STATE OF THE ART REVIEW
Covers

Revision: 28 June 2006
Authors: Maritz Rykaart and Jack Caldwell
mrykaart@srk.com and jcaldwell@infomine.com
General
Sources of Information
Figure
3. Field Hydraulic Conductivities
Variations with Time
Acid
Generating Tailings Piles and Waste Rock Dumps
United
States of America Cover Practice
Covers
to Control Acid Mine Drainage
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.
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.
Here is a list of soil cover functions, somewhat different but valid.
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
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:
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.
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
· 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
·
Also in the Infomine Library are other
publications by
· 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”
Yanful and Lin (1998) present a flow chart for soil cover
design.
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.
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.
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

There are two approaches to evaluating and monitoring cover performance. These have been defined as “macro” and “micro” monitoring. Macro monitoring entails measurement of water quality and flow data from seepage or overland flow discharged from waste facilities (i.e. a measurement of site wide waste load), independent of the actual cover system, whilst micro monitoring entails actual monitoring of the cover system itself.
In most cases, the standard against which cover performance is evaluated is the downstream water quality. The onus is on the mining company to prove that their design will ensure compliance to the water quality standards set for that site. The South African coal industry is a good example of this system in practice. Receiving water bodies in watersheds impacted by the mining industry are monitored and an assimilative capacity for each is determined based on the downstream water use. It is therefore recognized that the mining industry has an impact on the environment, and the maximum allowable contaminant load that any receiving water body can receive without impacting the downstream water use is calculated. Each mine in the watershed is assigned a pro-rated portion of that waste load, based on production numbers. The system is regulated through measurement of the contaminant loads emanating from each site. The mine must ensure compliance with this assigned contaminant load, by whatever means, including possibly soil covers.
For some sites compliance are also measured via other mechanisms, i.e. achieving low radon emissions for radioactive waste (DOE 2002) or specific final land use requirements, i.e. pasture (Rykaart et al 2002).
Direct performance monitoring of the cover system in the form of flux measurements through the cover (MEND5.4.2d 2001) is generally used to evaluate the processes that influence the cover system performance, and to assist in setting target performance criteria for downstream water quality monitoring. The review did not identify any case studies where this level of performance monitoring is used for the purpose of compliance monitoring.
Many case studies list the use of lysimeters to directly measure infiltration though the cover system and these measurements are used to evaluate the cover effectiveness. Bews et al (1999) illustrated the complexities involved in lysimeter design, and showed that in order for a lysimeter to correctly measure infiltration though a cover it has to be specifically designed for each application. We conclude that many cover performance results are based on inadequate lysimeters, often making the interpretation of data difficult.
Some covers have specific maintenance plans, for example the
The general approach to maintenance is to “deal with the problems if and when they occur”. Furthermore, the only aspects of cover maintenance that are addressed are erosion and vegetation. Aspects that have found to require maintenance, but that are rarely acknowledged upfront include sediment transport, settlement (both from consolidation and thaw) and physical degradation (as a result of wet/dry or freeze/thaw cycles).
Complex Covers Simple Covers Reclamation Covers


Cover Construction Costs
The cover placed in 1984-1985 on the Rum Jungle uranium mine waste rock dumps has three layers: (1) a low-permeability clay layer to control infiltration; (2) a storage-release layer to provide moisture to the vegetation in the long, dry season and prevent the clay drying out; and (3) an erosion resistant layer that supports vegetation. The covers were designed to limit infiltration to less than five percent of incident precipitation. For the first ten years the covers performed as designed, but by 2003 the infiltration rate was in excess of the design criterion.
As described by Taylor et al “Determination of the Reasons for Deterioration of the Rum Jungle Waste Rock Cover” testing of the cover materials showed that they no longer met the construction specifications. The clay permeability was several orders of magnitude more permeable than specified. The authors conclude that the increased permeability is the result of galleries formed by termites and ant, root growth from pasture grasses and trees, and extensive shrinkage and desiccation cracking. The authors state that acidification of the soil at the base of the covers may also affect cover performance.
The authors also noted that at some locations the upper layers of the cover were not as thick as specified possibly due to poor construction. The authors attribute some of the poor cover performance to the thinner cover sections.
Measurement of the oxygen flux in to the heap indicted that the full cover reduces the flux to about 20 percent of that into bare rock. The flux is about four times higher at the end of the dry season than at the end of the wet season primarily due to the moisture content changes in the soil of the cover.
Following closure of the Falconbridge Mill,
You may significantly underestimate the infiltration through the sideslope cover if you consider only the one dimension effect; runoff down the sideslopes and downslope seepage in the soil of an evapotranspirative cover may increase the infiltration rate in the lower reaches of the sideslope cover relative to the upper reaches of the sideslope cover. Christensen and O’Kane “The Use of Two-Dimensional Soil-Atmospheric Modeling in the Design of Dry Cover Systems for Mine Waste” [Infomine Library] describes the effect of the long sideslopes on the infiltration through what they call a dry cover, or what is more commonly called and evapotranspirative cover. The authors used the computer code VADOSE/M to model sideslope evaoptranspirative covers and examine the increase in infiltration that results from the downslope effect of the cover—they conclude that one dimensional modeling of sideslope covers can significantly underestimate the infiltration through a cover and that the designer may have to change the cover design in the lower reaches of the slope.
Can you control the flux of oxygen through a cover to the
acid generating wastes below? Shelp et
al. “Electrochemical Cover for the Prevention
of Acid Mine Drainage—A Laboratory Test” [Infomine Library] describe the use of steel mesh cathode
installed in a cover’s oxygen barrier of the top layer of thickened tailings to
reduce oxygen migration to the acid generating wastes. Laboratory testing indicates this may work,
but confirmation awaits field tests in
The carbon in a wood-waste cover in a wet environment may
consume atmospheric oxygen passing through the cover and thereby reduce acid
seepage generation. This possibility is
described by Germain et al. “Treatment of Acid Mine Effluent Using a
Wood-Waste Cover” [Infomine Library].
The authors describe the efficacy of a wood-waste cover at the East
Sullivan mine tailings pile in
Ten years after placing a cover over uranium mill tailings
in
The construction of soil covers on very weak, compressible fine
tailings is complicated by the low shear strength, poor trafficability, and
large settlement of the unconsolidated tailings. Wels et al. “A review of
dry cover placement on extremely weak, compressible tailings” describe how
to analyze and how best to place a cover over unconsolidated tailings. The analytical and construction approaches
they described are based on practice at the Wismut GmbH uranium mill tailings
impoundments in the eastern part of
The base metal and iron ore mines of
The
The Cannon Mine was operated between the mid-1908 and
mid-1990 as an underground gold and silver mine on the outskirts of
The U.S. EPA is moving fill from part of the Anaconda Mine site in Yerington,

