Back in January 2004, monitoring data at Teck’s Highland Valley Copper mine (HVC) in British Columbia, one of the largest open pit copper mines in the world, showed signs of instability in the mine’s east wall. Until that point, the east wall had been considered generally stable, so the data was worrisome. After some consideration, Teck decided to postpone expansion plans to investigate ways to depressurize the east wall so that eventually the expansion could go ahead safely.
Pressure from all sides
“Part of the expansion involved a pushback on the east wall, which is filled with overburdened glacial sediments,” says Sebastien Fortin, chief geotechnical engineer at Highland Valley Copper. In order to stabilize the slope, Teck’s team had to determine how to flatten it out enough to be stable yet remain steep enough to be within the range of the economics of the project. “To achieve that ‘sweet spot’ in the wall angle, we had to decrease the groundwater pressure within the clay sediment,” explains Fortin.
The work began by conducting an extensive geotechnical and hydrogeological field investigation program on the east wall. This included characterizing the slope’s glacial sediments, as well as conducting confirmatory drilling, window mapping and photogrammetry of the exposed bedrock slopes, and determining the groundwater distribution and flow for the various areas of the east wall.
The mining company brought in experts from Golder Associates and Piteau Associates, who developed a strategy to depressurize the clay layer in the east wall using a system that consisted of more than 80 wells, all strategically placed to address practicable depressurization targets. Timing was key because the mine’s expansion depended on the depressurization project’s success and completion by May 1, 2011.
Due to its texture, the east wall clay sediment was resistant to releasing the water, so vacuum-assisted wells had to be used. That was not the biggest challenge the project faced, however. “The technology is pretty nuts and bolts for the most part,” says Fortin. “But because we were doing the work while mining was being done, we would have had to wait for the ground to be released. The problem was that if we had waited for the ground to be released, we would not have been able to complete the project on time.”