Excess water content in mine waste poses a particular set of challenges for closure of mine waste storage facilities. For well-designed and well-built structures high water content will not cause physical instability and catastrophic failure; however managing the residual leachate is a complex technical challenge to be dealt with at significant cost to the operators. This is particularly true in the case of heap leach facilities, designed to be operated at relatively high water contents while also retaining as little residual water as possible. The state-of-practice in developing leachate management plans for the closed facilities includes deterministic models such as Heap Leach Draindown Estimator (HLDE) developed in the state of Nevada, the US. Such models rely on an approximation of the unit draindown rate using theoretical formulations of unsaturated flow which is then extrapolated to a regular orthogonal geometry and a single defined date for cessation of irrigation. In SRK’s experience this approach, if adopted with due care and validated through calibration, can be reasonably used on smaller heap leach piles; however, operators must often rely on the experience of their employees to “gut check” the validity of the draindown durations predicted by HLDE.
An improved method is proposed in this paper, consisting of a combination of numerical modelling of unsaturated flow and analytical modelling to predict the time required to achieve passive water management. The heap leach pile is first discretized geometrically into domains, grouping areas of similar height. Then, in the second step, the unit draindown rate is determined for each domain through a rigorous finite element numerical modelling based on the physical and hydraulic properties of the leach material. The resulting draindown curve is then multiplied by the representative surface area to compute the expected volume of draindown from each of these areas. In the third step of the assessment, the individual draindown volumes are assembled in a time-scale taking into consideration the actual schedule of when irrigation has been completed in the various regions of the facility leading up to the final closure date. Various complexities can be added, like recirculation to some areas and not others, or enhanced evaporation methods for portions of the recirculation.
