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<RGC:Project Experience
Island Copper Mine
British Columbia, Canada
- Client:
- BHP-Utah Mines Ltd.
- Years of Service:
- 1978 to present
- Project Description:
- 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.
- Services Provided:
- 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.
Papers:
Mathis, J.I., Robertson, A. MacG., and Robertson, R.B. (1993)
Material
Mining Below Sea Level: Design and Performance of the South Slope Extension at Island Copper.
In: Proccedings of the International Congress on Mine Design, Kingston, Ontario,
Canada, August 24-26.
Robertson, A. MacG., and Robertson, R.B. (1990)
:
Geotechnical Considerations in the Design of the Island Copper Mine South Wall Pushback.
In: International Symposium on Mine Planning and Equipment Selection in Surface
Mining, the University of Calgary, November 7-9.
Robertson, A. MacG., Olsen, R. and Pierce, G. (1987)
:
Assessment of Weak Altered Rock Masses at the Island Copper Mine.
In: Proceedings from SME Annual Meeting, Denver, February.
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