While it is down to the engineers and consultants employed to assess design and provide the best possible facility for the given conditions on site, it is ultimately the responsibility of the mine owner to ensure that the right experts are selected for the job, that their involvement comes at the correct times through-out the life of mine, and that best practices, regulations and government guidelines are adhered to during construction, operation and, eventually, closure.
TSF failures have been reported by international media this year at the Mount Polley mine in Canada; Cananea in Mexico; Dan River Ash Pond in the US; and at Itabirito, Brazil.
In terms of financial liability, the physical costs associated with large-scale failures such as these average US$30-100 million, while environmental liabilities (clean-up costs) run into hundreds of millions; and that is before considering value losses for shareholders, the implications of business interruption and damages to local communities.
More worryingly, events such as these damage the integrity of the mining industry, a sector that, despite huge advances, still carries the unfortunate reputation of causing gross damage to the environment. The general public’s perception of the sector and that of national and international governing bodies is key to its sustainability, and that is why MM has gone back to basics with this article, examining the fundamental principles behind good TSF design, construction and operation with six of the world’s leading consultants and engineers.
WHAT IS A TSF?
Mines typically process vast quantities of ore, only a tiny fraction of which is the actual commodity. Most of the material that passes through the processing plant therefore needs to be stored or disposed of in an economical and sustainable way rather than released into the environment where it could potentially have a negative impact on the local surroundings.
The main purpose of a TSF is to do just that: provide a safe, environmentally sound, and cost-effective way of storing the residues issued from ore processing, not only throughout the active life of a mine, but also during the closure and rehabilitation phases.
Extracting minerals nowadays is principally achieved through hydrometallurgical processes where material is crushed and ground, and the desired mineral is extracted using physical separation or chemical leaching methods. These processes are conducted at a relatively high liquid-to-solid ratio, so that the residual tailings to be stored are usually at a liquid-to-solid ratio from 1:1 to 3:1. This waste material or ‘slurry’ (a mix of finely ground particles and water) typically comprises more than 99% of the ore and other solids that pass through the process plant.
Storage usually involves constructing a large dam or dams that straddle lows in the topography, but can also involve deposition into a previously mined-out open pit. The tailings can also be thickened or filtered to remove most of the water before discharge into the TSF or stacked in piles. Volumes generated over the life of a new mine can vary from tens of millions of tonnes to over a 1,000Mt.
TSFs can be designed to permanently retain the solids in the slurry after they settle and release the process water for reuse, or to discharge them under permit conditions and water quality standards with treatment as necessary. As such, they are also often used for site water management, including process water supply, containment for site-contaminated water and as a holding basin for flood control before treatment and release to the environment, or for recycling. Potential uses, along with the production volume at the mineral processing facility, will influence the size of a TSF.
THE RIGHT SITE
The key feature of a TSF is a confining embankment or dam that can be constructed in stages and raised as extra storage capacity is needed. These are usually built from local fill materials (sand, gravel, clay, waste rock) and possibly imported or manufactured materials where required. In some instances, the coarser particles are separated from the tailings using cyclones and these ‘sands’ are then used to construct the retaining dams. A TSF will also generally feature:
• A tailings distribution system (pumps, pipelines, conveyors, trucks);
• A spillway for flood control;
• A water management system (process water return pipeline), seepage controls (collection ditches, interception wells and water treatment system);
• Instrumentation to monitor seepage and stability;
• An overall operations, maintenance and surveillance plan; and
• A closure plan.
However, design elements will differ depending on individual circumstances.
Due to their potential impact on a project’s development and cost, TSF site selection assessments should be initiated as early as possible – usually at the conceptual design stage. These studies should also be carried out in consultation with any relevant authorities to prevent unnecessary delays in permitting.
It is often the case that detailed information covering the environmental conditions is not available at this stage of a project, and it is therefore important to ensure that the impact of long-lead items such as environmental permitting are considered early on.
Romain Girard, associate senior geotechnical engineer at Golder Associates, explains: “The first site selection process would consider three to 10 possible locations at scoping level. This would then be reduced to one to three possible sites at prefeasibility study [PFS] level. This would allow for the preferred site to be developed further before the final feasibility study.”
