The particular underground operation relating to this paper is a gold producer and has an orebody predominately consisting of rhyolite and marcasites pyrite. The ore has been identified as being reactive to nitrates.
Operations currently utilise two twin boom Tamrock™ Super-Drill Jumbos for development mining. Capital (waste) headings have a nominal size of 7 m wide × 6 m high, while ore headings are designed at 5 m wide × 5 m high, with the lower reaches of the mine narrowing down to a 4.5 m × 5 m design. Production drilling is currently undertaken with an Atlas-Copco L6 Solo. Due to the reactivity of the ore, inhibited emulsion phase bulk explosives are used for all blasting activities, with electronic detonators utilised in the production sequence.
Sublevel shrinkage stoping is utilised when the orebody is stronger than the host rock. After blasting the ore, and during extraction, the surrounding host rock cannot maintain integrity and it ‘collapses’ on top of the ore. Gravity forces this collapsing rock to follow the blasted ore through the drawpoints.
The highest risk involved with sublevel shrinkage stoping is the necessity for charge crews to work upon ‘rills’ at the drawpoints, which are occasionally unstable, to charge the next blast ring to be fired. To overcome this issue a loading technique called ‘precharging’ is used whereby two to three rings are charged, but only the first ring is fired. This technique can minimise risks by taking the charge crew away from the rill and brow.
As this mine contains hot and reactive ground, strict guidelines from the Australian Explosives Industry and Safety Group Inc (AEISG) are adhered to. These guidelines relate to the sleep time for the precharged rings.
This paper describes the issues faced with precharging in hot and reactive ground, the monitoring required and the systems and procedures that were put in place to enable a successful outcome.
Crowdey, B and Wall, J, 2014. Precharging sublevel shrinkage
production rings in hot and reactive ground with electronic blasting systems, in
Proceedings 12th AusIMM Underground Operators’ Conference 2014 , pp 283–288 (The Australasian Institute of Mining and Metallurgy: Melbourne).