Underground » Coal Burst
Rock and coal failure about openings in coal mines is common during mining, however bursting of material is considered uncommon. A burst requires energy to be supplied to propel the coal or rock from the rib or face of the roadway. Gravity is not considered to be an energy source for bursts. The potential sources of energy are:
- Gas pressure released within the coal;
- Stored strain within the coal and strata;
- Seismic transmission of waves associated with ground failure not daylighting into the mine opening.
This report seeks to review a range of causes for bursts on the basis of past and current work. The report focuses on the energy sources required to cause the rapid expulsion of coal. Once the energy sources are defined the risk of various site conditions can be provided.
A burst occurs when the energy within the coal ribside exceeds the resistance of the fractured coal and the kinetic energy requirement to propel the coal. The resistance of a ribside is a variable, dependent on the height of the ribside, nature of bands within the coal, low angle structure and the nature of the contact boundaries of the coal.
In thick seam operations, the effective rib height may be greater than the nominal cut height of the seam. If the floor is broken or the immediate roof is broken or fallen out during mining, then the effective rib height will increase, and the rib resistance will decrease accordingly. This is also true in situations, other than thick seams, of a weak band in the roof/floor along which the coal or stone breaks. This barrier can break during mining or shortly after and increase the effective rib height and/or change the boundary conditions of the seam to the strata. Mining conditions and potential changes to the boundary conditions need to be included in such risks. Therefore, burst threshold criteria risks (e.g., energy, gas pressure) are a variable depending on geological characteristics.
The severity of a burst is related to the magnitude of energy. Energy is related to the stress, gas pressure and stiffness of the ground. The severity is also related to the volume of ground able to be mobilised by the energy. Coal bursts are typically associated with stress induced fracture of the coal or slip along bedding planes which have already occurred during mining operations. Therefore, the dynamic energy related to a burst interacts with, and may progressively weaken, the pre-fractured coal ribside and the resultant rib resistance.