Covers are constructed on these facilities at mines sites: tailings impoundments, heap leach pads, waste rock dumps, sludge ponds, and solid waste disposal units (the equivalent of a landfill). Generally the cover is constructed as part of the closure and/or reclamation works. A wide range of cover types have been designed and constructed on mine facilities world wide. The specifics of the cover are dictated by the waste covered, the environment of the mine and in particular its climate, and the governing regulations. Accordingly, in this paper, we survey cover types by county, facility including ore type, and climatic zones.

Cover Design Criteria

General Criteria

Design and performance objectives for a mine waste disposal facility cover may include:

· Limit infiltration

· Control air entry

· Resist erosion by wind and water

· Remain stable statically, seismically, and in the long term not creep or slide down the sides of the pile

· Support vegetation including the climax vegetation and the biotic regime associated therewith (e.g., ants and termites)

· Endure for a defined period.

Cover Functions

Here is a list of soil cover functions, somewhat different but valid.

Control Radiation
Stabilize the Waste Pile
Limit Seepage and Leachate
Limit Thermal Influences
Promote Vegetation
Limit Access

Regulations

Relevant and appropriate laws and regulations are generally the basis of cover design criteria. The facility owner may impose additional criteria on the basis of corporate objectives and standards. The consultant has the obligation of knowing, advising on, and designing to standards and criteria established for a specific project.

With the exception of coal mining in the United States (1977 SMCRA Legislation; Nawrot et al 1988), the review did not find reference to prescriptive regulations for the design and construction of soil covers associated with the mining industry. In Australia, the Australian Best Practice Guidelines (Australian EPA 1995) presents a set of common closure objectives that a mining company planning waste rehabilitation should consider. The South African coal mining industry is subject to a set of closure recommendations posted by the Chamber of Mines (DWAF 1992). The MEND program in Canada has produced a number of reports that document cover practices (MEND5.4.2d 2001). MEND has provided guidelines based on these reports but these are considered to be guidelines, not prescriptive regulations. Similarly there are guideline regulations under which Wismut must operate. None of these guidelines prescribe what the cover should look like, but rather state certain minimum standards that must be achieved though the placement of a cover system.

Cover Design Life

MEND5.4.2d (2001) suggests that long-term cover performance integrity should be ensured though appropriate design for a period of 1,000 years. In Australia, Normandy Mining Ltd. has specified physical stability of their waste containment facilities for between 200 and 500 years. The Wismut practice is to design cover systems that will ensure physical stability for a 200 year life.

Most mine sites acknowledge that some form of cover maintenance and repair will be required; however, generally the understanding is that such commitments would be temporary, i.e. immediately following cover construction there would be an intensive monitoring period; however, over time, say 10 to 20 years no more maintenance will be required.

In countries with no relevant regulations and generally weak standards of practice, the cover design criteria may reflect or be influenced by one or more of these mine closure objectives:

Remove human and animal health and safety risks
Prevent, remove, or minimize environmental impacts
Reclaim the area to reasonable social and economic land value
Secure a release of bonds.
Enhance corporate image.

Cover Components

Covers may be constructed of soil borrowed from suitable sources that may include other mine wastes piles. Cover may include geosynthetics such as geomembranes. We identify the following common components of a cover, some or all of which may be present in a cover—note that sometimes a soil layer may function in two or more of the following capacities:

· Vegetation support layer

· Erosion resistant layer

· Percolation control layer

· Moisture retention layer

· Foundation layer

· Reinforcing layer.

Ancillary Facilities

The design, construction, operation, and performance of a mine waste facility cover are affected by these common ancillary facilities that are often an integral part of the cover itself:

· The waste, including its shape, topography, chemical characteristics, response to consolidation, performance in the event of an earthquake, and the requirements for limiting infiltration and air entry.

· Surface water management facilities that control run-on from precipitation and snowmelt, and which direct run-off of precipitation and snow melt.

· Access roads that may result from waste deposition or which may be required to maintain the cover.

· Irrigation facilities that may be installed to establish and maintain the design vegetation.

· Monitoring instruments that may be installed to measure the performance of the cover including soil moisture probes, deformation monitoring stations, and earthquake response instruments.

General Sources of Information

The literature on mine waste disposal facility cover is immense and growing daily. The follow are some of my favorite sources of information, available via Infomine Publications Pages, Infomine Links, or web searches:

· The Proceedings of the International Conference on Tailings and Mine Waste. The proceedings of this annual conference generally held in Colorado are an excellent source of papers on covers (and other topics) for tailings impoundments and mine waste dumps.

· The many publications on mines and waste covers that may be accessed through the University of Reno’s Mining Life-Cycle Center

· O’Kane and Wels “Mine Waste Cover System Design – Linking Predicted Performance to Groundwater and Surface Water Impact” describes a method to develop site-specific criteria for covers for acid-forming mine waste.

