Pillars have been used as stope support since the early days of mining, and they remain the main support component in most present-day shallow tabular mines. The compelling need for using pillars is dictated by the prevailing considerations of rock mechanics at shallow depths, namely the large tensile stresses in the hanging-wall, and geological weaknesses in the hanging-wall rockmass. The hangingwall of any mining excavation is subjected to vertical 'deadweight' tensile stresses. In the case of tabular excavations, according to elastic theory, the extent of the tensile zone becomes larger with increasing ratio of mining span to the mining depth (L/H) and smaller with increasing Yatio of the horizontal to vertical components (k) of the virgin stress tensor. The variation in the extent of the tensile zone over a 200 m span stope, as a function of increasing depth, is shown in Figure 1, where the k ratio is assumed to be high near surface but falls off realistically with depth. It can be seen that large portions of the hangingwall (50 m or more) can be subjected to vertical tensile stresses. In contrast, at great depth (>1000 m), not only is the tensile zone smaller but horizontal 'clamping' forces generated by face fracturing tend to render the hangingwall virtually self-supporting. At shallow depths, there are usually joints and bedding planes which weaken the hangingwall rockmass. For example, in the south-west region of the Bushveld Igneous Complex (BIC), the hangingwall of the Merensky Reef typically contains two major weak parting planes. At 2 to 4 m there is a weak pyroxenite-norite transition, and at 10 to 15 m there is a well-defined parting at the Bastard Merensky Reef contact. In many cases, these parting planes are segmented by vertical joint systems which are mostly developed sympathetically to the faults and dykes.
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