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Our mining methods are very different to most South African deep level underground gold mines.
At the depth of mining at South Deep, rock stresses can become exceedingly high. Stresses are aggravated dramatically with the large high excavations associated with massive mechanised mining, as they could surpass the rock strength itself, increasing the risk of rock bursts. Massive mining refers to mining that takes place on a large scale, as opposed to narrow seam mining. A rock burst is a spontaneous and often violent rock failure – a seismic event that is induced by mining - which results in a damage to the mine infrastructure.
To combat the build-up in rock stress, the area is mined through a series of smaller excavations. These move the high stress zone away from the large excavations and create a lower stress zone above and below the smaller excavations. A destress cut is mined horizontally through the orebody on a grid pattern around pillars. These pillars then yield and transfer pressure in a reduced and controlled fashion to the footwall (the floor). The effect on the surrounding rock mass is to create an in-situ stress similar to shallower mines, where large excavations can be mined.
The de-stress cut is accessed via an access ramp system in the footwall of the orebody. The reef horizon (a near horizontal layer of sediment in which the gold is located) is reached through two main access drives. Stope access drives are developed at a 70-degree angle off the main access drive in a staggered pattern. The stope is the excavated area where the actual mining of the orebody takes place. The destress cut progresses in an arrow shape towards the hanging wall contact of the orebody. Connections are developed between the stope access drives at regular intervals. The destress cut provides access to the orebody to perform long-hole stoping as described below.
Long-hole stoping is the primary method of excavating the reef at South Deep. This is an efficient method to extract the large volumes required to mine the thick Elsburg reef package. The method can be described as a fan drilled radially from the stope access drives, with hole lengths designed so that the resulting area for excavation takes on the shape of a block. Multiples of these fans, or rings, are then drilled and charged up. The fans are blasted sequentially to break slices of rock, which forms the retreating face. The face is the surface at which mining is advanced, so the retreating face is the roadway that forms as this face is mined.
To remove the ore that is mined large, remote-controlled underground load-haul dumpers scoop up between 10 and 15 tonnes of ore at a time in large buckets.
The large mined-out cavities are then backfilled using mined material post-processing and pumped back underground into the large open stope cavities. These cavities can be up to 18,000m3 large – the size of 10 Olympic swimming pools.
Backfilling is a very simple concept where the cavity creating by mining is refilled using waste mining material. Backfilling is both efficient and environmentally friendly. It results in less waste being deposited on surface, is more energy efficient and leaves a smaller environmental footprint; and it is an excellent filling material, so improves stability and safety of the underground workings. At South Deep, backfilling provides the support needed so that an adjoining 30-metre-high gold-bearing area can be mined, without leaving pillars of unmined ore behind.