Strength of the coal seam
The strength and elastic module of the coal seam influence the fracture mechanics of an outburst. Stored elastic strain energy is released, and the failure is usually brittle indicating that the strain energy is not being used to distort the strata and a large proportion is still stored at the moment of breakage. Therefore, the ability of the seam to accumulate elastic strain energy is a large factor in assessing the liability of an outburst in the seam. (Lama & Bodziony, 1996)
A commonly used index for the ‘bursting liability’ of a coal seam is the strain energy index,known as “Wet”. Wet is the ratio of accumulated elastic strain energy to the permanent strain energy. The Wet values for a particular seam may be found either through laboratory experiments or by in-situ evaluations, with a Wet value greater than 5 indicating a seam is severely prone to outburst. The strain energy for an entire seam can be found by taking the mean of the values obtained for samples in key areas of the strata.