The Jwaneng and Orapa Mines of Debswana Diamond Company have reached depths of approximately 300 metres (m), and resource evaluations indicate that the pits will extend to depths of approximately 900 to 1,000 m. These depths will be achieved in approximately 35 years, requiring aggressive planning for how water will be managed to optimize pit slopes. Groundwater, surface water, and pore-pressure controls are paramount if stable slopes and safe mining are to be achieved. In the case of both mines, geologic units that constitute the pit slopes are generally of low permeability, dictating that dewatering/depressurisation systems must be implemented expeditiously.
Numerical groundwater flow modelling for both mines demonstrated that pit perimeter dewatering boreholes will be essentially ineffective in achieving depressurisation goals (that are established by stability analyses). Low-permeability geologic materials prevent effective propagation of low pressures within the dewatering borehole. As such, the focus of depressurisation is on in-pit dewatering boreholes along with sub-horizontal drains. Additionally, emphasis is also placed on preventing enhanced recharge from reaching the perimeters of the mines. Natural recharge from precipitation at the mines is low; however, artificial recharge (for example, from leaking pipes at the processing plants, leakage from tailings and slimes storage facilities, and surface-water runoff from nearby waste rock piles) is high and requires engineering intervention measures in order to maintain slope stability as pit depths increase.
Management concepts for addressing depressurisation include under-drainage utilizing tunnels and drains, eliminating enhanced recharge and future disposal areas located farther from planned pit expansions. In addition, surface-water control systems to intercept and direct precipitation runoff away from the pits are an integral part of effective pit groundwater controls.
The depressurisation and water-management techniques listed above require close cooperation and integration between hydrogeologists, geotechnical engineers, mine planners, and, lastly, mine pit operations personnel.
