CLIENT: Taseko Mines Ltd.

LOCATION: McLeese Lake, British Columbia, Canada

The Gibraltar Mine, Canada’s 2nd largest copper mine, is located approximately 45km north of Williams Lake, BC near the community of McLeese Lake. The mine began operation in 1972 under Placer Development and was later bought by Westmin Resources Ltd. in 1996. It was subsequently closed due to low copper and molybdenum prices, however in 1999, Taseko Mines Ltd. Acquired Gibraltar. Increases in copper prices by 2004 justified re-opening the mine. The mine comprises four open pits: Granite, Gib East, Gib West, and Pollyanna. Pit slope heights up to 300 meters are typical; mined in 15 meters high benches. The inter-ramp slope angles vary from 33° to 46°.  Waste rock dumps surround the open pits – The waste rock dumps are up to 140 meters high with overall slope angles of approximately 37°.

The geology and operations of the mine result in some complex challenges for the stability of the waste dumps and pit slopes:

  • The tills and glacial fluvial soils are often saturated. Fluvial and lacustrine soils stripped from the open pit footprints during early phases of mining were dumped just beyond the initial pit limits. The in-situ soils and stripped soils are now located in the foundations of more recent waste rock dumps
  • The tonalite rocks of the open pit slopes are generally strong, foliated, and cross-cut by complicated systems of faults. The faults combine to form complex slope instabilities that may be affected by high pore water pressures and blast disturbance
  • The high production rate of the mine must be supported by large blasts and high bench turn-over rates; the excavation has limited time to settle into equilibrium prior to the next geometry change and groundwater has limited time to drain from the rock mass
  • Slopes are mined through previously dumped waste rock and previous rock slope failures

By 2011, and after seven years of intensive work on several major mine development projects, BGC’s rock mechanics team was facing a reduction in feasibility studies. Our team shifted focus from studies to mine operations. We secured a small scope of work to review Gibraltar’s pit slope designs for phase 4 of the Granite pit. By 2012 the team was designing the pushback for phase 5 (Figure 1) while helping Gibraltar’s geotechnical team manage slope instabilities in phase 4. BGC was also subsequently asked to take over the designs for proposed waste dump expansions. Key projects completed by BGC between 2012 and 2021 are:

  • Field investigations and pushback designs for phases 5 and 6 of the Granite pit
  • Field investigations and designs for re-starting and expansion of the Pollyanna and Gib East pits and Mines Act permitting support
  • Groundwater model construction and predictions for the Granite pit, Connector zone, and Gib east pit
  • Design and field reviews of the south perimeter ditch capturing surface water runoff from the waste dumps
  • Foundation investigations and designs for the 7 centre, 7 east, 5 extension dumps, and overburden stockpiles
  • Quarterly and annual reviews of the pit slopes and waste dumps with annual regulatory reporting

Figure 1: Overview of the Granite Pit (looking west, from the crest of the east wall); the Phase 5 pushback of the south wall is in the background.

BGC has completed many smaller projects to assist in the management of pit slope and waste dump instabilities; a few examples are:

  • Investigation, instrumentation, and analysis of waste dump instabilities . The team developed dump advance rate recommendations to reduce the likelihood of inducing excess pore water pressures in the foundations
  • Characterization, numerical modelling, monitoring data analysis, and horizontal drain hole recommendations for complex G5C south wall instability. The team undertook numerical modelling with Phase2 and Flac3D
  • Mapping and interpretation of faults from photogrammetry mapping data and Specific Energy data from blast hole drills. This fault modelling has been important to the interpretation of slope instabilities in the pit slopes
  • Planning horizontal drain and vertical depressurization well targets for the pit slopes
  • Rock fall analysis and berm/barrier designs as part of managing active instabilities (Figure 2)
Figure 2: Managing rock falls with step-ins and catchment berms

We have helped the mine’s geotechnical team manage some challenging and complex slope failures. We helped them keep mining by:

  • Making slope design changes, supported by field reviews and stability analyses
  • Identifying surface water management needs; field visits to provide recommendations for re-grading of dump crests, pit crests, and filling of cracks to reduce the infiltration of surface water to unstable areas
  • Planning slope depressurization via horizontal drains and vertical wells; including the optimization of the programs through the 3D visualization of drain paths and intercepted flows with the modeled faults
  • Working with the mining engineers to reduce dumping rates and avoid the generation of excess pore pressures in the foundations via rapid loading and reducing mining advance rates until the slope deformation rates were within manageable ranges