Excessive groundwater can be one of the greatest and most costly problems faced in underground mining. Maurice Jones reports.
In planning any mine it is vital for both safety and overall mine economy that the location, pressure and likely flow from any groundwater, and even surface water inflow, be determined so that effective measures can be taken.
Commonly information of water can be obtained from historical records including neighbouring mines and from logging exploratory boreholes. Geological structures that may hold or transmit water must be noted and may include permeable strata (aquifers), whether through jointing or porosity, major faults and old mine workings.
Particularly troublesome may be ‘perched’ aquifers or faults that are not intersected by borehole programmes nor reach the surface. Even once mining has begun it may be deemed necessary to probe ahead of any development drives to detect such structures rather than come across a large ‘make’ of water unexpectedly. When a mine or district is closed it is likely to be necessary to plug and seal access drives and shafts to avoid excessive water pumping and ventilation of disused workings. Such seals can also protect potable aquifers from pollution from mine and natural mineral contaminants, such as sulphurous acid water.
Once the water hazard has been determined it will be necessary to implement a strategy to deal with it. Depending on the water pressure, likely flows, and possible damage to the rock structure from large flows, the strategy may emphasise exclusion, handling water within the workings, or a combination of both. For very large pressures and flows the emphasis may have to be on handling the water from within the workings, if the rock structure can handle it. However, the cost of such an operation has to be balanced against the earnings of the mine. In some mines the tonnage of water pumped out exceeds that of ore and waste extracted.
Setting aside the pumping/extraction option, this article emphasises exclusion, usually involving a probe drilling and grouting programme.
A special case is where the mine’s access route passes through a large aquifer, such as by a shaft or main drift/incline/ramp. In such cases temporary exclusion of water by groundfreezing of the hazardous zone is a much-used option. Once the construction is complete the waterproof lined structure of the shaft or incline is relied upon to eliminate or minimise water inflow.
The alternative of forming a permanent grouted ‘plug’ across the aquifer is that most used within the mine workings. It is often a point of great debate whether it is best t go straight for the usually successful but costly groundfreeze option, or to use a pattern of grout holes which, depending on the ground conditions and type of grout used, can sometimes fail, necessitating groundfreezing in any case, so increasing total cost.
Image: IME 2 20130312 Grout Boart LX6.jpg – 256kB) Caption: The versatile Boart Longyear LX6 reverse circulation geotechnical drilling package capable of operating various types of drilling method.
Video 1: Boart Longyear is a provider of mineral exploration drilling services and drilling products
Video 2: The Mosaic potash K2 mine in Saskatchewan, Canada
Video 3: A scientific report questions whether falling water levels in Thirlmere Lakes National Park near Sydney, Australia, are due to coal mining and asks what future impacts mining and Coal Seam Gas production may have on groundwater