This review gives an overview of how covers are used at mine sites, and gives resources such as online courses, papers, and books where you can obtain more information on covers technology. Two case studies are also given of covers used on soft tailings and covers used in cold climates.
A Technology Review is a piece written about a single topic, for the benefit of the reader, to provide them with readily accessible, current knowledge in that area. Covers is such a vast field that it is difficult to bring together the information that interests us, in a single document so as not to lose the reader in the sheer volume. So this review is an attempt to bring together the known documents and information sources available to us, and present them in a short, inclusive and interesting manner.
OVERVIEW OF COVERS
Covers are constructed in mines sites over facilities such as 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 worldwide. 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. The objective of covers range from:
- 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.
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.
There could easily be a full course on covers included in the EduMine repertoire, but for now this field has been separated out into a number of specialities:
In keeping with the diverse nature of this piece, here are a number of suggestions for papers relating to a wide range of cover projects in mining. In the InfoMine Library there are over 20 cover related papers but on my shortlist I would have the following.
In my opinion the most interesting, current and innovative cover project is that on Suncor's Pond 5 tailings impoundment. Here is the most current paper on the topic: An Update to the Construction of the Suncor Oil Sands Tailings Pond 5 Cover
Advances from the Wismut Project on Soft Tailings Cover Technologies - While this paper offers less detail, it give a historical context of the work being done in Pond 5 as to how a very similar problem was overcome in Wismut in eastern Germany.
But there are also more illustrative presentations on the cover, such as in the presentation Suncor Pond 5 Coke Cap
Another aspect of building your cover to the environment is given in this paper which brings seismic deformation to our attention: "Newmark Seismic Deformation Analysis for Geosynthetic Covers"
One publication that examines covers extensively is "Principles and Practice of Waste Encapsulation
". It takes us from the philosophy of cover design i.e. what do we want to achieve? Though design, assessment and the final twin stages of monitoring and maintenance. This is one book that I have found to be comprehensive, technical and helpful.
William H. Albright, Craig H. Benson, and W. Joseph Waugh wrote about the topic of water balance within cover systems for waste containment in their publication "Water Balance Covers for Waste Containment". The authors "give detailed analysis of the fundamentals of soil physics and design issues introduce application ecological concepts and revegetation practices, and then move on to construction, modelling, and maintenance".
Maritz Rykaart is an eminent professional in this field and has given us permission to share some of his presentations and papers.
- This presentation is an illustrative, easy going, and all round fun piece on various aspects of covers from the designers viewpoint.
- He also has a much more detailed paper in the InfoMine library regarding the design and construction on covers, which was presented at the 2006 International Conference on Acid Rock Drainage
Ward Wilson is an academic who features quite frequently in cover design. One of his papers evaluates the "integrity of cover systems from a global perspective".
Michael O'Kane has also published a number of papers on the topic. His paper on mine waste cover system design tries to develop a "site specific performance criteria for a cover system which is designed to isolate acid-forming mine waste".
Craig Benson and William Albright (as mentioned together in the books section) also contributed to the Alternative Cover Assessment Program (ACAP). Under this program a number of cover designs (conventional and alternative) were set up in landfill sites around the United States. Here is a brief description of the methodology of ACAP:
"At ten sites, the test facility includes at least one lysimeter equipped with hydrogeological and meteorological monitoring equipment. At the other sites, a flow monitoring device and weather station were installed to monitor existing leachate collection systems and meteorological conditions. The owner of each site designed the alternative and conventional landfill covers and constructed the test sections with assistance by members of ACAP"
The full USEPA document can be found here.
Zornberg has also published a vast number of papers from his areas of interest in covers, studying evapotranspiration and Monocover design. Here are two of his papers:
GEOSYNTHETICS IN COVERS ON SOFT TAILINGS
Geosynthetics have been used in constructing covers and access roads over soft tailings at the Cannon Mine, Wenatchee, at impoundments at Wismut, and most recently at Pond 5 at the Suncor oil sands tailings mine in Alberta. Here are two separate publications that can be found on that project, the latter a more illustrative presentation.
There are many ways to analyze covers and access roads consisting of Geosynthetics and fill constructed over soft tailings. These include:
- Anchored centenary on a yielding foundation. The geosynthetics are assumed to be anchored on firm dikes surrounding the soft tailings and are analyzed as catenaries using conventional structural formulae more commonly used in bridge design.
- Floating facilities that in essence displace their own weight of soft fluid tailings. This involves a simple application of Archimedes principal.
- Slope stability analyses. Use a commercial computer code to work out the factor of safety of the perimeter of the advancing face of the cover or of the access road.
- Deformation analyses using FLAC or PLEXIS. This involves a complete evaluation of the flow and deformation of the soft tailings in response to the superimposed loading resulting from cover and/or access road placement.
As in all engineering, analyses are but a pale replica of reality. They are but an aid to judgment: decide how to proceed on the basis of your gut feel, but do this only after doing all the analyses you can do.
COVERS IN COLD CLIMATES
The success of a cover is measured by its continuing effectiveness in the face of the environmental conditions it must overcome. Often, cover designs are based on the experience from temperate regions and our focus is on certain set of well documented temperate issues but in other regions these may not arise. Here is a publication that deals with cover design and construction in cold climates which is a prime example of tailoring our cover requirement to its environment.
This is the long report in summary from the MEND site:
Several dozen features or processes affecting soils have been identified in cold regions. The most widespread processes are ground freezing and ground ice formation, ground thawing and thaw settlement, and freeze-thaw cycling. Cryoturbation, solifluction, gelifluction and convective cooling can also occur in limited conditions. Combinations of these processes with specific soil or hydrologic conditions can result in terrain features such as ice wedges, pingos, thermokarst, patterned ground, boulder fields, mounds or hummocks, and mudboils, as well as a number of less common features. The rates of these processes can be slow enough that they would not be obvious in current observations of soil covers, but fast enough that they might have significant effects over a cover's design life.
Some covers are designed to include vegetation as an integral part of the cover. Some covers are designed primarily to be erosion resistant and hence to include more rock than vegetation. Regardless, in the fullness of time, vegetation will establish in all covers.
Sources of information on the topic of vegetation in covers includes:
- The conference proceedings of the thirty years of Tailings and Mine Waste
- The five or so years of conference proceedings on Mine Closure.
- A Google search with appropriate keywords.
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