Island Copper Mine
British Columbia, Canada
Years of Service
1978 to present
The Island Copper Mine is a 50,000 ton per day open-pit copper mine located along the shoreline of Holberg Inlet, on the west coast of Vancouver Island, British Columbia. Operated between 1974 and 1986 this open pit reached a pit bottom level of 1,330 ft. below sea level making it the lowest point on the earth's surface on which you could land a helicopter. The maximum slope height was over 1,600 ft. Waste from the pit was substantially placed into Holberg Inlet extending out into the inlet for about 3,500 ft. In the last stages of mine development the pit rim on the south side of the pit was extended by about 300 ft. beyond the original shoreline placing the pit crest in the natural overburden at about 60 to 80 ft. below sea level, but within the rock waste dump. To avoid huge inflows of sea water through the dump a 128 ft. deep slurry wall was installed for about 4,000 ft. and between 50 ft. and 100 ft. from the pit rim.
The pit was mined to minimize the stripping ratio by maximizing the pit slope angles. Variable alteration of the pit wall rocks resulted in highly variable pit wall strengths and slope angles. A method for soft rock strength estimation was developed for the mine based on back analysis of failing slopes. This enabled the slope angles to be optimized and to be cut at close to limiting equilibrium, with about 15% of the pit slopes failing in shallow failure that could be tolerated by employing appropriate mining methods and monitoring systems.
A slope failure on the south wall of the Island Copper open pit mine initiated the formation of tension cracks both around and through, the slurry wall during the last two years of mining. Steps-outs and de-watering systems were designed to buttress these failures.
The pit was flooded in 1996 creating, temporarily, the highest salt water waterfall in the world.
From 1978 to 1996 when the pit was flooded, Dr. Robertson was the rock slope stability consultant for the Island Copper Mine. Services provided ranged from setting up and assisting with joint and major discontinuity surveys; defining drilling programs for oriented core drilling; sampling and laboratory testing; performing stability analyses for the pit through the various stages of pit expansions; back analysis of failed slopes; design of buttresses and step-outs; design of horizontal drain hole de-watering programs; development of a soft rock strength estimating method for Island Copper rock; analysis of pit wall stability and breakback during and following pit flooding.