Depth of mining
As the magnitude of vertical stress increases with the
depth of mining, it seems reasonable to say that these stresses will increase
the probability of outburst at greater mining depths. However, this does
not exclude the chance of outburst at small depths. While many parts of
the world have not experienced outbursts at depths less than 180 metres,
the Moura mine in the Bowen Basin, Australia has experienced three gas
outbursts at a depth of approximately 130 metres. These outbursts were
attributed to shear zones present at that depth and it is accepted that
with the existence of certain geological conditions, outbursts may occur
at depths as little as 100 metres (Lama & Bodziony, 1996). There is
also little record of outbursts occurring when the depth of mining exceeds
500 metres. This is probably due to a change in the rank in coal as depth
increases. Therefore, it can be surmised that while mining depth plays
a role in the majority of outbursts, there can be no general rule found
for the likelihood of outburst at a given depth. |