Risk-cost-benefit framework for the design of dewatering systems in open pit mines
Creators: name: Sperling, Tony
Publisher: name: University of British Columbia
Year: 2011

description: Control of groundwater plays an important part in operations at many open pit mines. Selection of an efficient and cost effective dewatering program that will improve slope stability of the pit walls is frequently complicated by the complex and somewhat uncertain hydrogeologic environment found at most mine sites. This dissertation describes a risk-cost-benefit (RCB) framework that can be used to identify the most effective dewatering strategy under such conditions, because the stochastic framework explicitly accounts for uncertainty in hydrogeologic and shear strength parameters in the groundwater flow, slope stability and economic analyses. In the framework, the monetary worth of each design alternative is measured in terms of an economic objective function. This function is defined in terms of a discounted stream of benefits, costs and risks over the operational life of the mine. Benefits consist of revenue generated from the sale of mineral concentrate. Costs include normal operating and dewatering expenses. Monetary risks are defined as the economic consequences associated with slope failure of the pit wall, multiplied by the probability of such a failure occurring. Selection of the best design strategy from a specified set of alternatives is achieved by determining the economic objective function for each design and then selecting the alternative that yields the highest value of the objective function. Estimation of the probability of slope failure requires an accurate assessment of the level of uncertainty associated with each input parameter, a forecast of how dewatering efforts are expected to affect pore pressures in the pit wall in light of the uncertain hydrogeologic environment, and an evaluation of the effect that the pore pressure reductions will have on improving stability of the pit wall. Prediction of the pore pressure response to dewatering efforts is achieved with SG-FLOW, a steady state, saturated-unsaturated finite element model of groundwater flow. Slope stability is evaluated with SG-SLOPE, a two dimensional, limit equilibrium stability model based on the versatile Sarma method of stability analysis. To account for input parameter uncertainty, both the groundwater flow stability models are invoked in a conditional Monte-Carlo simulation that is based on a geostatistical description of the level of uncertainty inherent in the available hydrogeological and geotechnical data. Besides documenting the methodology implemented in the framework to conduct the geostatistical groundwater flow and economic analyses of the objective function, this dissertation also presents a sensitivity analysis and a case history study that demonstrate the application of the RCB framework to design problems typically encountered in operating mines. The sensitivity study explores how each set of input parameters, including hydrologic data, shear strength parameters, slope angles of the pit wall and dewatering system specifications impact on the profitability of the mining operation. The study utilized a base case scenario that is based on overburden conditions at Highland Valley Copper; therefore, the conclusions cannot be applied blindly at other sites. However, the framework can be used to formulate site specific conclusions for other large base-metal open pit mines. After the objective function was calculated for the base case, the aforementioned input parameters were systematically perturbed in turn to study how each parameter impacts on profitability of the mine. The sensitivity study showed that in the particular case analyzed changes in the slope angle and dewatering efforts can improve profitability by many millions of dollars. In particular, steep slope angles can be utilized in the early stages of mine development while the pit walls are relatively low, and then flattened as the pit wall height increases and the monetary consequences of slope failure become more pronounced. Furthermore, the sensitivity results indicated that pit dewatering is likely to be effective over a range of hydraulic conductivities from lxlO"8 m/s to lxlO'5 m/s and that accurate estimation of the mean hydraulic conductivity is much more important than estimating other statistics that describe the hydraulic conductivity field, including the variance and the range of correlation. Results of the sensitivity study clearly demonstrate that the RCB framework can be used effectively to identify the most effective dewatering strategy given a limited amount of geologic and hydrologic information. Also, it is shown that the framework can be used to identify the most important input parameters for each specific dewatering problem and to establish the approximate monetary worth of data collection. The case history study documents how the RCB framework was applied at Highland Valley Copper (HVC). Groundwater control is recognized as an important component of mining operations at this mine site; dewatering measures utilized on the property involve both high capacity dewatering wells and horizontal drains. The benefits of pit dewatering include improved slope stability, drier operating conditions in the pit, and a convenient production water supply. These benefits do not come cheaply, HVC is expecting to spend in excess of six million dollars on groundwater control in the next ten years. Before investing such large sums in groundwater control, mine management should be confident that the capital investment is justified, i.e. that the resulting economic benefits will significantly exceed the costs of the dewatering effort. Using historical data provided by HVC, the case history study documented in this dissertation shows how the RCB framework is used to identify the most profitable combination of slope geometry and groundwater control in design sector R3 of HVC's Valley Pit. By considering three possible slope angle and groundwater control options it is shown that by continuing to implement an aggressive dewatering program, HVC can expect to reduce operating costs by as much as nine million dollars in this design sector.