This paper illustrates the closure data that was collected in the Gold Mine shaft pillar during the extraction process of this pillar. The use of closure measurements as a diagnostic measure of rock mass behaviour in deep tabular stopes have been described in a number of publications. The data collected over a period of five years, when different support types were used, indicated that cemented backfill reduces the closure rate substantially. This reduces the amount of seismicity per area mined according to earlier work done in the industry. As a drawback, the backfill increases the induced stress on the footwall access tunnels. These excavations show signs of deterioration and additional support is required to ensure long term stability. The rate of closure was too low to analyse the time-dependent closure component observed in other mines and no conclusions can be drawn in terms of the effect of mining rate.
The use of closure measurements as a diagnostic measure of rock mass behaviour in deep tabular stopes have been described in a number of publications (Napier & Malan, 1997; Malan, 1999; Malan et al. 2007, Malan & Napier, 2018). It was found that in the deep gold mines of South Africa, the rock mass undergoes significant time-dependent deformation and the closure data is useful to identify different geotechnical areas and areas prone to face bursting. This time-dependent closure is observed even though the rock mass consists of hard brittle quartzites and lavas and it is caused by the time-dependent fracturing in the rock under high stress conditions. The value of continuous closure measurements was also demonstrated for the platinum industry (Malan et al. 2007). An example of typical closure data is shown in Figure 1 to illustrate the time dependent component recorded in some of the areas. For the gold mines, it was proposed that the data may be useful to identify remnants that may be safely extracted, but further work is required to test this hypothesis. The shaft pillar at the Gold Mine in the Free State is currently being extracted and a decision was made to conduct "continuous" closure monitoring in the stopes to determine if the data will be useful to investigate aspects such as the effect of mining rate and the effectiveness of backfill as support. The data collected and lessons learnt from this study is described in this paper. Of particular interest is a comparison of the data profile in Figure 1 compared to recent data collected. The layouts in the mining industry have become more conservative, with more regional pillars and smaller spans, and it appears as if the typical rate of closure being recorded has decreased.