Civil Engineering August 2021 | Vol 29 No 7

32 August 2021 Civil Engineering it challenging to identify and apply ap- propriate levels of safety when designing the dimensions of water channels and dams on mines. At the same time, some older mines have found that their existing infrastructure for water management may not cater adequately for the growing intensity of rainfall events. Mining companies are now applying the concept of probable maximum flood (PMF) in their design processes for mine infrastructure. This is a clear recognition that the approach to water management is changing, and that a much higher margin of safety is called for, which in turn increases the size of these water management controls. A significant impact of this engineering shift is the added cost that is expected with these larger structures with greater capacity for flow and storage. Managing the PMF could raise the capacity and size of a water channel, for instance, by anything from three to six times – adding considerably to excavation, concrete, and other costs. TRAPPING SILT An important aspect of water infrastructure related to mine waste facilities are silt traps, which are also impacted by higher water flows. These are crucial in preventing the silting up of pollution control dams, whose HDPE lining makes it risky to apply any form of mechanical cleaning – as the liner could be punctured. Removing as much silt as possible from the inflowing water is imperative to ensuring that these dams perform their control function effectively. In the course of our audits of these facilities, however, it is not uncommon to find silt traps that are not performing this task adequately due to changing climatic conditions. An effective silt trap provides the nec- essary retention time for the fine material to settle, before the water flows into the dam. Maintenance is also imperative, to remove material mechanically from the concrete-lined silt trap at regular inter- vals. For this reason, two compartment traps (one operational and one standby) will typically be constructed in these ap- plications. This allows one to be cleaned while the other continues to operate. The cleaning process is also time-consuming as material needs to dry out to a large extent before it can be practically loaded and hauled to another location. This drying process can also take time, as the fine material will by its nature tend to trap moisture. UPGRADING INFRASTRUCTURE Where mines realise that their existing water management infrastructure around a waste facility – or even around a processing plant – is no longer fit for purpose, upgrading may be required. The upgrading of existing infrastructure may be inevitable for mining operations in regions where rainfall intensity is increasing New channel capable of handling a PMF under construction