This review discusses the design, construction, and operation of waste rock dumps. Different types of mine fills are described. Other topics discussed include codisposal of waste rock, air flow in dumps, and rock dumps in British Columbia. Several waste rock dump consultants are also listed.
The briefest description of waste rock dumps is from Western Australia-all you need to know in a page. The most comprehensive description of a waste rock dump is in the EduMine course Design and Operation of Large Waste Dumps. In practice no definition is needed; simply see the picture on the front page of this review.
As humans we instinctively categorize things as a first step in trying to understand them. That is the only justification for repeating this categorization of waste rock dumps from the EduMine course. In practice mines generally put the waste rock dump as close to the pit or shaft as possible, except maybe as stated by BHP Billiton: "We will not commit to a new mining project that disposes of waste rock or tailings into a river. This position does not apply to the disposal of waste rock and tailings materials in conventional waste rock dumps or tailings dams, which may be constructed within the catchments of a river system where such structures are designed to retain and store the waste materials. It also does not apply to the discharge of water from tailings dams or waste rock dumps that are of a quality acceptable for downstream beneficial uses."
Valley fills partially or completely fill the valley. The surface of the dump is usually graded to prevent impoundment of water at the head of the valley. Valley fills which do not completely fill the valley may require construction of culverts, flow-through rock drains or diversions, depending on the size and characteristics of the upstream catchment. Valley fills which completely fill the valley are sometimes referred to as "Head-of-Hollow" fills. Head-of-Hollow fills are common in the coal fields of the southeastern U.S., and often incorporate chimney drains for collection and conveyance of seepage and runoff.
The Cross-Valley fill is a variation of the Valley fill. The embankment extends from one side of the valley, across the drainage, to the other side of the valley. The upstream portion of the valley is not completely filled, and fill slopes are established in both the upstream and downstream directions. To avoid impounding water, Cross-Valley fills usually require specific provisions for conveying water through or around the fill (e.g. diversions and/or culverts or flow through rock drains).
Sidehill fills are constructed on sloping terrain and do not block any major drainage course. Dump slopes are usually inclined in the same general direction as the foundation. Toes of Sidehill fills may be located on the slope or on flat terrain in the valley bottom.
Ridge Crest Fills
Ridge Crest fills are a special case of Sidehill fills, wherein fill slopes are formed on both sides of the ridge line or crest.
Heaped fills, and also referred to as Area, Stacked or Piled fills, consist of mounds of waste with slopes formed on all sides. Foundation slopes are generally flat or gently inclined.
Here is a picture taken from the EduMine course Design and Operation of Large Waste Dumps which shows the waste dump classification:
Also, here's a presentation I've found very informative in the design and application of different waste dumps.
The design of mine waste rock dumps is described in these publications:
- Engineering Design Manual for Disposal of Excess Spoil (OSM (1989))
- Design of Non-Impounding Mine Waste Dumps (SME (1985))
- Development of Systematic Waste Disposal Plans for Metal & Nonmetal Mines (USBM (1982))
- Pit Slope Manual, Chapter 9 - Waste Embankments (CANMET (1977)
- Engineering and Design Manual for Coal Refuse Disposal Facilities (MESA (1975))
In summary the following steps are generally involved in the design of a waste rock dump:
- Establish mine rock and overburden characteristics and quantities.
- Assemble and review possible disposal site information, and hence select a site
- Characterize the selected site
- Establish the potential impacts of the dump on the environment
- Develop plans for disposal, operation, and closure
As with all mine waste disposal facilities, including heap leach pads and tailings impoundments, the following are issues to be addressed in the design of the waste rock dump:
In designing a waste rock dump, the performance of the dump at various stages of its proposed life may be modeled using numerical and/or computer models. This may include:
- Water balance studies of the site and the dump
- Geochemical modeling of potential acid rock drainage
- Groundwater impact modeling
- Slope stability modeling of static, seismic, and runout performance
- Long-term geomorphic studies to establish how the dump will behave in the long-term (1,000 years and more) as an integral part of the topographic of the site.
An integral part of the design of a waste rock dump is the preparation of the following documents for the dump or at least parts of the overall mine documents that relate specifically to the waste rock dump:
Construction of a waste rock is considered to entail the preparation of the site to receive the waste rock as part of the overall mining process. A contractor may be employed to construct access roads, strip the site, prepare foundations, place underdrains, and install surface water management facilities.