During the removal assessment, EPA determined that the PCB contamination and fugitive dust needed to be addressed as soon as possible. Because the sulphide tailings were exposed, there was the potential the wind could spread the dust across the property or even carry it off-site. The emergency response was necessary to prevent the contaminants from migrating. To address the PCB contamination, EPA removed and disposed of 119 PCB-containing electrical transformers. EPA asked potentially responsible party Atlantic Richfield Co (ARCO) to complete the removal actions, but the company declined. EPA is completing the emergency cleanup and seeking reimbursement from ARCO and other potentially responsible parties.
EPA is using a number of safeguards to prevent the spread of contamination during the placement of the cap, including air monitoring and using water trucks to help reduce the risk of the tailings becoming airborne. Once complete, the soil cap and sealant will cover eight basins across nearly 120 acres at the site. The emergency response has cost approximately $600,000 to date.
Anaconda mine site is a massive 3,400-acre site
approximately 105 km from
Covers to Control Acid Mine Drainage
Covers to control Acid Mine Drainage should control infiltration of water and also inflow of air. Here is a brief discussion of some covers proposed and/or used for Acid Mine Drainage control.
This is a simple cover of soil over the rock. The permeability depends on the soil gradation and the density and crack pattern in the soil.

A slightly more complex cover where a relatively high density soil is placed directly over the rock, followed by a lower density soil selected and placed to support vegetation.

The cover used at the Gray Eagle Mine Tailings Impoundment.
Below we summarize come comparisons of the infiltration and runoff for a variety of covers.

A comparison of runoff and percolation from a variety of covers.

Yet more covers and calculated infiltration and runoff.

