Mine Dewatering

Groundwater is a common problem when mining, and developing a mine below groundwater level
presents many challenges. Poorly controlled groundwater will have negative impacts on the safety,
efficiency and economics of mining operations.
If groundwater can be controlled by a planned programme of dewatering, the mine site will typically see
several benefits, including;
• More efficient working conditions: better trafficking and diggability, reduced downtime due to pit flooding
• Reduced blasting costs: lowering groundwater levels in advance of working will provide dry blast holes, reducing the need for more costly emulsion explosives
• Lower haulage costs: Dry ore and waste rock weigh less than wet material, so dewatering of rock provides a haulage cost saving
• Improved slope stability and safety: lowering of groundwater levels and reduction in pore water
pressures can allow steeper slope angles to be used, while maintaining or increasing factors of safety.

Successful dewatering requires a hydrogeological assessment of the mine site. This may be achieved through desk study, borehole investigations, pumping tests and numerical modelling. It is also essential that the most appropriate dewatering techniques are selected and used, because the choice of technique can have a huge influence on the effectiveness of the dewatering.
Mine dewatering can be achieved using a range of groundwater control techniques, depending on the geology and the type of mine:
• In-pit pumping – used to pump from sump areas within the pit
• Perimeter dewatering wells – used to intercept lateral groundwater flow into the pit and to lower groundwater levels in advance of mining
• Pit slope depressurisation drains – inclined or horizontal drains used to provide permeable pathways to allow trapped or slowly draining groundwater behind pit slopes to bleed off into the pit
• Cut-off walls – slurry walls used to exclude groundwater from shallow alluvial or drift deposits, or to seal off preferential flow along permeable strata
• Grouting and artificial ground freezing – used to seal off preferential groundwater pathways, or to reduce ground permeability in advance of shaft sinking or roadway development.

Surface water must also be controlled to allow efficient mining operations. Runoff from the surrounding land surface must be diverted away from the mine, for example by using collector drains and diversion bunds. Within a mine any surface water must be controlled by the drains and sumps to collect the water away from working areas.

Groundwater Investigations

Groundwater investigations play a key role in the development of mine dewatering systems. Investigation options include:

Hydrogeological desk studies

Hydrogeological desk studies

Desk top studies and research into existing information such as geological maps, and information from neighbouring mines, can be a very cost effective way to identify groundwater problems at an early stage.
Numerical groundwater modelling can be used to assess likely flow rates, distance of influence and the potential for adverse environmental impacts.

Installation of monitoring wells

Installation of monitoring wells

Monitoring wells and specialist piezometers installed in advance of dewatering can provide valuable data on hydrogeological conditions, and can be used for permeability testing or groundwater quality sampling.

Pumping tests

Pumping tests

Pumping tests are a reliable way of determining representative mass hydraulic conductivity of soils and rocks, and of providing other information on groundwater conditions. The tests involve pumping a well at a controlled rate, while monitoring flow rate and drawdown of groundwater levels. Pumping tests may involve between a few days to several weeks of continuous pumping.

Borehole permeability tests

Borehole permeability tests

A range of tests can be carried out in individual boreholes, including rising and falling head tests, constant head tests, Lugeon tests, Lefranc tests and packer tests. When carried out in accordance with relevant published standards and interpreted appropriately, such tests can provide some indication of hydraulic conductivity values and groundwater conditions.

Measurement is the first step that leads to control and eventually to improvement. If you can’t measure something, you can’t understand it. If you can’t understand it, you can’t control it. If you can’t control it, you can’t improve it 
– H James Harrington

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