Geology

Geology plays a very important role in the manifestation of Outbursts. Seams having complex geological structures are more prone to outbursts if high gas conditions are prevalent. The following points need attention when geological factors are being considered.
• Any structure related to horizontal movement of beds within the coal seams, presence of soft coal beds in the seam(s) and the thickness.
• Presence of thrust faulting, thickness of the gauge and width of the fracture zone. Thrust faults of<0.1m thickness are reported not outburst prone in some mines, but this is not a definite conclusion as the thickness of the gouge can vary suddenly at the same place through the seam thickness from roof to floor(Lama, 1980).
• Outbursts have occurred in seams of varying thickness including seams as thin as 0.5m. Outbursts are more frequent where the seam starts thinning out or maximum thickness is reached.
• Normal faults which have a throw greater than the seam thickness result in complete stoppage of gas flow (gas trap) and are more dangerous, than those with throw less than the seam thickness. Clear faults without any gouge are less dangerous and greater the thickness of the gauge the greater are the chances of outbursts. Strike slip faults generate more gouge than thrust faults and gauge thickness is lower on the normal faults.
•The presence of cleat in coal, particularly shearing of the cleat and presence of fine sheared material between the cleat surfaces (Wood and Hanes, 1982) is an indication of high stresses and deformation to which the seam is subjected.
• Seams which are surrounded by sandstone layers have a higher probability of shearing. Shale and softer rocks tend to protect the seam from shearing and hence reduce the chances of an Outburst.
• Micro-porosity of coal seams also affects the Outburst pronenss of the seam. Micro-porosity determines the capacity of the coal to sorb gas as well as its permeability at matrix level. Coal containing mean pore size radius <75µm are thought to be more liable to Outburst. Coal seams liable to Outburst have higher percentages of Clarain. These have highly fractured coal plys(layers) with high density of micro-cracking (>7mm/mm²). Seams not liable to Outbursts have crack density <3mm/mm². Seams where the structure has been damaged or completely obliterated are most liable to Outbursts. (Lama & Bodziony, 1996)
• Determination of the presence of structures such as faults, dykes and sills is a common practice in general geological investigation prior to the opening of the deposit. Sheared zones due to folding and faulting usually have upto three metres of cintred coal (a brownish and weak formation known as Mylonite). This needs to be investigated carefully
• Seams with Prodyakonov strength index of f<0.5 are highly liable to Outburst.(Lama & Bodziony, 1996)
• When a coal bearing area is split into two areas with a major fault, it is essential that both these areas be examined separately. It is quite possible that the area on the dip side may have quite different conditions related to gas regime.