Modern Mining September 2016

COAL

the conveyor system can empty the bunker – assuming no inflows – within 27 minutes. Describing the bunker, Wells says it has a finished diameter of 14,7 m and extends 24 m down from the 4 Seam horizon (200 m below surface) to the soffit floor of the box front chamber. The bunker has a cylindrical section of 16,5 m and – below that – a conical section of 5,5 m. The box front chamber extends a fur- ther 10 m to the footwall of the decline on the 2 Seam horizon. According to Wells, the first task was to create a collar ring beam at 4 Seam level to facilitate excavation of the bunker. “To do an excavation of this type, you need a decent infra- structure,” he says. “So we excavated a top cut to a depth of 1,8 m using an LHD and an exca- vator. The ring beam itself has dimensions of 1 m x 1 m and – including approach slabs – required 200 m 3 of concrete to be poured. To excavate the barrel of the bunker, we opted to use drop raising to sink a 4 m diameter cen- tre core which was then slyped out to the full diameter of 14,7 m. We supported the side- walls as we advanced using mesh, shotcrete and rock anchors. In all, the bunker excavation amounted to 5 800 cubes. “To establish the profile of the conical sec- tion – which we only did earlier this year once the box front was completed – we used sand- bags to form a back shutter and then backfilled behind these bags with mass concrete. In all, we used 6 800 sandbags during this phase of construction.” He adds that the volume of shotcreting was huge with 450 cubes of mass fill shotcrete being required for backfilling with an additional 126 cubes of 70 MPa andesite shotcrete, 300 mm thick, being placed on two layers of mesh as the final step in the operation. “This andes- ite shotcrete was very unforgiving in terms of pumpability and flowability,” notes Wells. While the excavation of the bunker was by no means an easy task, the construction of the box front – which houses the chutes and vibrating feeders controlling the discharge of coal to the decline conveyor system – proved equally demanding. “Put simply, this is primar- ily a steel structure but with massive amounts of concreting required for the floor slab, the 16,5 m long, 10 m high and 1 m thick east and west side walls and the 1,5 m thick soffit or roof,” he notes. “Altogether we poured over 1 800 m 3 of concrete. Given the dimensions of the side walls, we elected to make use of wide scaffolding. These walls, incidentally, each required 30 tons of reinforcing.” The main elements of the steel structure (the

had at every step of the way from our client, Sasol Mining.” The bunker feeds ore to the decline con- veyor which has a capacity of 2 900 tph and a belt speed of 4,6 m/s. With this capacity,

Above: Simplified cross section through the bunker and box front.

The decline conveyor seen from the box front. It has a belt width of 4,6 m/s and is rated at 2 900 tph. The extension of the conveyor under the box front was undertaken over four days in May this year.

24  MODERN MINING  September 2016