Underground » Strata Control and Windblasts
The phenomena of coal shrinkage due to gas desorption is well-known in the field of coal gas testing. The impact of this on roadway stability has not been widely recognised, as in the past most gassy seams were relatively thin (<3m) and the roof and floor were typically non-coal. Over the past 10 years, more thick coal seams have been mined. Roadways typically leave a significant thickness of coal in the roof as a function of stability or coal quality. Where these seams have had high gas contents, the impact of coal shrinkage on roadway stability has been recognised.
It was noted in field monitoring at Dartbrook Mine that gas desorption (primarily C02) caused a significant change in roof and rib displacement characteristics about the roadway.
It became evident that analysis of stress conditions within thick coal seams needed to be related to tectonic stress and lithostatic stress, but was significantly modified by the effects of fluid pressure and volume change caused by gas desorption.
Coal fabric has a relatively high component of adsorbed gas within its structure. This gas content can vary depending on the fluid pressure within the pore space. If the fluid is below the desorption pressure of the gas in the coal, then gas will migrate out of the coal. The migration of gas reduces the actual fabric volume of the coal and it shrinks.
As a general correlation, the laboratory stress drop in the coal (in MPa) from shrinkage is approximately half the gas content reduction as measured in m3/t. That is 2.5MPa stress reduction will occur for desorption of 5 m3/t.
The effect of gas desorption in a mining environment has the added effects of strata readjustments to the coal shrinkage. Therefore the stress changes in the mine are impacted by strata readjustments to the coal shrinkage and as such they are different from the laboratory results. The mine scale effects are anticipated to occur about mine roadways and gas drainage holes.
It was found that the overburden will subside and reload the coal in a vertical direction. This will occur in response to a pore pressure loss and due to coal gas desorption.
The ground readjustments are such that the vertical stress in the coal is not significantly modified, however the horizontal stress in the coal is reduced. The amount of horizontal stress reduction approximates to the stress change related to pore pressure drop and gas removal less ground readjustments.
The primary ground readjustment of the coal in a horizontal sense is created by the “Poisson’s Ratio” effect of the vertical reloading.
The vertical stress in the coal on a mine wide scale is essentially overburden load. The horizontal stress in the coal (MPa) is reduced by approximately 0.28 times the gas content reduction as measured in m3/t.
The overall opportunity of drainage is to reduce the horizontal stress in the coal to improve roadway stability and support requirements.
The noticeable feature of gas drained areas is that roadways can be driven with good conditions at greater depths than would be expected otherwise.
A general guide for Australian conditions, the impact of gas drainage is to increase the depth at which roof deformation is initiated (under development mining) by 75-150m for every MPa of stress loss in the coal.