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Depressurization of the north wall at the Escondida Copper Mine, Chile

Abstract Concurrent dewatering and slope depressurization operations have been underway at the Escondida open pit since 1996. A hydrogeological investigation has been undertaken as part of the depressurization operation on the north wall. It has shown that there are two distinct hydrogeological units in the slope, the altered and unaltered Escondida Porphyry. The hydrothermally altered Escondida Porphyry is clay rich and has relatively high matrix permeability. The underlying silicified, or unaltered porphyry, has very low matrix permeability and groundwater flow is in steeply dipping NW-trending fracture systems. Pore pressures in the silicified porphyry are higher than the altered material. A system of horizontal drains and vertical wells has been in operation to reduce pore pressures in the altered porphyry. To investigate effective drainage measures for the silicified porphyry, a groundwater model of the slope was constructed. This analysis showed that a drainage tunnel with horizontal drains was the most effective method of draining significant areas of the slope to the required pore pressure targets.