Many South African gold mines have reached the end of their life and have closed or are in the process of closing. Dolomitic aquifers overlie the gold-bearing reefs of the Witwatersrand Supergroup, and many of the gold mines have been disrupted by the inflow of extraneous water into the underground workings during their operational life. Expensive underground pumping has been necessary to dewater the mines. When mines cease operating, the cost of pumping is a major burden on the owners. As a result, some mines have stopped pumping and rewatered the abandoned mine workings. Where there are adjacent mines that are still operating, it is necessary to ensure that appropriately designed water barrier pillars and concrete water plugs are installed to prevent the ingress of water. SRK has assessed several water barrier pillars and carried out detailed concrete water plug designs and supervised the construction.
The first step in the process is to scrutinise mine plans and identify existing pillars that are wide enough to function as a water barrier pillar and extend across the entire mine boundary. Pillar stability needs to be assessed using numerical modelling, available geotechnical data and empirical design guidelines, which are based on a century of Witwatersrand gold mining experience and research. Potential seepage through major geological structures that traverse the pillar also needs to be considered.
It is then important to identify tunnels that traverse the boundary pillar and to assess suitable sites for the construction of concrete water plugs, which can prevent water flow through these tunnels. Major geological structures, weak rock mass conditions and adverse stress conditions should be avoided. Detailed mapping of the joints and minor faults, permeability testing and pre-grouting must be completed. The concrete plugs must be designed to resist shear along the interface between the concrete and rock, under the maximum water pressure when the mine is fully rewatered. Simple analytical models assuming a planar interface are normally used for design. More detailed numerical modelling can simulate the increased shear resistance due to interlocking as a result of the irregular rock surface, which demonstrates that the true safety factors are much higher. Ensuring that the hydraulic gradient along the interface is less than 50 and high-pressure grouting after casting the plugs are important for minimizing seepage. Aggressive water conditions also must be taken into consideration in the selection of materials for construction.
Analysis of historical mining and fluid-induced seismicity during rewatering is essential. The plugs must be capable of resisting the water hammer and damage due to extreme seismic events.