The performance of covers on tailings
piles to limit oxygen migration to the acid generating materials beneath is
the topic of an abstract in the latest CIM
magazine. In brief, the abstract tells
us that the measurements of a cover in
Sadly we cannot access the papers—need to
be a CIM member for that. Or see a paper
on the same topic from 2003, unless you subscribe to the Canadian Geo
Natural
Resources Canada describes the site closure. See similar information about the site and
others in
One of the delights of trawling the web
is to find a superb volume you had not previously seen—here are my picks from
this trawl—not much to do with the topic of the CIM paper, but worth you time,
nevertheless:
Ø
Sustainable
Improvement in Safety of Tailings Facilities (2004) and see page 27 for
information relevant to moist covers designed to limit air flow.
Ø
Engineering
Guidelines for the Passive remediation of Acid and/or Metalliferous Mine
Drainage and Similar Wastewaters (2003).
See page 91 and following for information of covers.
Aubertin, M.,
Bussière, B., Barbera, J.-M., Chapuis, R.P., Monzon, M. and Aachib, M. (1997).
Construction and Instrumentation of In Situ Test Plots to Evaluate Covers Built
with Clean Tailings. Proceedings of the
4th International Conference on Acid Rock Drainage,
Australian Environment Protection Agency (EPA) (1995). Rehabilitation and Revegetation. Module in series on: Best Practice Environmental Management in Mining. Australian Federal Environment Department, June 1995.
Ayres, B.K.,
O’Kane, M.O., Christensen, D., Lanteigne, L. (2002). Construction and
instrumentation of waste rock test covers at Whistle Mine,
Aziz, M.L. and
Bennett, J.W.,
Harries, J.R. and Ritchie, A.I.M. (1988). Rehabilitation of Waste Rock Dumps at
the Rum Jungle Mine Site. Proceedings of
the Mine Drainage and Surface Mine Reclamation Conference,
Bews, B, Barbour, S.L., Wickland, B. (1999). Lysimeter
Design in Theory and Practice. Proceedings of Tailings and Mine Waste,
Cabalka, D,
Department of
Energy (2002).
Department of
Water Affairs and Forestry (DWAF) (1992). Rehabilitation Principles for Coal
Waste Dumps Arising from an Audit of Rehabilitation Works on Coal Wastes in
Department of Water Affairs and Forestry (DWAF) (1998). Minimum Requirements for Waste Disposal by Landfill. Waste Management Series, Second Edition, ISBN 0620-22993-4.
Drummond, S.,
Smith, M., Robison, N.E. (2003). Equatorial Tonopah, Inc. Copper Heap Leach
Closure. Presentation delivered at Heap Leach Closure Workshop, March 25 and
26,
Gardiner, R.T.,
Haug, M.D., Wong, L.C. and Johnston, K. (1991). Design and Construction of a Compacted Earth Test Cover for a Potash Tailings Pile. Proceedings of the First Canadian Conference on Environmental Geotechnics. 14 May. pp. 185-192.
Lee, N.
(1999). Evaluation of Cover Materials
for a Large Scale test Facility at
Lundgren, T.
(1997). Bersbo Pilot Project Physical Behavior Seven Years after Covering the
Waste Rock Piles. Proceedings of the 4th
International Conference on Acid Rock Drainage,
McKenna, G.T.
(2002). Sustainable mine reclamation and
landscape engineering. PhD Thesis in
Geotechnical Engineering, Department of Civil and Environmental Engineering,
Meiers, D.E., Barbour, S.L. (2002). Monitoring
of Cover and Watershed Performance for Soil Covers Placed over Saline-Sodic
Shale Overburden From Oilsands Mining. Paper presented at 2002 National Meeting
of the American Society of Mining and Reclamation.
MEND 2.22.4a (1999). Construction and Instrumentation of a Multi-Layer Cover Les Terrains Aurifères. February.
MEND 5.4.2d (2001). MEND Manual, Volume 4 – Prevention and Control. Ed. G.A. Tremblay and C.H. Hogan, February.
Münnich, K.
(1993) Reduction of the Migration of Contaminants through Mineral Barrier
Systems by Hydraulic Means. Proceedings of Fourth International Landfill
Symposium, Sardinia,
Nawrot, JR,
O’Kane, M.,
Porterfield, D., Weir, A. and Watkins. A.L. (2000). Cover System Performance in
a Semi-Arid Climate on Horizontal and Sloped Waste Rock Surfaces. Proceedings
of the 6th International Conference on Tailings and Mine Waste ’99,
O’Kane, M.,
Stoicescu, J., Januszewski, S., Mchaina, D.M. and Haug, M.D. (1998). Design of
Field Test Plots for a Sloped Waste Rock Surface. Proceedings of the 15th
National Meeting of the American Society for Surface Mining and Reclamation,
Rykaart E.M.
(2002). A Methodology to Describe
Spatial Surface Flux Boundary Conditions for Solving Tailings Impoundment
Closure Water Balance Problems. Ph.D. thesis submitted to Department of Civil
Engineering,
Rykaart, E.M.,
Fredlund, D.G.,
Shepperd Miller
(2001). North Waste Rock Storage Facility Acid Rock Drainage Mitigation and
Final Phased Permanent Closure Plan, Rain Mine, Carlin,
SRK-Robinson
Inc. (1994a). Construction Report,
Vangorda Rehabilitation PWGSC Project 760831, Vangorda Plateau, Faro Mine,
SRK-Robinson
Inc. (1994b). Final Design Report
Report, Vangorda Rehabilitation GSC Project 760831, Vangorda Plateau, Faro
Mine,
Suter, G.W., Luxmoore, R.J., Smith, E.D. (1993). Compacted Soil Barriers at Abandoned Landfill Sites are Likely to Fail in the Long Term. J. Environ. Qual. 22:217-226.
Van Zyl (2002).
Wates, J.A.,
Rykaart, E.M. (1999) The Performance of Natural Soil Covers in Rehabilitating
Opencast Mines and Waste Dumps in
Yanful, E.K. and St-Arnaud, L.C. (1991). Design, Instrumentation, and Construction of Engineered Soil Covers for Reactive Tailings Management. Proceedings of the 2nd International Conference on the Abatement of Acidic Drainage. Montréal, Québec. 16 to 18 September. Tome 1, pp. 487-504.
Yanful, E.K.,
Yanful, E.K.,
Lin, M. (1998). An Integrated Approach to Designing Soil Covers for Reactive
Mine Waste. Proceedings of the 51st
Canadian Geotechnical Conference,
Zhan, G, Mayer,
A B , McMullen, J, and Aubertin, M, (2001), Slope Effect Study on the Capillary
Cover Design for a Spent Leach Pad. Proceedings of Tailings and Mine Waste ‘01,