4.2 Open pits

Open-pit operations involve both the creation of a mine pit and the placement of waste and sub-economic materials either on the adjacent ground surface (out-of-pit) or within the pit (in-pit). Monitoring of the waste rock types removed from the pit and their selective placement in dumps or as backfill in mining voids is part of an effective management plan for minimising acid and metalliferous drainage (AMD).

Other key aspects requiring monitoring are the geotechnical stability and safety of pit walls, groundwater ingress and drawdown, and groundwater quality. Geotechnical stability and safety are monitored by daily inspections by qualified geotechnical personnel, controlling access to the pit, and the use of slope stability monitoring equipment such as radar scanning and survey prisms to monitor wall movements. The website http://www.smenet.org/store/mining-books.cfm/Slope-Stability-in-Surface-Mining/194-0 contains relevant information on pit slope design, case studies of pit slope stability, and the stability of waste rock dumps and tailings dam stability, and in each case includes monitoring. Performance assessment and monitoring are also discussed in Read & Stacey (2009).

Groundwater ingress is monitored and controlled by in-pit pumping, and groundwater drawdown is monitored by piezometers installed in boreholes around the perimeter of the pit and beyond. Boreholes are sampled to monitor groundwater quality surrounding the pit.

Prior to mining, the impacts on water of creating an open pit are quantified using various modelling tools, as described in the Water management leading practice handbook (DRET 2008a). The modelled parameters enable pit dewatering requirements and associated impacts to be predicted before mining so that mitigation measures can be planned and implemented. The extent of interactions between surface water and groundwater and the pit are based upon assumptions about the staged development of the pit and adjacent landforms scheduled in the life-of-mine plan.

As a consequence of the limitations of modelling, leading practice requires the model, as well as the dataset and assumptions that are used as a basis for modelling, to be verified and amended according to data collected during the operational phase (Kuipers et al. 2006). Modelling should not be limited to its use as a once-off tool that is run only with initially limited input data. It should instead be regarded as an iterative process; monitoring should focus on the collection of data to which the model is particularly sensitive, and for which there is initially less data than needed to produce a well-calibrated and robust model. This will enable the model to be revalidated and recalibrated, and its reliability and accuracy as a predictive tool to be continually improved.

The requirement for predictive modelling goes beyond estimating likely water inflow rates to include predicting water quality based on key geochemical characterisation parameters, as well as monitoring the effectiveness of various control measures (such as seepage barriers). Where open-pit mines are close to water resources with identified beneficial values (such as potable drinking water supplies, grazing and groundwater-dependent ecosystems), additional attention is required in relation to the lateral extent of groundwater drawdown and the potential for contamination.

Post-mining objectives for the open-cut pit also influence which key investigations and what data gathering are needed during the operational phase. Operational monitoring, efficiently combined with life-of-mine pit management, enables timely and effective closure strategies to be developed in consultation with regulatory and community stakeholders. Questions to be considered include the following:

  • Will there be impacts on nearby rivers and groundwater resources during or after mining?
  • Will dewatering or stream diversions around pits and underground workings affect groundwater- dependent ecosystems?
  • Will the water in a flooded open pit be of adequate quantity and quality to enable access and use by others for grazing, recreational or urban use?
  • How will the water levels and seasonal fluctuations in those levels affect pit wall stability?
  • Could valuable water resources drain to the final void and become contaminated, rather than remain accessible and usable for downstream and adjacent water users?
  • Could contaminated water from a flooded pit contaminate adjacent groundwater and surface water systems?The contamination of groundwater could occur if the flooded pit level rises above the surrounding groundwater level to become a source, rather than a sink, for contaminated pit water; the contamination of surface waters could occur if the contaminated pit water overtops the pit.

After mine closure, geotechnical slope stability and safety must be maintained and groundwater recovery and surface water inflows to the pit must be assessed following the cessation of dewatering. Geotechnical slope stability and the safety of pits generally require a minimum of perimeter bunds and fences around the pit, and usually the flattening of pit edges down to the estimated final low water level. Pit water quality should be estimated using data gathered during operations and by the use of the refined hydrological and hydrogeological models that have been developed using that data. Groundwater monitoring will be needed to verify predictions and, if objectives are not met, intervention with control measures will be required. Mine operators therefore have a strong imperative to undertake leading practice rehabilitation, which minimises or eliminates the need for ongoing inspections and maintenance (of course, this should apply to all domains).

At the very least, monitoring will be needed to provide input into predictive modelling of the chemical limnology and water quality of a future pit lake. Further details on the assessment and monitoring of seepage water quality are in the leading practice handbooks Preventing acid and metalliferous drainage (DIIS 2016d), Mine closure (DIIS 2016e) and Mine rehabilitation (DIIS 2016g).

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