Inducer shotfiring is considered to be the oldest and most widely used method to control outbursts in mines, and it was initially developed in France by the end of 19th or beginning of the 20th Century in the Gord coalfield. Blast holes are drilled into the face and loaded with charges. The charge delivers a short intense shock wave to the strata, which may trigger outburst, Stress relief is likely to occur by the fracturing of the blasted area, thus transferring high stress concentrations to an area farther in advance of the face. Danger from outburst occurring under such circumstances is minimal since mine personnel and equipment are evacuated from the area.
The Inducer shot firing can be very effective in reducing outburst hazards, and since its development the methods found applications in various European coalfields, Canada, Japan and also in Australia. The application of the inducer shotfiring in Australia has been practiced in a number of Australian mines, and in particular, there is several documented reporting of the practice by various researchers from Metropolitan Colliery, NSW and Collinsville, Queensland (Chap 5, Lama and Bodziony, 1996) (Click).
A major concern with this technique as used throughout Europe and as was used at the Metropolitan Colliery is the apparent increase in the number of outbursts in a particular mine over and above what was previously or usually the case, and irrespective of the gas type. Such reporting is in agreement with the perceptions held by various researchers that inducer shotfiring has negative impact on coal permeability particularly outside the fractured zone surrounding the induced hole.
Two of the key elements associated with outbursts as identified are mine gases and tectonics. Just as gas drainage involves the prevention of outbursts in the seam and or associated rock strata to prevent outbursts. Currently, there are four different mechanisms used to de-stress a seam of outburst preventions, these being stress relief drilling, stress relief mining, inducer shotfiring and water infusion.
Stress relief techniques serve to reduce outburst potential, by reducing the ability of the immediate face to store the energy necessary for outbursts. The fracturing of the immediate face insures that high gas pressures or structural stress conditions do not exist in the next cut. Structural stress is shifted to the solid coal inbye the face. Gas pressure is reduced by coalbed gas flow through the induced fracture systems. Stress relief boreholes of about 100 to 300 mm in diameter are drilled into the face to mechanically distress the coalbed by removing large volume of drill cuttings (Click). Stress relief slot cutting on longwall faces is reported to be highly successful for reducing outbursts. Relaxation slots are systematically cut from 2.0 m to 1.0 m deep into the face, distressing the immediate face area.
Water infusion, the injection of water at high pressure (>7 MPa), has been explored as a stress relief mechanism to prevent outbursts. Te water pressure fractures the coal in advance of the face and provides for slow relaxation of stress through lateral movement enhanced by the presence of the water. However, the effectiveness of water infusion decreases as the depth of cover increases, owing to the reduction in coalbed permeability with depth.
For water infusion to be applied effectively, the water content about 10 l /m3 of coal is required to suppress dust ( in contrast, the spray systems used when cutting coal mechanically, utilises more than 30 l/m3 of mined coal). Injecting water at low pressure and flow rate over a long period gives better results than by injecting the same amount of water at high pressure. Hole diameter in the order of 70 mm drilled to a depth of 130 m in a panel 200 m wide would be an acceptable practice. In friable coal beds the infused zone ten to be circular, while in the blocky coal the infused zone is elliptical. The level of dust generation reduction is dependent upon the effectiveness of the infusion system, and the USBM survey showed that the water infusion reduced dust generation during mining by 58 %. In Australia, a recent study in a local mine resulted in a respirable dust reduction at the longwall work up to 73 % (Click).