Whistle Mine, Sudbury, ON Canada
Whistle Mine, located approximately 60 km north of Sudbury, ON, Canada (Fig. 1), was a satellite nickel orebody that was developed by Inco Ltd. (now CVRD Inco Ltd.) between 1988 and 1998.
Approximately 6.4 Mt of waste rock was stockpiled on the surface during the life of the open pit mine. Due to its acid-generating potential and proximity to local surface water receptors, all waste rock was relocated to the open pit between 2000 and 2001. The climatic setting for the site is characterized by generally wetter conditions in the fall, winter and spring (annual precipitation of 870 mm with 30% as snowfall), and drier conditions during the hot summer months (annual potential evaporation of 520 mm).
Okane designed a multi-layer soil cover comprised of a levelling course, a barrier layer, and a growth medium layer was selected for capping the 9.7 ha backfilled pit. The cover system design project comprised the following major tasks:
- Determination of the preferred material for the barrier layer through evaluation of field performance monitoring data obtained from test covers and review of estimated costs for full-scale construction of various alternatives;
- Physical & hydraulic laboratory characterization of the barrier layer and growth medium materials;
- Soil-atmosphere numerical modelling for determination of the minimum cover layer thicknesses based on predictions of net percolation and oxygen ingress, including 2-D simulations to address the potential impact of the sloping cover on saturation levels in the barrier layer;
- Slope stability analyses of the preferred pit cover system;
- Landform evolution and erosion numerical modelling for the development of a sustainable pit cover runoff management system design;
- Design of a performance monitoring program for the pit cover system; and
- Consideration of the potential impacts of various physical, chemical and biological processes on sustainable performance of the preferred cover design.
The final landform design for the backfilled pit consists of several catchments oriented parallel to the slope with progressively higher levels of erosion protection in the channels downslope. This landform is more analogous to natural systems and provides a micro-topography that will aid in the success of revegetation efforts. A series of ponds at the base of the slope will be actively managed in the short term until a mature grass cover establishes, and over time, a wetland area will establish to provide long-term attenuation of peak surface flows and diversified habitat for wildlife.
Construction of the pit cover system occurred during the snow-free periods of 2004 and 2005. A local contractor completed all the earthworks, while OKC provided construction supervision and on-site QA/QC testing services. A quality assurance / quality control (QA/QC) program was in place to monitor construction of the pit cover system and ancillary infrastructure, including a surface water management system and performance monitoring system. Key aspects of the construction project were:
- Blasting and cleaning along the pit perimeter to blend the cover into the surrounding landscape and limit O2 ingress;
- Installation of a robust geotextile prior to constructing the barrier layer to prevent migration of clay-size particles into the underlying coarser materials over the long term;
- Cross-slope ripping of topsoil into the upper granular cover material to reduce soil loss until a mature grass cover establishes and ensure, at least in the short term, runoff waters from the catchment slopes flow along the rip lines to the erosion-protected channels; and
- Seeding the cover system with a variety of native grass and legume species and finally, installation of a performance monitoring system.
Okane provided a record of construction for these items including the results of QA/QC testing and inspections conducted by the Engineer. Based on the records of construction, the Whistle Mine pit cover system and ancillary infrastructure were constructed in general accordance to the Design and Specifications.