Operation of the waste rock dump, generally done by the mining employees, involves these activities:
- Ore transport from the mine and-or mill to the dump.
- Off-loading of the ore at the dump in accordance with the planned dump development and operating plans, including lift height and location
- Access road construction and maintenance
- Clearing of new areas for dumping, foundation preparation and drain construction as required in new areas
- Maintenance, upgrade, and expansion of surface water management facilities
- Environmental monitoring of conditions at the dump including seepage water, surface water, groundwater quantities and quality.
- Dump performance monitoring and documentation including stability, erosion, consolidation, and creep.
The idea of disposing of the waste rock together with the tailings is intuitively attractive, but practically difficult. I have heard is succinctly described thus: "use up the space in the rock voids by filling with tailings; give those weak tailings some strength by mixing in rock." The photo below is worth looking at simply because it appeals to our senses. But there is a more relevant reason: the picture shows, moving from bottom-left to top-right: a decant pond; the tailings beach; and the codisposal beach where the waste rock is mixed in with the tailings.
This picture is lifted from what is, in my opinion, by far the best and most informative presentation on the topic: Codisposal by Mike Gowan of Golder Associates takes a while to download, but I promise it is worth the wait. I cannot resist replicating additional diagrams from this paper to whet your interest. They follow; first one showing a simple codisposal approach, and second a photo showing waste rock end-dumped into an open pit.
The best paper and presentation (for my money) at the 2006 SME conference in St Louis is that by Brian Ayres and his colleagues at O'Kane Consultants. I am much indebted to them for allowing me to post on InfoMine the presentation they made. I greatly and enthusiastically recommend you spend some time reading both the paper Incorporation of Natural Slope Features into the Design of Final Landforms for Waste Rock Stockpiles and the PowerPoint presentation.
On the basis that you may be busy, and because I believe in what they say and want to encourage you to read their paper and peruse their presentation, here is a precise of their main points:
- Design the final landform using natural analogues.
- The reclaimed landform can be no more stable than adjacent undisturbed areas; therefore use gentler slopes, higher density drainage, and smaller drainage basins.
- The preferred reclaimed slope is a "spur-end" slope plan with a concave of complex (convex-concave) profile.
- Avoid terraces and contour banks.
- Use computer codes to predict erosion profiles over at least 100 years.
- Avoid man-made materials such as pipes, gabions, and concrete.
- Design conservatively for extreme events
- Incorporate wetlands and small lakes into the design to attenuate runoff and peak flows.
McPhail and Wilkinson in Planning- Landform Design Concepts discuss the application of similar concepts in Australia. In Waste Rock Dump Decommissioning at Mine in Northern Western Australia they describe the issues, the solutions, and case histories of closure on mine waste rock dumps.
I also recommend these Australian Best Management Practice Guides which deal inter alia with waste rock dumps:
When rock that contains sulfide minerals is exposed to air and water, the weathering of the minerals causes the water to become acidic. When the acidic water seeps from the rock, the resultant seepage is called Acid Rock Drainage (ARD). Biological activity may contribute to and accelerate the formation of acidic conditions in the rock. The low pH of the acidic seepage may mobilize heavy metals in the rock. Acidic seepage has the potential to detrimentally affect groundwater or surface water into which it flows.
If you suspect that the rocks that will go into your waste rock dump are likely to contain sulfide ores you should prepare the design of the rock dump and undertake its operation and closure in a way that controls and ultimately minimizes the generation of acidic seepage.
Air Flow in Waste Rock Dumps
Wels et al in their publication Control of Air Flow in Waste Rock Dumps describe how air moves through a waste dumps and thereby contributes to the oxidation of pyrite-bearing ores and hence acid mine rock drainage. They believe that air flow can be limited by "judicious" placement of reactive rock to control the "internal structure" of the waste rock pile. See also Lamontagne et al Layered Co-Mingling for the Construction of Waste Rock Piles as a Method to Mitigate Acid Mine Drainage for more ideas. Christensen and O'Kane in The Use of Two-Dimensional Soil-Atmosphere Modeling in the Design of Dry Cover Systems for Mine Waste provide more insight into the mechanisms and how to control them.
BC Rock Dumps
Looking through an old 1991 magazine called International Mine Waste Management I read an article by Alistair Kent called Coal Mine Waste Dumps in British Columbia Stability Issues and Recent Developments I was left wondering what happened subsequent to the 1991 "recent developments".
