Groundwater flow in the vicinity of open pits and underground mines varies 3-dimensionally and with time. Assessing mine dewatering commonly requires the development of 3-dimensional (3D) numerical groundwater models, based on 3D geological, structural, and hydrogeological data, to fully characterise the groundwater flow. SRK often applies the finite-difference code Visual MODFLOW-SURFACT in mine dewatering projects. This code goes beyond the standard MODFLOW code to simulate saturated/unsaturated conditions (multiple water tables), open pit excavation (using seepage face cells and collapsing model grid), and dewatering wells using the fractured well package. The hydrogeological team at SRK Denver has used numerical modelling as an integral tool for the wide variety of mine dewatering projects listed in the Table above. All of these models were used to simulate passive inflow to open pits and underground mines. Active dewatering options were modelled at Elegest, Orion South/Star, Silangan, and Hycroft; while pore pressures for slope/roof stability analyses were also used at Cerro Matoso and Livengood.
Potential environmental impacts to groundwater levels and surface water flows were modelled at Olovskoye, Goldfields, Paredones Amarillos, Orion South/Star, Cerro Matoso, Livengood, and Hycroft; with post-mining conditions, including pit lake formation or flooding of the underground mine modelled at almost all of the environmental projects.
Where active dewatering is required, the groundwater model was used to evaluate the most efficient dewatering option to reduce residual passive inflow to the mine (Cerro Matoso), and to define the optimal pumping rates and well spacings for the dewatering system (Orion South/Star). Where hydrogeological conditions are complex, the model was used to reduce both pumping costs and hydrogeological risks to the project (Elegest), optimising the mine plan.
In addition to predicting dewatering requirements and mining impacts, we have used 3D modelling to:
• Guide field investigations to test the most sensitive hydrogeological units and parameters (Goldfields, Orion South/Star, Livengood, and Hycroft)
• Analyse results of comprehensive testing programs where analytical formulas do not work (Olovskoye, Elegest, Orion South, and Cerro Matoso)
• Model block cave (Silangan) and longwall (Elegest) operations, where hydraulic conductivity values change in time and space above the mine area
• Conduct uncertainty and sensitivity analyses (in all of the projects in the Table above).
