This excerpt taken from the VLKAY 6-K filed Nov 10, 2009.
AVOCA MINING METHOD
The Avoca method will allow stopes to be advanced according to the prevailing wall stability conditions. Typically stopes will advance at least 20 to 25 m before filling is required. If stopes require tight filling against the face the next phase of stoping starts with choke blasting against the backfill. If wall conditions are good and access is available from both ends of the stope, backfilling can follow after mining, maintaining a gap of 15 to 20 m between the advancing mining and backfilling faces, thus reducing potential backfill dilution and avoiding a cyclic mining system where production must stop for backfilling.
Stopes will be 30 m high and over the full vein width, with an average width of 8 m.
The stopes will be backfilled with un-cemented waste rock. Mine development waste rock and imported waste from surface will be used for backfill. Surface waste will be dumped through a waste pass located near the central ramp. Mine development waste rock will be dumped into the waste pass system at convenient underground locations.
The ramp locations have been selected to provide access from either side to the majority of Avoca stoping. Crosscuts from each ramp will access the ore and then sub-levels will be driven in ore to connect the crosscuts.
The Avoca method provides high productivity from a small number of working faces and will be suitable for most of the vein zones dipping over 55°, accounting for about 40% of the mining inventory.
A geotechnical study is required to determine the maximum allowable span, the size of the stope, and required ground support.
The stopes will be mined out on retreat and from the lower level or sill up to the upper sill above in 25 m high panels, sub-level floor to sub-level floor. Cycle times for stoping operations were estimated based on average stope parameters and productivities of utilized equipment.
Blasthole drilling will use a longhole drill for 25 to 30 m long blastholes from the drilling level on top of the stope. Blastholes will be 75 mm in diameter drilled on a 2 m burden x 2.5 m spacing pattern. Ammonium Nitrate and Fuel Oil (ANFO) will be the bulk explosive initiated by non-electric (NONEL) detonators and pentolite boosters.
The stope production sequence will commence with the development of a slot in the corner of the stope. A 2 m x 2 m slot raise will be developed by longhole drilling and blasted in short stages from the bottom up. The slot raise will then be enlarged to form a slot across the full width of the stope. Subsequent rings of drillholes will then be blasted into the open stope.
The broken ore will be mucked from the bottom sub-level by remote control LHDs to a remuck bay. The ore will then be loaded into trucks, and hauled to the surface.