Resource World Magazine

a u g u s t / s e p t e m b e r 2 0 1 5 www.resourceworld.com 47 MININGWORLD Tailings storage facilities – no mistakes allowed by Laura Barker T ailings storage facilities (TSFs) are a major contributor to the environmen- tal mining footprint in Canada. An estimated 120 active TSFs reside in British Columbia, and TSFs cover approximately 176 km 2 in Alberta. The security of tailings dams in North America was brought to the forefront of the mining industry after the August 2014 breach at the Mt. Polley Mine in BC, prompting the industry to take a closer look at design standards. Storage of tailings, the solid mineral- processing waste often mixed with water to form a slurry, is an inevitability of mining. Earthworks materials comprising borrowed soils, cyclone sand (the processed coarse tailings fraction), and/or waste rock fill are typically used in the construction of tailings dams which, for 'conventional' facilities, contain large bodies of slurried water and tailings. Factors including changing climatic conditions, dam foundation characteris- tics, and the settling and drying of tailings must be considered in design, from startup through closure. The wide range of con- siderations and potential unknowns pose geotechnical challenges for engineers. Klohn Crippen Berger (KCB), a Vancouver-based engineering consulting company, is in the specialized business of tailings and mine waste management. Formed in 1951, and with offices across Canada and internationally, KCB dedicates approximately 130 engineers and scien- tists in their Mining Environmental Group (MEG) and over 100 in their Oil Sands Group to tailings issues, their primary focus. KCB's active BC tailings projects include those at Highland Valley Copper and Gibraltar, where they have monitored construction since the 1970s. They have also recently carried out feasibility tail- ings studies for Quintette, KSM, Morrison among others in BC. KCB's Vice President of MEG, Howard Plewes, has been working in the tailings industry for 35 years. He's observed chang- ing trends in tailings disposal including the adoption of paste and filtered tailings tech- nologies in the mid-1990s and onward (both methods recommended by the Mt. Polley expert review board to be considered for tailings disposal); and increased consider- ation of geochemical properties of tailings and waste rock in their design. KCB provides a wide range of tailings alternatives studies for clients, including optimization studies on tailings dewater- ing, enhanced water recovery and dry stacking. A few of their related experi- ences include helping to implement paste tailings at the Myra Falls Mine (2003) on Vancouver Island, working on filtered tail- ings at the Greens Creek Mine, Alaska, since the mid-1990s; and studying co-disposal of tail- ings with waste rock for the Porgera Mine in Papua New Guinea (2000). KCB has also implemented 'anoxic collec- tion drain systems' at Vale in Sudbury to prevent clogging of dam toe drains due to Acid Rock Drainage precipitates, thereby mitigating potential destabilization. Looking forward, regula- tory changes, and changes to site investigation will likely evolve from Mt. Polley. According to Plewes, "Tailings dams are frequently long linear structures many kilometres in length. For tailings dam design, you need to understand the regional geology of the area and design a site investigation program so you don't overlook anything. In the future, I think there are going to be protocols for what is an acceptable level of site investigation. Nevertheless, we never have complete certainty of the geology and design and operations must reflect this uncertainty." "It is most important that the founda- tion geology be evaluated for presence of brittle, strain-weakening or contractive materials (materials that can lose strength with strain). Those materials are the ones that can be dangerous to you, as your foundation can actually weaken as you build on it rather than gain strength. Commonly used factors of safety may not apply to these situations and higher factors of safety may be required. Strain- weakening soils were a factor in the 1998 Aznalcollar tailings failure in Spain and the recent panel report suggests it's been a factor at Mt. Polley. " KCB's senior staff include leaders in tail- ings design who are involved in guiding industry practice changes at multiple levels: Harvey McLeod currently chairs the tail- ings committee for ICOLD (International Commission on Large Dams); Bryan Watts helped develop the guidelines for Operations, Maintenance and Surveillance manuals for tailings dams; Howard Plewes and Lawrence Clelland participate on the Mining Association of Canada (MAC) Tailings Working Group; and a variety of individuals have been involved with dam safety guidelines for the Canadian Dam Association, MAC and independent review boards for tailings dams. Looking to the future of TSF design, Plewes' comments, "You have to be well aware of the performance of tailings dams and where things have gone wrong in the past and the lessons learned. There are many defensive measures we have incorpo- rated in our designs in recognition of these past incidents." n A tailings beach in northern British Columbia. Photo courtesy Klohn Crippen Berger.

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