Underground » Strata Control and Windblasts
Caving is a fundamental aspect of most modern underground coal mining operations. However, in some mines where the roof comprises strong and massive rock, caving does not occur readily, resulting in an extensive mass of unsupported rock spanning a large void in the goaf. Such a 'hang up' may collapse, suddenly and often without warning, compressing the air beneath and forcing it out of the goaf through surrounding openings giving rise to a phenomenon known as wind blast. The force of the wind can, and sometimes does, cause injury to mine personnel, disruption to the ventilation system and damage to plant and equipment. It may also increase the hazard of explosion if methane in explosive concentrations is expelled from the goaf and mixed with raised coal dust.
The Wind Blast Research Group in The University of New South Wales (UNSW) School of Mining Engineering has been monitoring wind blasts in underground coal mines since 1992, most recently at Moonee Colliery, and has achieved a 'world first', the successful instrumental 'capture' of wind blast velocities and overpressures and the elucidation of their characteristics. The definitive body of data obtained has contributed to the development of a fundamental understanding of the phenomenon.
The objectives of the extension project described in this report have been achieved. Findings have included the following.
- Confirmation of the necessity of measuring both air velocity and overpressure time histories in order to fully describe wind blasts.
- The identification and quantification of four key parameters that characterise wind blast intensity, namely peak wind blast velocity and maximum excursion, which may be derived from the air velocity time history, and peak overpressure and impulse, which may be derived from the overpressure time history.
- Establishment of the 'laws' relating wind blast intensity to distance from the fall and to roof fall area.
- The effect of panel geometry in the vicinity of the fall upon wind blast intensity.
- The effects of changes in the geology of the Teralba Conglomerate Member, which formed the main roof overlying the longwall panels at Moonee Colliery, upon wind blast intensity. The data suggested a slight tend towards decreasing wind blast intensity as mining progressed towards the edge of the Conglomerate 'channel'. Whenever the thickness and competence of the main roof were sufficient for it to bridge the goaf, intense wind blasts occurred which were associated with the failure of the lower part of the Conglomerate. When the thickness and competence of the main roof were insufficient for bridging to take place, 'normal caving' ensued and there were no significant wind blasts. There was no apparent transitional behaviour.
It is anticipated that the results of field monitoring work carried out as part of the extension project will facilitate the drawing up of further guidance for the industry regarding panel design & layout and safe working practices in mines subject to wind blast.