TAILINGS DAM VERSUS A WATER DAM, WHAT IS THE DESIGN DIFFERENCE?

ICOLD Symposium on Major Challenges in Tailings Dams, June 15, 2003

Harvey McLeod, Vice President Len Murray, Manager of Mining, Asia-Pacific

Klohn Crippen Berger Ltd., Canada

Summary The technology of tailings dam design is based on the same geotechnical principles as water dams, however the presence of saturated tailings solids as the stored medium, versus water only, presents unique challenges and design benefits. The gradation of mine tailings typically varies from silty medium fine sand to clayey silt. Tailings dam design sections vary considerably. For example, the dam can be made entirely out of unprocessed tailings with upstream construction, or the dam may be made out of borrow material with little or no reliance on the tailings. Impounded tailing solids have hydraulic conductivity and shear strength properties that can be used to the advantage of the designer. On the other hand, sulphide rich mine tailings have a potential to oxidize and leach metals through acid rock drainage. Seepage control, for environmental not dam safety, becomes a critical design parameter which can lead to much lower tolerances for seepage losses from the impoundment, compared to water dams. Unlike water dams, tailing dams are closed at the end of mine life (typically 20 years) but the tailings cannot be removed for decommissioning. Therefore design must allow for safe decommissioning and low or zero maintenance in perpetuity. This paper presents an assessment of the differences in design details and approach between water dams and tailings dam and illustrates these points with case histories from major international projects. The main design issues of piping, drainage, structural fill, seepage control and closure are discussed. Introduction Design, construction, operation, and closure of tailing dams have some fundamental differences when compared to conventional water storage dams. Some of these differences work to the benefit of the dam designer, and some increase the complexity and difficulty. A guiding note, irrespective of the type of dam, however, is that all dams are engineered structures and that the principles of soil mechanics still apply. A tailings dam must safely contain mine tailings and process water not only for operations, but for perpetuity.

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Table 1 Summary of Differences between Water Dams and Tailing Dams Component Tailings Dam Water Dam Stored material Tailings solids, process water

(various contaminant levels) & runoff water

Water

Regulatory regime Ministry of Mines, Ministry of Environment

Ministry of Public Works, Regional Authorities, National Dam Associations

Operating Life Finite operating life (5 to 40 years)

Typically designated as 100 years, but as long as required by society.

Construction Period

Staged over mine life of 2 to 25+ years.

Usually 1 to 3 years.

Closure Infinite closure period, try for walk away design

Often not addressed, but facility may be decommissioned.

Engineering Medium to high level High level Continuity of engineering

Varies: Owner and engineer may change frequently during the construction life

Usually one engineering firm for design and construction.

QA/QC Generally good for starter dam and variable levels during operations. Can be at a low level for some mining companies.

High level

Consequence of failure

Tailings debris flow resulting in physical damage and environmental contamination.

Water inundation damages.

Dam Section Can vary during the design life, e.g. transition to centerline, or downstream.

Usually a consistent section

The following sections discuss some of the differences between the design of water dams and the design of tailing dams against a framework of the main design considerations, namely piping, drainage, tailings as structural fill, seepage control and closure. Design for Piping The design for control of piping in tailings dams needs to consider the stored tailings and the location of the free water pond, as well as the zonation and grading of materials in the dam. There is an opportunity to minimize the use of costly processed filters in drains by controlling the hydraulic gradients. The tailings can be used to form part of the seepage barrier to water flow and thereby reduce the hydraulic gradients along the upstream face of the dam. By reducing the hydraulic gradient, the risk of piping of tailings, or other fill zones, through the dam can be reduced. This approach is commonly applied on an empirical basis where it is often specified that the upstream tailings beach should be a minimum width to keep the free water pond away from the dam. This reduces both the hydraulic gradient and the quantity of seepage flow. An example of maintaining a tailings beach to mitigate piping was observed with the dam failure of the Phoenix tailings dam in southern British Columbia (Klohn,

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1979). The tailings dam was constructed by the upstream method, with a decant water pipe located in the dam. Twenty-five years after closure piping occurred between an elevated water pond and the deteriorated decant pipe. The solution to stop the piping was to distribute the surface tailings and push the pond away from the dam crest. This eliminated subsequent piping development. The authors are also aware of a number of reported cases where sinkholes have been mitigated, or seepage controlled, by moving the pond away from the dam. Seepage analysis programs are readily available to allow modeling of the seepage gradients in response to percolation, spigot location or climatic variations. However filter criteria for low head conditions are not as readily available. Sherard does provide insight into how low head filter criteria can differ from conventional criteria, which has been developed for high gradient conditions. The seepage analysis of the dam, therefore, should include the tailings mass, and the sensitivity of the pond location should be assessed to optimize both the seepage control and piping control design for the dam. The tailing dam designer should use caution in the placement of pervious rip rap zones on the upstream face of an impervious dam, and with the use of upstream rockfill shell zones. These zones have the disadvantage of introducing the full hydraulic head on the upstream face or core of the dam and negating the advantages of the tailings beach.

