Groundwater and surface water control at Highland Valley copper mine (HVC) (in rock and soil both)

Abstract

This paper is about dewatering of the two large open pit porphyry copper-molybdenum mines of Highland Valley Copper in BC, Canada. These two pits occur within the well differentiated Guichon Creek Batholith with phases progressively more acidic and coarser grained towards the center and intersected by intense fracturing within the ore bodies and also concentric alteration haloes. Also, they are both cut by the steeply dipping Lornex fault which is an average 155 m wide faulting zone with frequent sympathetic faults adjacent to it. As the two pits have mined down at depth, they have exposed bedrock especially with frequent toppling-type faults and shears which have exhibited varying degrees of toppling and ravelling of the pit walls. Also, on the east wall of the Valley Copper pit, there is up to about 350m of glacio-fluvial overburden, originally with artesian flows in two or three aquifers. The toppling is exacerbated by the hydrostatic pressure of the groundwater in the pit walls. Both pits have significant groundwater and run-off in the spring. In order to be able to continue mining down to depths of about 600 to about 750 meters it has been necessary to de-water the walls and divert the surface run-offs to prevent infiltration into the toppling faults. So two water diversion projects were necessary at the crest of the highwalls and were completed to drain both by gravity and pumping the spring run-off water away from the two pits. Also, a major well drilling project is ongoing in the glacio-fluvial overburden aquifers to dewater them with upwards of 4500 US gpm flows. In addition, an annual series of horizontal drainholes were drilled and are ongoing approximately every three benches to dewater and depressurize the bedrock. Minor bedrock wells in the crest of the Lornex Pit also contribute to the dewatering. All the dewatering has allowed the two pits to mine down to close to design depths without significant disruption.