Mechanics of Gas Related Coal Bursts in Mining

Rock and coal fracturing is common about roadways, particularly under the influence of elevated stresses, either tectonic or related to mining abutment stress. Shear fracture is very common, however, it is very uncommon that the coal is propelled from the face or ribs as a result of fracture of the material.

A coal burst is defined as a rapid expulsion of coal (and potentially gas) from the boundary of the roadway. A burst typically requires extra energy to be supplied to propel the coal or rock from the rib or face of the roadway.

The depth of material impacted may vary from 0.5m to greater than 3m inside the roadway boundary. Burst volumes up to 50m³ are noted and cause significant disruption to operations and risk to personnel.

In this report the role and nature of gas energy within coal bursts is undertaken. The analysis is largely related to rock mechanics concepts rather than detail of the organic chemistry of coal.

The overall relationship for coal bursts noted in this study relates to a combination of factors. Gas pressure and gas volume are the active energy factors and coal rib strength is the resisting energy factor. Therefore, factors that are risk factors for a burst are those which contribute to gas pressure, gas volume and factors that reduce the resistance of the coal in the ribside.

The results indicate that elevated porosity and micro fracture density are risk areas which allow increased gas volumes to exist and to generate as a result of fracture propagation. Micro fractures can have a high density about fault structures and intrusions.

Fracture propagation can occur when the pore volume is greater than approximately 10% where the internal gas pressure is greater than the confining stress and coal tensile strength in areas close to a ribside or free boundary. Micro factures within the coal can propagate when internal gas pressure is greater than the confining stress in areas close to a ribside or a free boundary. Propagation of closely spaced fractures can allow significant volumes of gas to be diffused and initiate a burst event. Finely spaced fractures are commonly noted at the back of burst cavities indicating that this phenomenon had occurred as part of the burst event.

The energy required to cause a gas coal burst has been assessed on the basis of modelling a burst generated from a pocket of high gas content coal located at various distances into the rib. The energy required was found to be dependent on the seam height, distance of the source into the ribside and factors which may reduce its overall resistance to movement such as the existence of clay bands.

A key finding is that the threshold energy level will be different for different seams and structural/depositional domains in the coalfield.