By Sue Longo

Paste, a combination of ground-up rock (tailings or overburden or other waste materials), water and chemical additives, is being mixed in Canmore, Alberta, to help support tunnels in a worked-out coal mine, to allow for safe construction on the surface.

Many coal producers are looking for a “win” in the struggle for public support. One way to do that is by building positive relations with communities affected by current or previous coal mining. This includes managing the effects of subsidence, which can cause depressions and cracks at the surface. This not only damages structures, roads, pipelines, railroads, etc., it can affect streams and rivers, triggering serious environmental consequences such as fines and other sanctions for the companies responsible.

Coal operators that develop effective ways to prevent the collapse of closed mines, or worked-out parts of operating mines, can realize several benefits, including:

Positive relations with stakeholders — With community consultation, such a large part of the environmental permitting process, having that “social license to operate,” is increasingly important in business success.

A good track record with environmental regulators — Managing the environmental effects of current and closed operations means a better chance for trouble-free regulatory approval.

Sound management of potential legal liabilities — Less risk of damage claims from owners of buildings as well as linear property such as roads, railroads and pipelines help to manage legal risk as well as possibly improving access to capital.

Public health and safety — Helping to alleviate these long-term physical liabilities will manage the risk to the public and local wildlife and potentially “give back” the land for recreational activities.

One possible solution to coal mine subsidence comes from the world of hard rock mining: “paste” technology. Paste usually involves tailings — the sand-like reject produced by the mine’s mill after the target mineral has been extracted from the ore. To make paste, the tailings are mixed with water and other additives. These elements are engineered into a “recipe” suitable for the application, and the result is a product that resembles toothpaste in consistency, and which can be pumped through a pipeline.

In an increasing number of mines, paste is pumped underground to backfill worked-out stopes to prevent their collapse. After the paste cures into a hardened mass, the ore body next to it can then be safely mined.

Paste is a mature technology, and its growth has brought greater efficiencies in operation, a wide range of equipment able to produce paste, and growing ranks of mining professionals with experience in designing, maintaining and operating paste plants.

One example of a former coal mine property turned housing development dates to the early 2000s, and the intervening years have demonstrated that the paste solution utilized has stood the test of time.

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
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.

Canmore, Alberta, began life in the 1800s as a coal mining town. Since the mine closed in 1979, the underground workings posed a risk of collapse, and the town’s legacy of coal mining was impeding residential growth east of the main community.

Remediating this site involved using on-site piles of o