A phased approach to mine dewatering

Abstract

The construction of an excavation often means penetrating the local or regional water table. This causes inflows, which if the country rock is significantly permeable can become at best a nuisance to operations and at worst a hazard. Dry working conditions are preferable as they reduce wear and tear on machinery, reduce earth moving costs and often improve slope stability and therefore safety. Options available to mine management are dewatering, diversion, sealing or a combination of methods. To achieve the most effective, least cost method it is essential that the origin of the ground water is determined. The success of a dewatering exercise is directly linked to an understanding of the ground water regime. Mines cannot afford to use “blanket methods” when dealing with ground water. It is essential to target the actual inflows and not divert or seal off water indiscriminately. The potential impact of ground water inflow to a mine can often be assessed at the pre feasibility stage. A hydrogeological investigation is best tackled in three phases. The first phase is a desk study and borehole census and can be initiated by the mine or quarry developers. The main objective is to determine water levels and regional hydrogeology. It is very important that the Phase one hydrogeological investigation is started at the same time as the geological investigation. All exploration drilling records should include comment on where water was encountered and in what volume. The objective of Phase two is to indicate at a first level of confidence the probable impact of mining on the ground water and vice versa. The level of confidence is determined by the quality of data collected in Phase one. At the end of Phase one management will be able to assess if water is going to prove a hazard to mining or not. If it is going to be a hazard then Phase three will be activated. The objective of Phase three is to plan how to reduce or remove the hazard and either handle or divert the probable inflows. Phase three can be accomplished through trial dewatering, computer modelling or through the application of practical experience. The level of sophistication at Phase three is determined by the potential costs and risks involved.