An additional piping concern has been emerging with water storage dams that use glacial till core zones, with filters, to control seepage and piping. Long-term internal erosion, such as has occurred at the Bennett Dam in northern British Columbia raises concerns that the conventional filter criteria may not provide long term security in design of seepage barriers and piping controls. The tailings dam designer has the opportunity to consider the potential use of tailings to minimize long-term reliance on impervious zones and filters. This can significantly reduce the risk of the long term piping potential and internal erosion.

The Omai tailings dam failure also illustrates the case where a tailings beach may have prevented a major dam failure. Failure of the dam occurred by piping of the saprolite core zone into the rockfill shell zone, with subsequent piping of tailings and failure of the dam. In this case the full hydraulic gradients were acting across the core zone, and water was available for continued transport of piped material. Design for Filters and Drainage The ability of tailings to reduce hydraulic gradients in the dam can also be used to the advantage of the tailings dam designer by allowing the use of geosynthetic filter fabrics for filters and drains. It is commonly debated that geosynthetics may not last the hundreds to a thousand years that closure criteria may impose and therefore they should not be used for permanent structures for closure. However, in cases where the closure condition will be a dry cover (e.g. no free water pond), or where the final water pond will be hundreds of meters upstream of the dam, the resulting hydraulic gradients into the drain may not be high enough to cause piping in the long term, thus relying on the geosynthetics only during

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operations. The authors have used this analysis to show that a geotextile/gravel drain could be used for a centerline cyclone sand dam recently designed in Peru. In concrete faced rockfill tailing dams there is a risk of piping of tailings through damaged concrete (e.g. cracks). The standard designs for concrete faced rockfil dams (CFRD) includes a relatively coarse bedding-transition zones downstream of the concrete face, which, while acceptable for water leakage, these coarse transition zones encourage piping of tailings. This has been considered to be a low risk because of the potential size of the cracks and the likelihood that the hydraulic gradient, through the tailings, will be low enough to control piping. However, in cases where the water pond is near the concrete facing, piping through damaged concrete could occur. In most cases it is probably prudent to place a geotextile filter fabric between the concrete and the bedding layer to provide additional assurance. The potential for chemical degradation of drains needs to be considered for tailings dams. For example, the INCO R-4 tailings area (Plewes & MacDonald, 1996) stores tailings with elevated sulphide levels and mineral precipitates from oxidation have plugged the granular drain system. The design solution for this case was to design inverted saturated drainage systems where oxidation would be minimized. Chemical precipitates can also cause filters and drains to become cemented and thereby prone to cracking with a consequent reduction in piping protection and potential for loss of fines.

Many tailing dam designs include pipes, either as underdrains or near the dam face, to reduce water pressure and allow capture of seepage water for treatment. While these drains can be effective in lowering the phreatic surface, they also introduce a potential piping pathway and increase the risk of piping failure. There are a number of case histories of tailings piping within PVC pipes installed in the dam.

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Design for Tailings Loading Tailings has a higher specific gravity than water and therefore imposes a higher stress onto the dam. Under most design conditions, this is not a controlling factor, however there are several cases where the loading from the tailings been shown to be important. The Los Frailes dam failure, in Spain, was due in part to the higher specific gravity tailings (in this case greater than 4.0), which increased the static loading to the foundation (resulting in higher pore pressures) and the static load on the dam. The other condition where it can be important is in the dynamic loading case for centerline dams. The liquefied tailings mass acts as a heavy fluid against the dam during the earthquake.

Figure 1 Los Frailes Dam Failure, Spain: High specific gravity of tailings contributed to loading on dam and foundation. Design for Seepage Control Seepage control for tailing dams and the tailings impoundment is an important environmental consideration in the design, where in some cases the allowable seepage rate (for environmental protection) may be a fraction of a L/s. Seepage control can be improved through the use of tailings as part of the seepage barrier. For example, this could involve designing the depositon sequence to provide a central free water pond with wide tailing beaches around the perimeter of the impoundment. The permeability of the tailings can be further reduced by thickening to produce non-segregating tailings. Minimizing segregation results in more uniform, low permeability tailings. The permeability of the tailings can also be reduced by promoting consolidation, either by surface drying, or by