A good site selection study should thoroughly evaluate a variety of aspects and should include social, environmental, engineering and economic considerations. It would include such items as:
1. Environmental, social and permitting considerations, which would include an Environmental and Social Impact Assessment (ESIA). This would also evaluate the management of hazardous materials, if present, and take into account decommissioning and closure considerations. Operators must be aware of variations in local legislation and regulation in the areas in which they are planning to mine.
2. There are also key site requirements that should be taken into account, including an analysis of the topography and geology of the site, as well as the hydrological and hydrogeological setting. A thorough knowledge of previous and current land ownership and use is essential.
3. A storage capacity assessment and plans for efficient and safe disposal methods. An outline of the physical size of the facility (footprint and maximum height) should be put together alongside broader financial plans for capital expenditure, operating expenditure and closure costing. Similarly, plans for tailings transportation and water management should be outlined, and an assessment of the availability of construction materials should be made.
4. A risk assessment on screening-level dam safety should also take place in parallel with the siting study to identify the main risks and liabilities associated with a particular site. This allows for adequate mitigation measures to be developed during all stages of the design, and subsequently during construction and operation.
Site selection planning should take place at the same time as planning for the mineral processing facility, and it should take into consideration the plant’s long-term tailings storage needs. It is often beneficial to locate the TSF at a lower topographic grade than the mineral processing facility to take advantage of gravity flow of tailings slurry. Condemnation drilling should also occur to ensure there is no ore under the selected TSF site.
“There are significant benefits of utilising a whole life-cycle approach from the outset,” says Irwin Wislesky, technical director of tailings and mine waste at SLR Consulting (Canada), “with appropriate emphasis given to the closure phase as this is the longest part of the life cycle.”
WORKING WITH THE ENVIRONMENT
Every TSF is unique and the design will largely depend on two factors: local conditions and tailings characteristics, both of which are directly influenced by the commodity being mined and how it is processed. For example, the style of operation will dictate the method and rate used to raise the TSF embankments.
The local setting and topography can greatly affect the type of containment required (flat topography may result in a larger footprint and more significant dam volume due to a larger perimeter length), as well as the deposition and tailings management strategy employed. Major factors that influence design include: climate, geology, potential seismicity, topography and proximity to areas of human habitation, forest reserves, freshwater or marine ecosystems and downstream infrastructure.
The use of 3-D modelling software allows designers to select locations for development which minimise embankment fill requirements and disturbance to the local catchment area. Girard gives an example: “We [Golder] recently designed a dam in northern Europe utilising thickened tailings deposition which allowed us to reduce the size of the embankments by increasing the beach slope angle of the tailings mass. This provided significant cost-savings for the company where storage space was at a premium.”
Stuart Schmidt, manager of technology at engineering firm Bechtel, says: “Probably the best setting and topography is provided by a valley, where the TSF dam can be built across the low end.” In these situations it is particularly important to minimise impacts to the natural hydrology regime; inappropriate siting of the facility can lead to significant risks, such as disruption of the natural catchment drainage regime and contamination of local water supplies through excessive run-off or seepage from the TSF.
Climate can also affect the construct-ion and design of a TSF, particularly the dam. “It is very difficult to build an earth dam in wet environments, and thus a rockfill dam may prove a better alternative. Very cold climates can impact the delivery of tailings to the TSF,” adds Schmidt.
Very dry areas require maximum water recycling, while very wet areas may require additional water management measures. In these circumstances, it is worth considering thickening and filtering to reduce water losses within the facility and provide additional seismic stability. Wislesky described an example of uranium TSF in Namibia where SLR established an approach to both thickening the tailings and depositing them in smaller paddocks created inside the larger TSF, thereby almost halving the water losses.
Jamie Spiers, senior consultant at SRK Consulting in the UK, outlines a recent project that SRK worked on in West Africa which considered a valley fill impoundment TSF. “Due to the extreme run-off calculated for the site, it was necessary to design a river diversion to allow dam construction to commence and ensure that run-off inflow into the facility was minimised. This prevents clean water contact with contaminated tailings material,” he explains.
“Ideally, the TSF would have been placed in a smaller catchment area, but there was limited availability of land. A trade-off had to be undertaken between the installation of a water-retaining embankment which would require liners to prevent excessive seepage through the embankment, and costly perimeter drainage canals to minimise inflow into the facility. The latter was selected as the risks associated with seepage management in the main dam were minimised.”
