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Another example of how paste technol- ogy can help manage the subsidence of a closed mine comes from Yellowknife, Northwest Territories, in northern Canada, a city that grew up on mining and became the territorial capital in 1967. The former Giant mine and its network of underground mine workings have been the focus of intensive stabilization efforts. Since the mine's closing in 2004, these voids have begun to weaken and collapse, causing negative effects on the surface including subsidence and sinkholes. Considering its close proximity to the site to Yellowknife, various unrestricted access points for the public, the adjacent Ingraham Trail, and numerous on-site fea- tures, including a creek, it was important to find effective (and cost-effective) ways to support the mine workings to prevent the propagation of failure through the mine. Some of the general challenges faced at orphaned or abandoned sites, which may be relevant to coal-mine remediation, include: Hazardous substances — The main con- cern is the possibility that groundwater flowing through the open, abandoned mine workings might pick up hazardous materials, where it would likely spread fur- ther. This hazardous material could be left over from the mining operations or be nat- urally occurring, but the voids under- ground provide a natural pathway for these materials to migrate. Infiltration of surface water through cracks, voids or other path- ways into the subsurface could exacerbate the migration of contaminants into the environment. This has parallels to the sele- nium, mercury and naturally occurring radioactive materials (NORM) that can be an environmental issue in coal mine water. Inaccessible mine workings — Three Sisters was completely inaccessible and only part of Giant mine is still safe for human access. This means that under- standing the conditions underground and the areas of potential failure becomes a complicated undertaking. The solution in both cases involved a drilling and 3-D mapping program to model the under- ground drifts and stopes, with some of the information coming from the mine plans made during the pre-1960s. The drilling program was extended to include observa- tion boreholes to monitor the paste as it was delivered underground to ensure leaks into other areas of the mine were mini- mized. Multiple stakeholders — Many govern- ment bodies, including regulatory authori- ties, have a stake in these properties, meaning that many facets of both projects required local, regional and federal approval. This can apply in coal mine remediation as well, when the property owners must work with local and state/provincial political leaders, regula- tors and environmental groups to find solutions that address the issues. How Coal Companies Can Build on Their Social License Coal producers who seek to build good rela- tions with communities affected by coal mining can get support from gaining good skills in managing subsidence issues for closed underground mines. This can include: Community research — Find out which communities have issues with subsidence. Then, see if there is potential to build good relations with community leaders through offering to work with them on remediation of the property. This could include current or former coal mining towns that want to diversify their economies as retirement or recreational havens — and have been stymied by areas vulnerable to collapse, and where it was unsafe to build hiking trails, golf courses, residential communities and the like. Regulatory authority research — The next step is to find out whether helping to remediate the worked-out mine can help build offset credits with which to support future coal extraction operations. It may help if the remediation work is to be done in the same service area, generally based on being within the same watershed, as the new extraction site. Build relations with qualified profes- sional support — There is a growing body of professional services firms that can offer good skills in paste technology. However, that is only one piece of the puzzle. It was the team approach that made Three Sisters and Giant mine successful — the team of paste engineers, underground mining engineers, rock mechanics engineers, geotechnical engineers, 3-D modelers and drilling specialists who worked together to develop the underground "picture" along with the elements of the solution e.g., bore- hole layouts, strength requirements, pumping expertise. It is important to con- sider each piece and seek out partners that can bring their expertise to bear on the problem. The effect can be a "social license to operate," which results in community sup- port, rather than community opposition, to new coal-extraction proposals. Sue Longo, P. Eng, MBA, is an associate in the paste engineering and design division of Golder Associates, based in the company's Calgary, Alberta office. Tel: 403-299-5600; sue_longo@golder.com. p a s t e t e c h n o l o g y c o n t i n u e d 46 www.coalage.com February 2015 A mixer (center) is used to combine water, overburden and chemical additives, with the resulting paste being pumped underground to prevent the collapse of underground coal workings in Canmore, Alberta. This area, previously unsafe to walk on or build on, has now been developed as a combined residential/recreational/retail community.