· Also in the Infomine Library are other publications by Wels who describes design, construction, and performance monitoring of covers at mine waste disposal facilities form New Mexico to Canada. Not all his papers are discussed in this State-of the-Art review but they are recommended for clarity and insight.

· Although rather outdated, Caldwell and Reith in “Principles and Practice of Waste Encapsulation” discuss in detail covers for uranium mill tailings and other very low level radioactive wastes. (Still available to my surprise from Amazon)

· Any of the many sites accessed by keywords landfill covers; the design, analysis, construction, and monitoring of covers for landfills is of interest to those charged with covers for mine waste. I particularly like the University of Nevada’s Desert Research Institute Alternative Cover Assessment Program, the Sandia National Laboratories Alternative Landfill Cover Demonstration, the California Integrated Waste Management Boards Alternative Final Landfill Cover Program, the EPA fact sheet on evapotranspirative covers, and the volume “Landfill Covers for Use at Air Force Installations”

Cover Design Approach

Yanful and Lin (1998) present a flow chart for soil cover design. Wels and O’Kane (2003) present a “typical” approach to the design of soil covers. Wismut developed an in-house design approach that focuses on the selection and testing of suitable cover soils, and on the development of appropriate quality assurance and quality control procedures for the cover placement.

As a result of the review undertaken by SRK, a formalized approach to cover design is proposed.

Cover Design Procedures

The complete formalized approach (Steps 1 through 12 in Figure 1) has been followed successfully in a number of actual case studies; Kidston Gold Mine (Durham 2002), AA Heap Leach Pad (Zhan et al 2001), Les Terrains Auriferes (MEND2.22.4a 1999), Whistle Mine (Ayres et al 2002) and Wismut being good examples. Sites where this approach has been adopted, but not yet fully implemented (i.e. only up to Steps 7, 8 or 9), include Mt. Whaleback (O’Kane et al 2000), Grasberg, Kestrel Coal, Syncrude (Meiers et al 2002), Kaltim Prima Coal, Questa Mine (Wels et al 2002) and Greens Creek.

Beyond these case studies, pilot-scale work appears to be limited to research studies in the form of experimental test plots which has not led specifically to a detailed full scale design of any particular waste facility cover; for example at Waite Amulet (Yanful and St-Arnaud 1991), Sullivan Mine (Gardiner et al 1997), Heath Steele Mine (Yanful et al 1993), Myra Falls (O’Kane et al 1998), Bersbo Mine (Lundgren 1997), Key Lake Mine (Lee 1999), East-Sullivan Mine (Aubertin et al 1997) and the Potash Corporation of Saskatchewan (Haug et al 1991).

There are a number of case studies where the cover construction has been completed without pilot scale work, with the cover performance based solely on uncalibrated numerical modeling (i.e. skipping Steps 6 though 8). For these case studies cover performance monitoring is implemented in tandem with cover construction with a view to proving the design. Examples of this approach include Equity Silver (Aziz and Ferguson 1997), Golden Sunlight (Wilson et al 1995), and Rum Jungle (Bennet et al 1988).

Most full-scale covers are constructed without pilot scale testing or calibration monitoring (i.e. moving straight from Step 5 to Step 9). Examples of this approach include the Vangorda waste rock pile (SRK 1994a, 1994b), Yankee- and Coral Gold heap leach pads, and the Glamis waste rock dumps and heap leach pads. Drummond et al (2003) reports on the cover designed for the Tonopah heap leach pad, where no design was done at all. Their approach was simply to adopt a design similar to those in the surrounding areas and apply that – the premise being that if it works elsewhere, it is good.

Cover Performance Analysis

Erosion

Control of cover erosion is generally best effected by:

· Vegetation which may be preferable in a moist climate.

· Placement of rocky soil which may be preferable in a dry climate.

· Contouring to limit runoff lengths—benches at 20- to 50-ft vertical intervals are commonly used.

· Disking to create paddocks, i.e., a series of vertical and horizontal surface on the otherwise overall slope sideslope.

Infiltration

There is a considerable body of data and many technical papers on quantifying infiltration on mine waste facilities in the InfoMine technology sections and systems. In addition, the procedures and practices developed by the landfill industry for infiltration control and cover construction are readily available and applicable to mine waste disposal facility covers and infiltration control.

In 2003 we looked at 177 case studies in 14 countries of covers for mine waste. We subsequently updated the study to include more than 200 case studies. This paper summarizes our findings.

Cover performance data (Figures 2 &B 3) illustrate reliable cover performance data for four case studies. In all cases there has been a significant increase in infiltration over time. The increased infiltration appears to be as a result of increasing hydraulic conductivity of the cover material. Differences of one to three orders of magnitude between design and measured values are not uncommon: soil properties should change over time, especially when these soil covers are subjected to wet/dry and freeze/thaw cycles.

Figure 2. Long-term Infiltration Through Four Covers

Figure 3. Field Hydraulic Conductivities Variations with Time

Cover Performance Monitoring

There are two approaches to evaluating and monitoring cover performance. These have