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underdrains in the tailings impoundment. Great care must be taken with underdrains, however because as previously discussed, they can also provide a pathway for piping of tailings. The permeability of tailings typically reduces by an order of magnitude due to consolidation. Design for Closure The design of the tailings dam cannot be decoupled with the tailings impoundment. The physical and chemical properties of the tailings must be accounted for in the design. For example, high sulphide content tailings need to be stored to minimize the potential for oxidation and acid drainage. This imposes constraints on the dam design both for operations and closure. For example, on closure the tailings may need to remain saturated for perpetuity and this will require that the dam and liner system have a low enough permeability to allow the natural surface inflow to maintain saturation of the tailings. The selection of construction materials should be carried out in the context of longevity and closure. For example, till cores, coarse drains and pipes should be avoided (if possible) and replaced with tailings core zones and graded filter/drains. This can provide more flexibility (and hence less risk) for long-term internal erosion. The potential for cumulative seismic displacements can also be reduced, by avoiding narrow width, zoned fill sections. Closure of tailings facilities that contain a high percentage of very fine tailings presents a problem with consolidation of very fine sludge tailings. This is one of the most significant issues facing closure of the oil sands tailing impoundments. Thickening of tailings, to produce a non-segregated tailings mix assists consolidation and final closure of the tailings impoundment. The closure requirements for tailing dams, where the objective is to construct a walk away solution require integration of long term geologic processes to simulate natural structures to resist erosion, seismic loadings, floods, landslides, snow avalanches, etc. This leads to the desire to design tailing dams that simulate natural landforms and are naturally resistant to the geological and environmental conditions that they are constructed in. Case Examples Illustrating the Use of Tailings as Structural Fill The slow rate of construction and the tailings material provides a unique opportunity for optimization of a tailing dam, which is not the case for water dams. The use of tailings as a hydraulic fill material has formed the basis for spigotted and cycloned sand construction for tailings dams for over 50 years and many papers are available that document good practice in the use and placement of tailings as part of the dam structural fill. Typical examples of high dams using tailings as a structural fill are summarized in Table 2, along with other high tailing dams.

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Table 2 Partial Summary of Large Tailings Dams (Greater than 80 m high) DAM HEIGHT

PROJECT

LOCATION

CURRENT DESIGN

DAM TYPE

MINERAL

Highland Valley Canada 130 150 CS Copper

Gibraltar Canada 100 120 CS Copper Kemess South Canada 145 165 CS/ECRD Copper Brenda Canada 120 150 CS Copper Kennecott USA 30 80 CS Copper Fort Knox USA NA 115 ECD Thompson USA NA USS Copper Montana Tunnels USA NA 250 Modified

Centerline

Newmont Mill 2/5 USA NA 100 ECD North Block USA NA 137 Antamina Peru 120 232 CFRD Zinc-copper-lead Southern Peru Peru 70 110 DSS Copper Candelaria Chile NA 163 FRD Disputada Chile NA 120 Copper Los Leones Chile 160 160 Chuquicamata Chile NA NA DSS Copper Foskor Selati South Africa 45 140 CS Phosphate/Coppe

r Anglogold Ergo South Africa 84 90 DSS/USS Gold Impala Platinum South Africa 40 120 USS Platinum El Teniente Chile NA NA DSS Copper Alumbrera Argentina NA 120 Modified

Centerline/Rockfill Copper

Dexing China NA 210 CSS Copper Hongjiadu China NA 183 ECRD Ok Tedi Interim Dam

Papua New Guinea

130 130 ECRD/CS Copper , Gold

Medet Bulgaria 105 105 USS Copper, Gold Elatsite 1 Bulgaria 145 145 DSS Copper, Gold Elatsite 2 Bulgaria 117 160 DSS Copper, Gold Assarel Bulgaria 125 211 USS Copper, Gold

CFRD Concrete faced rockfill ECRD Earthcore rockfill FRD Filter rockfill CS Centerline cycloned sand USS Upstream cycloned sand DSS Downstream cycloned sand EFD Earthfill dam

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Centerline Construction Centerline construction relies on the deposited tailings to form the main upstream support for the tailings dam. Additional local support to the dam raise is provided with either spigotted tailings, cycloned tailings, or borrow material. The downstream zone may be constructed of conventional borrow materials or cycloned sand. This method can typically be applied to almost all tailing dams, although it can be limited in cases where large volumes of water are required to be impounded, resulting in a relatively high dam section above the tailings beach. In these cases the stability of the upstream slope (into the impoundment) becomes the critical design condition. Examples of major centerline dams include Highland Valley Copper (140 m high), Canada and Kennecott Utah Copper (140 m high), United States. Figure 2 Highland Valley Copper Tailings Dam, British Columbia, Canada: Centerline cycloned sand with glacial till core.