TSFs used for site water management may require extra considerations. “We recently undertook a conceptual design for a gold mine in Saudi Arabia where a key driver was reuse of process water,” says Richard Elmer, head of mining at Knight Piésold. “Our design focused on tailings dewatering options to maximise water savings with the added benefits of minimising the TSF footprint and improving stability. With very low in-country energy costs, the higher capital expenditure associated with filtering the tailings was offset.”
For another recent Knight Piésold project in Norway, there were very different drivers. “The principal concern of the client was to provide a TSF design that maximised progressive restoration in an area of steep topography to provide areas of new flat ground that could be reused for construction or agriculture,” explains Elmer. “This was achieved by careful phasing of the TSF construction.”
As tailings facilities are usually quite large, they require vast quantities of fill material to provide a containment barrier, so the availability of construction materials can be a limiting factor. At the Antamina site in Peru, where rock was easily available, Bechtel used a rockfill dam with concrete facing as the starting dam for the TSF, while at the Yarwun Alumina Refinery site in Queensland, Australia, where cohesive soil was easily available, earth dams were used.
Local clay or till is often utilised as a low-permeability barrier where available, but artificial liner materials may need to be included in the design if natural materials are not available. A flexible approach is often necessary in these situations; SLR recently designed an unlined TSF using Canadian shield bedrock which has low permeability, and seepage collection trenches to protect groundwater. The waste rock generated by the pit was used to create the containing embankment.
“It’s natural to home in on engineering issues like health and safety and stability at the design stage,” comments Nick Watson, technical director and head of geotechnics at Wardell Armstrong International. “But a good risk assessment should also look more widely at environmental and social factors, and potential effects on local communities and political fallout.”
Many of the TSFs that Wardell Armstrong has worked on have been in arid climates in less-developed countries where the availability of suitable construction materials was limited by long lead times. “This often means using less rockfill and more fine-grained material for construction,” explains Watson.
“The availability of plant and machinery may also be restricted. The design and width of a clay seal at a site in West Africa, for example, had to be modified when we discovered that the only available machines were refurbished ones originally used for building motorways in France in the 1950s!”
GROWING WITH THE MINE
A TSF is an active structure, usually built in a staged manner, which is continuously raised or expanded during the life of a mine to meet the needs of the mineral processing facility. Facilities commonly have a starter dam, which is followed by a series of raises or lifts to accommodate increasing amounts of tailings.
Girard says: “Any design should be cost-effective for the prevalent conditions at the mine, but new orebodies [and extensions] are frequently found, the processing plant expanded and throughput changed. Therefore, these parameters should be taken into consideration at the start of a project in order to design a facility that is flexible. Sometimes economic conditions dictate that a mine is put under care and maintenance, and this also needs to be considered during the design.”
Expansions are often required due to changing commodity prices too; a price rise lowers the grade of mineable ore and, in turn, increases reserves. Should production increase beyond the scope of the original design, then a further design will be required to accommodate the additional tailings or an additional site sought for a new facility.
“The choice of method for raising the TSF will be dependent on various factors, including availability of land downstream, consolidation properties of the existing tailings and the design of the existing impoundment,” says Elmer. “Raising an existing facility is usually preferred to finding a new site to minimise environmental and social impacts and to ease the permitting process. Provided the design is thorough, including detailed assessment of the existing facility, expanding a TSF shouldn’t necessarily be avoided.”
The cost of expanding the capacity of a TSF can range from almost nothing, where the current operational system accounts for regular raising of the facility with available materials, to quite a significant amount if new facilities have to be designed and built. Errors at the design stage can be extremely costly to remediate during the operational phase.
TO STORE OR STACK?
While traditional slurry TSFs are widely used, cost-effective and well understood, it’s always good to explore other approaches for managing tailings.
“At the beginning of a project, all mines should be looking at alternative storage methods, both on the dewatering spectrum (from thickened tailings to dry stacking) and for backfill, or other options such as tailings and waste rock co-disposal,” states Girard.
The choice of tailings storage method is rarely controlled by a single factor. Some mines do consider methods such as dry stacking or backfill, but these options are not always possible or practical regardless of the costs to build, operate or close the facilities. “Regulatory drivers may contribute to the choice of final tailings solution, but we are not currently seeing directives from governments for a particular type of tailings disposal system,” adds Elmer.
