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 extremely high. These stresses are aggravated with the large excavations associated with massive mechanised mining, as they could surpass the inherent rock strength, 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 can result in damage to the mine’s infrastructure.
To combat the build-up in rock stress, areas are mined through a series of smaller excavations. These help 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 supported by pillars. These pillars then yield and absorb pressure in a controlled fashion. The effect on the surrounding rock mass is to create an in-situ stress similar to shallower mines, where large excavations can be safely mined.
The destress cut is accessed from an access ramp system in the footwall of the orebody and the reef horizon is entered with two Main Access Drives (MADs). The MADs progress into the orebody and Stope Access Drives (SADs) are mined on either side and parallel to the MADs in the dip direction. The destress cut progress in an arrow shape, maintaining a lead/lag of 0 to 6 metres, towards the top contact of the targeted orebody. Holings are developed between the SADs on intervals of 20 metres forming the yield pillars. The destress cut provides access to the orebody to perform longhole stoping as described below.
Longhole 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.
To remove the ore that is blasted large, remote-controlled underground load-haul dumpers load and move 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 in size – 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.