy dissipation index increases, effectively reducing the energy concentration and resulting in less violent bursting. During loading the hollow coal samples with pre-conditioning holes, the fractures first occurred at the vertical and lateral ends of the boreholes. The fracture initiations at the locations provide conditions for the combined pressure relief boreholes. When the major principal stress is approximately 85%, the coalescence of the initial fractures in adjacent boreholes becomes a significant factor in measuring the influence of drilling pressure relief on coal burst impact. The greater the density and range of fracture penetration, the better the stress relief effect from the combined boreholes. This greater intensity is manifested in the larger initial ejection velocity of coal fragments and larger burst pit.