For underground mines, using tailings to backfill mined-out voids has been common practice for many years. The tailings undergo further processing with the reduction of water content and addition of binding material such as cement before being pumped underground. This provides two key advantages: minimising surface disposal while providing support for underground openings. Some mining methods are dependent on backfill, but will usually only require 20-50% of the total waste tonnage generated by the operation, so while a tailings facility is always needed, the size can vary.
SRK is currently conducting studies at sites in Australia, Central Africa and Chile to investigate the viability of thickened tails and paste technologies. The advantages of thickened tails include water conservation, efficient run-off management, minimised dam construction costs, enhanced stability and minimised infiltration of supernatant fluids and oxygen which could lead to acid generation. “The increased processing costs incurred by thickening tailings can often be offset by reduced capital costs for embankment construction and water treatment facilities,” explains Spiers.
Dry stacking techniques are being increasingly applied to minimise the footprint of TSFs and hence the environmental liability of an operation. By dewatering the slurry and recycling water back to the process plant, less water is lost in storage and mines can better meet their abstraction and discharge limits. But every method has its pros and cons, and while dry stacking can save a considerable volume of water, it would typically have a much larger carbon footprint than hydraulic tailings disposal.
Schmidt says: “We have looked at dry stacked filtered tailings to reduce water makeup by up to two thirds and the concomitant supply costs of that water. The real challenge is applying the tailings filters required at a very large scale to the process – this is first-of-a-kind technology. The overall cost advantage of dry or backfill system versus wet will depend on several factors, including the proportion of tailings as a function of the ore stream, project location, project-specific terrain, and facility locations and layout.” The two main challenges with dewatering are the difficulty of drying out some material, and the extra cost that might make the project less viable.
MONITORING, AUDITS AND INSPECTIONS
TSF dam monitoring equipment and procedures are similar to those used for water storage dams. Routine monitoring through the dam and its foundation, including regularly scheduled visual inspect-ions, is important to validate design assumptions and ensure dam safety.
Monitoring systems vary from site to site, and considerable technological progress has been made in this area in the past ten years. Current technology includes the installation of seepage and dam movement devices that can trans-mit data via the internet, thus allowing for remote management from a centralised location. These are generally coupled with a predefined alarm system that can also be linked to safety devices such as shut-off valves or the start-up of emergency pumps. A common pitfall is to instigate a monitoring programme, but not have appropriate trigger levels or action plans should specified levels be reached. This is something that should be addressed from the outset.
Usually monitoring comprises a range of instrumentation, including piezometers to measure groundwater level, inclinometers to measure any movement in the impoundment walls, and survey points to measure any vertical movement/settlement. The tailings management system is typically integrated with the overall water management system at a mine site, and monitoring strategies to ensure a good understanding of the site water balance are essential.
Monitoring programmes are developed based on key risks, objectives and routine controls, and a risk assessment carried out at the design stage will enable monitoring to be targeted at the areas of highest risk. If stability is affected by embankment settlement, for example, it will make sense to place settlement monitoring stations along the crest. The site operating manual typically specifies where and how often the monitoring should be done, and what action should be taken on the results.
Ideally, all TSFs should have standpipe piezometers installed around the embankment to allow monthly water levels to be measured. Should the position of the phreatic surface within a particular embankment rise above an acceptable level, this may pose a stability risk to the outer embankment structure and remedial measures should be taken immediately.
Groundwater monitoring bores should be installed around the dump perimeter to ensure seepage from the TSF does not adversely affect local groundwater supplies. Water samples should be tested regularly to ensure that pH levels, sulphides and selected metal concentrations fall within internationally recognised safe limits.
Visual inspections of the structure should be carried out on a daily basis during the operational phase to ensure that all drainage structures and embank-ments remain intact and function correctly.
Event-driven monitoring may be required on a more frequent basis in regions with a high level of seismic activity or rainfall, and emergency action plans (EAPs) should be developed and regularly updated.
AVOIDING COMMON PITFALLS
Ultimately, the successful design, construction and operation of a TSF is linked to the mine owners’ commitment to environmental stewardship, safety and sustainability. Every consultant and engineer inter-viewed for this article agreed that most pitfalls associated with operating a TSF are due to poor water management. This includes:
• Management of process water stored in the TSF;
• Run-off from upstream areas;
• Seepage through the dam or foundation; and
• Pore water from eventual tailings consolidation.
Issues can be avoided by using appropriate fill materials for embankment design, through strict quality control and assurance during construction, and ongoing review of the tailings deposition strategy throughout the operational phase.
