Dealing with a coal-mining legacy at a mountain site in Canada

The Three Sisters Mountain Village is thriving as a mixed-use development with tourist appeal, after engineers were able to stabilize underground voids left by nearly a century of coal mining. The site won a Canadian Brownfield Award in 2006. Image courtesy of Three Sisters Mountain Village Ltd.

Nestled just inside the western folds of the Rocky Mountains in Alberta, Canada, is Three Sisters Mountain Village. Located in the town of Canmore, the site sits in the shadow of the three-peak mountain from which the developer derived its name. The property is adjacent to the Bow River, one of the leading trout-fishing watercourses in North America, and a few minutes’ drive from world-renowned Banff National Park.

In this setting, Three Sisters wanted to offer a health, wellness, and adventure resort and residential community, including two golf courses, located just an hour’s drive from the booming city of Calgary.

The government of the Province of Alberta encouraged development of the 2,000 acre (800 hectare) site, looking to create a tourist attraction and community similar to the development at the base of the ski slopes at Whistler, in neighboring British Columbia.

One of the challenges affecting Three Sisters’ plans lay in Canmore’s origins—not as a tourist paradise, but as a coal-mining town.

Beneath the planned development, east of the main Canmore town site, lie extensive underground coal workings dating back to 1887. Seven coal seams were mined on the westerln half of the site using an underground room-and-pillar method, which leaves large “voids,” or open areas, underground. The workings rise and fall along with the coal seams, at times rising perilously near the surface. Operations continued there until 1979.

Some of the land above the shallower tunnels had already collapsed. Other parts of the property were relatively safe from sudden collapse, but would likely see slow subsistence over time as voids deep underground begin to collapse. Surface impacts included mine entrances and abandoned equipment, the former tipple site where waste rock and other overburden material was discarded, and a vertical strip mine.

Remediating properties on top of coal mines is a longstanding problem in many parts of the world. The redevelopment history of this abandoned site from brownfield into a community popular with Hollywood glitterati holds lessons for anyone seeking to develop a former mine site. Some of the former coal mines in states such as Colorado and West Virginia underlie property that is, like the Three Sisters site, desirable for real estate development. In other areas, roadways and railroads are at risk from subsistence.

While other types of underground mining may also cause subsistence, it is the shallow depth of many coal mines that causes the most immediate danger—in some mines, the workings actually break through to the surface.

Solutions developed in one place will not necessarily work in another, hence the need for customized solutions.

In the Three Sisters situation, one of the unique factors was found in Alberta Provincial Regulation (AR 114/97), which required a developer to have the situation studied by an undermining engineer, who prepares a report and recommendations. When these are carried out, the engineer certifies the results, which are in turn certified by an independent expert.

In its work remediating the mining property, environmental and geotechnical engineering firm Golder Associates considered pumping standard concrete into the underground voids to fill them up and prevent further collapse.

However, concrete would not have worked well in this situation given the fact that it would not flow well, and would require a large number (read expensive) of boreholes to make sure all necessary voids were filled. Using concrete also would have made large areas of the property unsuitable for development.

Accordingly, Golder decided to use a new technique involving “paste,” a mix of Portland cement, water and on-site overburden materials, mixed in a standard cement-mixing truck and then pumped down into the voids to fill them. Remotely-operated video cameras helped operators keep an eye on the underground processes to make sure that the job was done thoroughly.

When the paste had flowed into the voids and hardened, the risk of collapse was removed and the land was safe to build over. Additional benefits of using paste include:

• More of the voids could be filled cost-effectively, so that more of the property could be remediated economically and certified as safe for construction.
  
• Using onsite waste materials reduced environmental impact by negating the need to truck in aggregate materials. It also cleared up some of the surface stockpiles of waste rock left over from the mines, which covered valuable acreage.
  
• The ability to fill more voids increased the planner’s flexibility in designing the development, including the location of houses, roadways, parkland, golf courses, and wildlife corridors.

Although Three Sisters Mountain Village is not a brownfield in the sense that it contained large amounts of contaminated soil, its previous use did find developers facing many of the same hurdles presented by more common types of brownfield sites.

One of these, of course, is risk assessment and management.

In the case of Three Sisters, one of the main risk factors was whether houses, roads, golf courses, and other elements of the development were at risk from a catastrophic underground collapse or even a more gradual subsistence. This meant looking at both real and perceived risks from the future homeowners’ perspective.

Ways to deal with these risks include:

• Wherever possible, less-dense land uses were positioned over the highest-risk areas. One of the reasons that the golf courses were planned for the property’s south side was that these areas were among those most heavily undermined. Another was to provide a buffer for wildlife corridors, helping meet Three Sisters’ high standards regarding environmental sustainability.
  
• Good use of Geographic Information System (GIS) technology helped obtain the greatest possible benefit from the excellent records of the mine workings—some parts of the property were underlaid by as many as three different mines. GIS helped manage the vast amounts of data generated by the work, allowing engineers to provide lot-by-lot information on remediation work performed.
  
• Three Sisters obtained coverage under the Alberta New Home Warranty Program, which has proven a vitally important vehicle to not only promote marketplace acceptance and confidence, but also to advance the state of brownfield redevelopment awareness.

Three Sisters is gradually building units and bringing them to market. Since 2001, about 800 residential units have been developed and occupied, with another 400 units under construction or approved. A second 18-hole golf course, the first phase of a resort center core, is now under construction, and a shopping/mixed-use retail village is planned for the development’s entrance. Build-out to about 4,500 combined residential and accommodation units is planned for 2015.

For all parties involved, it was gratifying to gain external recognition for the project. Three Sisters Mountain Village Ltd. won a 2006 Canadian Brownfield Award (a “Brownie”) from the Canadian Urban Institute, for excellence in sustainable design and technological innovation in the development of the Canmore site. The Brownie awards program recognizes leadership, innovation and environmental sustainability in brownfield developments across Canada.

Chris Ollenberger is president of Three Sisters Mountain Village Ltd., in Canmore, Alberta. Jack Crooks is a principal of Golder Associates in the firm’s Calgary, Canada, office.