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Upstream Construction (Conventional) Upstream construction, with limited drainage provision, can be used in non-seismic areas. The slope of the dam and the rate of rise need to be controlled to allow pore pressures to dissipate to prevent static liquefaction. A case history of static liquefaction (Davies et al, 1998) is the Sullivan mine in southern British Columbia. In this case the dam failed by static liquefaction caused by placement of the fill material for the next stage of dam raising. Upstream construction methods can be enhanced to provide additional drainage through the use of upstream drain blankets & finger underdrains, or directional drilling drains. The drainage systems, in some cases, are relied upon to lower the phreatic level to provide seismic and static liquefaction protection. However, in many cases drains can depressurize but not desaturate the tailings and the use of drains alone to prevent seismic liquefaction is questionable. Figure 3 Sullivan Mine, British Columbia, Upstream Dam : Static Liquefaction Failure Upstream Construction (Compacted, Consolidated and/ or Air Dried)

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Consolidation of tailings through drying or compaction, are also used for protection against static and seismic liquefaction. Consolidation through air drying, however, is limited to arid sites and where the rate of rise of the tailings dam is low enough to allow complete drying of the placed layer. Typically, this rate of rise may be 1 m per year (equivalent to 2 cm/week). The authors are not aware of any case histories where consolidation, through drying, has been sufficient to densify the tailings to prevent seismic liquefaction (assuming the material is saturated). Machine compaction of upstream spigotted tailings is carried out on some dams to provide seismic liquefaction resistance. INCO R-4 Tailings Area showing Compacted Upstream Spigotted Beach Hydraulic Fill Cylconing of tailings to produce sand for dam construction is common, particularly with large open pit copper mines. Recent advances in this technology include the use of flotation circuits to remove sulphides, for acid drainage control, such as at the Kemess gold-copper mine in northern British Columbia. The use of cycloned sand in seismic areas requires careful control of compaction and/or

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drainage. It is the authors experience that the density required to preclude seismic liquefaction in highly seismic areas (above PGA > 0.2g), assuming saturation, can only be reliably achieved through mechanical compaction. The use of underdrains, while promoting strong downward gradients and maintaining a low saturation level, are not sufficient to densify the sand to preclude liquefaction. Thickened & Paste Tailings The relatively recent introduction of high density thickeners is providing the tailings dam designer with another tool to optimize the use of tailings as a structural fill. Thickening of the tailings results in a steeper beach slope (1% to 5%), which can allow stacking of tailings above decant pond and dam elevations. This can result in a lower height for the tailings dam, however it is often accompanied by a larger impoundment footrprint. Steeper beach slopes have been achieved for paste tailings (mix of thickened and dewatered tailings) and for small tailing piles. Paste tailings, defined as low slump tailings which will release little or no water, is being promoted by some designers as the solution to tailings disposal (Newman et al, 2003). An important benefit of the paste tailings is that the absence of a free water pond significantly reduces the risk of piping and flow failures commonly associated with conventional tailings storage. As with all tailings disposal methods and tailings dam designs, the designer must balance the cost and risk of different tailings disposal technologies to select the best available technology. (McLeod and Plewes, 2003). Dewatered Tailings Dry tailings disposal, where the tailings is mechanically dewatered, have been used on a few sites. In this case the tailings dam can be formed by compaction of dewatered tailings. If saturation and liquefaction are not a concern, the disposal could consist of simply stacking the tailings, without compaction. However, compaction could still be required to reduce permeability to limit seepage through the tailings mass. Dewatered tailings disposal is currently being carried out at several mines where a main requirement at the site is the environmental concerns. Conclusions While tailing dams and water dams share a considerable amount of similarity in design and construction they are two very different structures. The design of tailing dams needs to consider the role of tailings, both as a part of the structural component of the dam, as well as the environmental aspects of storing a mined waste product. Tailing dams have additional considerations regarding piping, filters, drainage, geochemistry, and structural support.

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References DAVIES, M. P., DAWSON, B.,TASAROFF, D. and CHIN,B., 1998. Static liquefaction Slump of Mine Tailings A Case History. Proceedings 51st Canadian Geotechnical Conference, Edmonton. KLOHN, E.J.,1979. Seepage Control for Tailings Dams, Proceedings of 1st International Mine Drainage Symposium, Denver, Colorado. MCLEOD, H.N. and PLEWES, H.D., 2003. Can Tailings Dams Be Socially Acceptable?. International Congress of Large Dams (ICOLD), Symposium on Major Challenges in Tailings Dams, Montreal. PLEWES, H.D. and MACDONALD, T., 1996. Investigation of chemical clogging of drains at Inco Central Area tailings dams. Tailings and Mine Waste 96 Proceedings: 59:72, Rotterdam :Balkema NEWMAN P., WHITE,R., and CADDEN, A., 2003. Paste The Future of Tailings Disposal?

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TAILINGS DAM VERSUS A WATER DAM, WHAT IS THE DESIGN DIFFERENICOLD Symposium on Major Challenges in Tailings Dams, June 1

SummaryIntroductionDesign for PipingDesign for Filters and DrainageDesign for Tailings LoadingDesign for Seepage Control

Design for ClosureDAM HEIGHTCURRENT

DESIGN

Hydraulic FillThickened & Paste TailingsDewatered TailingsReferences