Following a breach in August 2014, Mount Polley Mining Corporation (MPMC) asked Golder to assist their remediation and restoration efforts, including environmental assessments, dam design and reconstruction, river and fish habitat restoration, public and Indigenous communication and consultation, and permitting and water management.
Mining operations resumed within two years and stream recovery is well underway after the tailings dam breach at the Mount Polley Mine in central British Columbia, Canada. The breach was the result of a failure through a glaciolacustrine layer in the foundation of the perimeter embankment of the tailings facility, releasing tailings, water, and dam construction material into the downstream environment, including Quesnel Lake.
Meeting nature’s deadline with an innovative technique
After the breach, the immediate objective was to reduce the risk of additional releases of tailings, and there was significant urgency involved. This is because each spring, streams and lakes in the area swell with melted snowpack, leading to high flows and water levels known as “freshet.” This annual increase in surface water volume had the potential to cause further erosion of impacted watercourses or to wash more tailings into water bodies.
To resolve the problem, Golder took on the task of designing an embankment at the location of the breach. Standing in the way was winter, which comes early in the high, mountainous Cariboo region of British Columbia. Freezing conditions made many conventional construction techniques unworkable. However, the problem needed be resolved during the winter, and before Mother Nature’s deadline, in the form of freshet, arrived.
At this stage, a senior mine waste consultant at Golder assumed the role of Engineer of Record for the stabilization and repair of the Tailings Storage Facility (TSF).
Golder’s solution involved using the cutter-soil mixing technique to build a crushed aggregates embankment with a central low-permeability wall during winter. This work started with constructing an embankment with crushed aggregates. Then a deep probe, fitted with counter-rotating cutter wheels cut its way into the soil down to the target depth in the foundation while injecting a bentonite slurry. As the cutter wheels were pulled back to the top of the embankment, a bentonite and cement slurry was injected down and the cutter wheels mixed the slurry into the crushed aggregates to create a column of flexible cement, which cured into a sub-surface column designed to meet the specifications. A series of overlapping columns placed in a row, form a low-permeability sub-surface cut-off wall along the newly built embankment. This technique allowed construction of the embankment during winter.
Completion of the TSF repairs successfully allowed management of the 2015 freshet flows. Later in 2015, Golder developed a design to raise the TSF embankment. Authorization for construction and restart of operation of the TSF allowed resumption of full mining operations by June 2016 – within two years of the breach.
Integration of engineering and environmental work
Another pressing issue was to meet government directives to stop the ongoing erosion of the exposed stream substrates, to reduce the turbidity of the water flowing into Quesnel Lake.
To do this, Golder worked with MPMC staff and other stakeholders as well as indigenous communities who were also part of the government-to-government regulatory environment. Achieving compliance with the directives, and at the same time rebuilding the habitat, required the integration of the engineering and environmental aspects of the project. This meant that the design of the stream had to be effective at preventing erosion, practical to construct and lead to the development of functioning stream habitat.
Restoring habitat involved re-creating the stream’s normal curves in the channel and adding other features such as areas of fast and slow-moving water, and a range of fish habitat suitable for uses such as spawning substrate and hiding places to allow growth of newly-hatched fish.
The need to complete the project also meant a need for a change from the traditional approach in which a firm such as Golder creates a design, applies for a permit, receives the permit, and then starts construction. Partly to meet the timeline, the work involved significant amounts of field engineering, in which decisions were made on site. This meant that changes to plans could be implemented quickly, but with full input of all representative groups responsible for carrying out that implementation through a working group.
Send in the drones
The Canadian federal government requires ongoing monitoring for five years after project completion. Traditionally, monitoring of physical stability has been carried out by a survey team on foot, but Golder’s team has pioneered the use of Unmanned Aerial Vehicles (UAVs, or drones) for this work.
Twice a year a team member visits the site and flies a drone about 15 metres (45 feet) above the ground, capturing images of the remediated area. These images are then stitched together using photogrammetry software, which generates a single image of the area.
Promising results for environmental restoration
Golder’s team has seen encouraging signs of restoration – plants and trees growing, the same fish species as before the breach, and other wildlife returning. Golder’s survey team reports that there have now been two years of rainbow trout spawning in the reconstructed habitat, as well as other wildlife including bears, lynx, and a variety of birds.
Golder’s monitoring of the project area over a two-year period has found that out of 770 spatially monitored features such as boulders and logs, 757 have shown no signs of movement – indicating that the design of the stream, and the restoration measures, have been successful.