I am indebted to Chris Carr, the only geotechnical engineer with the BC Ministry of Mines (MEMPR), who took time to bring me up to date. Here is his report (Thanks):
A series of reports have beenpublished following the work carried outby the British Columbia Waste Dump Research Committee.The committee wasformed in 1990 with representation by industry, CANMET, BC Ministry of Environment and BC Ministry of Energy, Minesand Petroleum Resources. Thefollowing reports have been completed to date:
- Investigation and Design Manual, 1991
- Operating and Monitoring Manual, 1991
- Review and Evaluation of Failures, 1992
- Failure Runout Characteristics, 1992
- Methods of Monitoring, 1992
- Consequence Assessment for Mine Waste Dump Failures, 1994
- Runout Characteristics of Debris fromDump Failures in Mountainous Terrain Stage 2: Analysis, Modeling and Prediction,1995
- Rock Drain Research Program, 1999
The "Investigation and Design Manual" and "Operating and Monitoring Manual" are referenced in the Health, Safety and Reclamation Code for Mines in British Columbia (2003).The guidelines were prepared by Piteau Associates Engineering Ltd.and Klohn Leonoff Ltd. respectively.Prominent geotechnical consultants and industry representatives reviewed the guidelines and many of their suggestions were incorporated.
Reports (1), (2) and (3) form part of theprofessional development course "Design and Operation of Large Waste Dumps" administered by EduMine. Copies of the reports are available for purchase from BiTech Publishers (http://www.bitech.ca/)
Most consultants to the mining industry have specialist who can help with the design, operation, and closure of waste rock dumps. Here is a quick look at how some describe their services or projects they have completed.
(If I leave your company out, please let me know so we may rectify an inadvertent oversight-keep in mind this is my personal selection based both on what I find interesting and what I hope will help you assess the consultant's approach, attitude, and experience.)
Golder Associates: Disposal system evaluation, contaminant migration assessment and measurement, rock dump design and stability analysis, construction inspection, monitoring, closure and rehabilitation design.
Klohn Crippen Berger: Investigation and design of waste rock dumps, including developing strategies and construction methodology for segregated dumps to store acid forming waste and treatment of ARD.
O'Kane Consultants: Newmont Australia, Woodcutters Mine (Australia): Development of a detailed closure plan for the waste rock dump and installation of performance monitoring gear for the full-scale cover system. (I have already noted my enthusiasm for their technical approach-see above-and I note again how quick and responsive they were to let me post the information. I know nothing more about them, but these brief encounters are enough to convince me they must be a well run firm.)
Vector Engineering. Vector has expertise and experience in all aspects of waste dumps including site selection, design, stability analysis, dynamic modeling, ARD problems and remediation, drainage and surface run-off considerations, closure and rehabilitation, environmental impacts and permitting issues, cost analysis and CQA.
Metago Environmental Engineers: Metago considers tailings and waste rock management issues within the mining industry to be first and foremost environmental engineering issue requiring sound geotechnical engineering of tailings and waste rock behavior and associated characteristics, seepage characteristics as well as sound environmental engineering to minimize and control surface and ground water contamination potential, dusting, rehabilitation commitments, erosion protection and maintenance as well as closure commitments. (A personal note: I have known Gordon McPhail for many years and respect his personal integrity, so I do not hesitate to repeat this statement that would otherwise be cut as cant.")
Pincock Allen & Holt: Waste Dump Design and Placement. (A very personal aside on this one: Kay Pincock was the first senior engineer I met in the US. He was unfailingly kind, considerate, and courteous. He was friendly and helped me and my family greatly as we moved into and settled down in Tucson. I owe him an unreturnable debt, and revere his memory.)
SENES Consultants Limited. SENES provided technical support to Equity Silver in the development of their reclamation proposal for high sulfide bearing waste rock piles. The Equity plan for closure of acid waste piles involved covering the piles with clay and treatment of residual runoff for as long a period as required. SENES developed a predictive, screening level model to assess acid generation rates with and without cover as well as for various types of cover material. The model results were used to support Equity's position that the current bonding cost provisions were adequate.
Western States Mining Consultants: D.H. Scriven was staff engineer for the geotechnical subcontractor to CDM on the Remedial Investigation/Feasibility Study at five Superfund sites in Clear Creek and Gilpin Counties, Colorado. The sites included mine waste rock and tailings dumps, several adjacent to perennial streams. Stable configurations were determined and alternative remedial methods were evaluated. A comprehensive geotechnical report was prepared in support of and complementary to CDM's RI/FS efforts.
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