The importance of a properly engineered TSF cannot be underestimated
By Steve Fiscor, Editor-in-Chief
From a mining engineering standpoint, designing a tailings storage facility (TSF) is one of the less glamorous assignments. It is, however, one aspect of a mining operation that can impact a project from beginning to end. When things go wrong with TSFs it can have a large impact.
The ways mines manage tailings systems varies by region and commodity. Obviously, a mine operating in an arid environment, especially those that pay to pump water to a site or have limited water resources on site, would want to recycle as much process water as possible. There are sites where containment and sedimentation is more important than water recovery. Sound structural integrity of the dam is a must and every TSF design has to take into account individual site conditions ranging from, but not necessarily limited to, its geotechnical setting, geochemical setting, geologic setting, construction material availability, hydrologic setting, permitting requirements, closure requirements and potential impact to the environment. For this reason, design of each TSF is unique and has to consider all elements that might affect long-term operation and functionality of the facility and the closure requirements for the facility.
Following the Mount Polley release, Al Hoffman, the chief inspector of mines for British Columbia’s Ministry of Mines, issued a directive for third-party inspection of all TSFs in British Columbia. The mining industry responded in a very responsible way, with a flurry of activity surrounding the engineering of TSFs and a desire to ensure that their individual TSFs were designed properly and that the facilities were being operated in accordance with design. As a result, many third-party reviews have been undertaken to evaluate operating facilities around the world.
Understanding the Approach
Many engineering consultancies offer design services for tailing storage and management. NewFields is one of those firms. Founded in 1995 to provide a cost-effective alternative to large institutional engineering and environmental consulting firms, it emphasizes a solution-oriented approach to consulting assignments. Now with nearly 20 years of experience and more than 300 employees located in 21 offices on three continents, NewFields believes its approach is working. In addition to new clients, they are retaining existing clients, an outcome that results from trust-based relationships that have spanned several decades.
Headquartered in Atlanta, Georgia, NewFields has several subgroups, which operate autonomously. NewFields is a different type of consulting service, explained Carl Burkhalter, an associate with NewFields. “Our group, NewFields Mining Design & Technical Services, specializes in mining with offices in Denver, Colorado, Reno, Nevada, and Elko, Nevada,” Burkhalter said. “We focus on engineering design, construction management, and construction quality assurance/quality control (QA/QC) of tailings facilities and heap leach pads, general civil engineering for infrastructure, and geotechnical services for mines.”
Another subgroup, NewFields Mining & Energy, which is based in Montana, specializes in the environmental and permitting side of the business, developing Environmental Impact Statements (EIS), baseline studies, Environmental Assessments (EA), permitting, closure plans, etc. There are 13 other groups within the company that specialize in different aspects of civil, environmental and geotechnical engineering.
Burkhalter has been directly involved with engineering TSFs for more than 25 years. More recently, he said he has seen a lot more emphasis on water recovery from tailings using thickened or filtered (dry-stack) technology. “These practices work particularly well for those mining operations that need to recover the process water or mines that have a limited tailings storage footprint,” Burkhalter said. “Practices vary widely across the ‘tailings continuum,’ which includes everything from impounding slurries to filtered (dry-stack) operations.”
The most common form of TSFs is for slurries that contain 35% to 55% solids by volume. Basically the tailings are discharged behind the dam. The tailings settle and the water is either recovered from the impoundment or it evaporates. With thickened tailings, more water is removed, the thickened tailings is pumped to the deposition area (or underground in paste backfill operation) and the tailings form steeper slopes because they are less fluid. For filtered (dry-stacking) processes, filtration is used to reduce the moisture content to a range that is near optimum moisture content as defined by ASTM D-698, Standard Test Methods of Laboratory Compaction Methods of Soil. The tailings are usually hauled and/or conveyed to the deposition area and compacted to form a dry stack.
“All of these processes have advantages and disadvantages,” Burkhalter said. “The capital costs associated with thickened, paste and filtered tailings are much higher than conventional slurry tailings. Water recovery is the key. If the need to have enhanced water recovery justifies the investment, then those technologies work well.”
When the lack of water is not a concern, Burkhalter explained, conventional impoundments are the least expensive TSFs to build and operate. These systems can be designed to be completely safe but special attention is required in the areas of focus cited above due to the process solutions that are typically contained with conventional tailings facilities. “Conventional TSF systems can vary, but the principles remain the same,” Burkhalter said. “Some mines may use a rotating discharge system for example, while others use a single-point discharge. Depending on the nature of the tailings or the jurisdiction, the impoundment may need to be lined. Most impoundments are designed with under-drains to drain and consolidate the tailing and/or to reduce potential driving head on liner systems.”
Costs and environmental controls are major considerations when it comes to handling tailings. Burkhalter said. “A well-engineered facility with proper construction quality control will not only minimize the cost associated with tailings management but will also minimize the risks.”
Working with NewFields allows Burkhalter to do one of the things he likes best, helping an owner/engineer build an environmentally sound facility without breaking the bank. “At the end of the day, everyone wants a facility they can be proud of,” Burkhalter said. “We recently helped a client in Mexico with their design of a small tailings dam. We reviewed the specifications for the dam and the available construction materials. We found that by using a coarse construction material, which was readily available, the mine could build a sound dam that met its needs and in doing so could save a bundle of money.”
Meeting the Guidelines
The directive in British Columbia orders each mine to conduct a Dam Safety Inspection (DSI) by December 1. The DSI must cover all dam structures for all TSFs and it must be conducted by a qualified professional engineer. The DSI must then be reviewed by an independent qualified third-party engineer from a firm that has not been associated with the original design of the TSF. Mine managers must submit any recommendations from either the DSI or the independent review to British Columbia’s chief inspector of mines.
NewFields has been actively assisting a number of mines with the directive, Burkhalter explained. “All TSFs have an engineer of record who designed and oversaw the construction of the facility,” Burkhalter said. “That person, who is either a registered P.Eng. in Canada or a P.E. in the U.S., stamps the report and plans. NewFields routinely provides these services and can also provide the independent third-party reviews.”
“We are currently working with several companies to comply with the British Columbia directive,” Burkhalter said. “Likewise, other engineering firms are reviewing the dams that we designed. An independent engineer reviews everything and makes recommendations. It is prudent engineering to have another set of eyes look at the facility.” Burkhalter also noted that “the third-party review process was a common practice among some of the larger mining companies even before the directive was issued.”
NewFields is also assisting Canadian companies operating abroad who are designing and operating all of their dams to Canadian standards. “We have provided assistance to our client’s in Latin America, helping them meet Canadian dam safety guidelines,” Burkhalter said.
In recent years, there have been a number of incidences that have highlighted the potential environmental and financial risks associated with a tailings dam failure. “Mine operators understand that these are high-liability facilities,” Burkhalter said. “They are very aware of the risks associated with their TSFs and know that it is vitally important to make sure that the facilities are designed and operated properly. The system’s design can affect mine closure plans as well. So these projects must be well-planned from the onset of the permitting process to the end of the project life.”