Whistle Mine

Figure 4 (a to d) Photos illustrating the key aspects of constructing the Whistle Mine backfilled pit cover system.

Figure 1 Location of Whistle Mine

Design, Construction and Performance Monitoring of the Backfilled Pit Cover System

Background:

Whistle Mine, located approximately 60 km north of Sudbury , ON , Canada (Figure 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 wette

Figure 2 Output from the Siberia model showing failure of the first landform alternative after 100 years.

r 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).

Although lime was added to the waste rock during backfilling, a cover system is required for the backfilled pit to minimiz

Figure 3 Rendering of the final landform for the covered backfilled pit.

e acidic drainage over the long term. Based on the results of geochemical modelling, limiting the influx of atmospheric oxygen to the waste rock is of greater importance than limiting the infiltration of meteoric water on the long-term water quality of the pit. A water cover is not feasible for this site because of the absence of a pit lake and a 13% slope from the north to south perimeter. Inco retained us in 2000 to provide engineering services for designing, supervising construction, and performance monitoring of a cover system for the Whistle Mine backfilled pit.

Design of the Cover System:

A multi-layer soil cover comprised of a leveling 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.

Based on the results of the above design work, the final cover system includes a compacted clay barrier layer, nominally 0.5 m thick, overlain by a minimum of 1.2 m of sand and gravel to protect the barrier layer and provide a rooting zone for vegetation. The primary design objective of the pit cover is to limit the influx of atmospheric oxygen to the underlying reactive waste rock by maintaining the barrier layer at or near saturation at all times, thereby creating an oxygen ingress barrier.

Two alternatives were evaluated to manage runoff generated from the spring snowmelt and rainfall events over the long term. The first alternative comprised a highly engineered system with lateral diversion berms and two heavily armoured channels to convey runoff waters off the landform. Output from SIBERIA , a 3-D landform evolution model, shows breaching of the lateral berms, development of gullies and rills, and in general, failure of the landform over a 100-year period (Figure 2). The second and ultimate final landform consists of several catchments oriented parallel to the slope with progressively higher levels of erosion protection in the channels downslope (Figure 3). This landform is more analogous to natural systems and provides a micro-topography that will aid in the success of re-vegetation 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.

Cover System Construction:

	Blasting and cleaning along the pit perimeter to blend the cover into the surrounding landscape and limit O2 ingress (Figure 4a).
	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 (Figure 4b).
	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; (Figure 4c)and
	Seeding the cover system with a variety of native grass and legume species and finally, installation of a performance monitoring system (Figure 4d).

Performance Monitoring:

Designed and installed a field-monitoring program for the pit cover system to evaluate its performance over time. The system includes a meteorological station, two weirs for measuring runoff flows, two automated stations for monitoring net percolation rates and in situ moisture and gas concentrations within and below the cover system, 13 secondary stations to monitor spatial performance, and four groundwater monitoring wells. Based on field data collected up to December 2006, the pit cover system is performing as expected; the influx of atmospheric oxygen and meteoric water to the waste rock backfill has been substantially reduced since construction of the cover system.

Value Added by Mineclosure.com to the Project:

Expertise in numerical modelling for development of an optimized cover system design based on long-term predictions of oxygen ingress, net percolation, and surface erosion.

Knowledge of the potential impacts of site-specific physical, chemical and biological processes on the sustainable performance of cover systems in a variety of climatic settings.

Design and installation of specialized equipment for monitoring performance of cover systems.