After project construction is complete, a critical element for continued success is that the mine operator maintains professional engineering oversight and inspection of the TSF, particularly when expansions are designed and undertaken; the second most common concern among interviewees was the level of involvement from a competent geotechnical engineer/consultant throughout the life of the project.
Mine personnel in charge of the facility are sometimes more focused on production than tailings disposal, and TSFs must be built and operated in exact accordance with the design if they are to function in line with the designer’s intent. Appropriate allowances for changing operations and for extreme conditions are essential. “Engaging the designer in the ongoing inspection and monitoring of the TSF is good practice, as well as having independent third-party review,” says Elmer.
It is also important to remember that TSFs are active structures and that sound design must be coupled with sound construction and operation to keep them safe during the entire life of mine and beyond. “The design must be tailored to the capacity of the mining company, its operators and contractors to build and operate the structure safely,” cautions Girard.
Wislesky believes that most common pitfalls can be avoided, or at least reduced by good site selection, design and financial robustness. “These, in conjunction with a dedicated mine policy, clearly demonstrate the need for knowledgeable tailings facility operators who understand their limitations and when it is necessary to call the consultant,” he explains.
Meanwhile, Watson says change is probably the biggest issue. “While the mining engineer concentrates on operational matters over the 20- to 30-year life of the mine, people come and go, production changes, regulations change and things are gradually done differently.
“The original design intentions can be forgotten or are no longer valid. That’s why regular audits are so important to make sure the containment practices are still safe, or to change them if necessary with the agreement of the regulatory body,” he says. “One specific pitfall is not being able to find as-built construction drawings in the event of a failure, which is why they need to be held securely. Some mines keep all their records in a secure data room for future reference, which clearly helps with carrying out reviews.”
BEST PRACTICE IN PRACTICE
With heightened coverage of TSF failure events this year comes additional pressure on both service providers operating within the global mining sphere to deliver safe and sustainable facilities, and mine operators to ensure they are doing all they can to adhere to best practice.
“There has always been pressure on tailings consultants to deliver safe and sustainable TSF designs that meet with client expectations,” says Spiers.
“The occurrence of large-scale TSF failures has generally declined over the past 50 years as the result of a shift towards specialist design teams being responsible for dam engineering, and the involvement of experienced practitioners in the review of existing facilities at operational mines.”
The principal effect of recent failures that Knight Piésold has seen is an increase in requests from mines to provide more regular inspections to check that their operations comply with the design intent.
“A TSF is never going to be ‘sustainable’ in that it is a facility for storage of a waste product, but designers can apply sustainability principles to post-closure design to minimise long-term negative impacts,” states Elmer. “A disused TSF need not become a liability if properly closed, and can even become an asset if the land becomes usable where previously it was not.”
Any high-profile tailings dam failure places pressure on the industry as a whole, but it also means accrued pressure on the regulators involved in the permitting process and, as a result, can lead to more stringent criteria for TSFs.
“In Golder’s experience, most of the technical challenges TSF designers are faced with can be overcome, although in some instances this can lead to significant cost increases for the disposal of tailings by the mining companies,” explains Girard.
“In such cases, the cost implications of regulations can be inhibitive and have been known to stall or halt projects altogether.” He reiterates that there is a need for the involvement of competent people throughout the whole process – designers, operators and regulators.
Most consultants are on board with sustainable TSF development, but may be influenced by pressures from some mining companies to limit their involvement with the project. Although mining companies are often concerned about sustainable development, other pressures such as mineral production and budget restraints can lead to shortfalls in the attention paid to a TSF.
“The consulting business should pick up, as failures tend to focus people on these high-risk man-made structures. In fact, we expect that third-party involvement and government-mandated reporting will be coming in the near future as a result of recent TSF failures,” adds Wislesky.
As poor water management has been linked to many of this year’s high-profile failures, there’s likely to be even more pressure in the future for mines to dewater their tailings.
“There will also be increasing emphasis on recycling waste for a more sustainable approach. This isn’t always possible, of course, especially with fine-grained material, but we’ve recently seen some positive examples such as waste material in Namibia reworked to make bricks,” says Watson.
In short, some lessons are learnt the hard way, but hopefully the learnings taken away from this year’s events will eventually strengthen the use of best practice from both mining companies’ and service providers’ perspectives, and the mining industry will emerge a better one because of it.