Many undermining mitigation works do not require ongoing maintenance. This is because some mining works are backfilled with materials using paste techniques that prevent future impacts. In other cases, the buildings overtop of such workings have structural elements within them like reinforced foundations, structural beams or columns and connection detailing that all form part of the structure of a building, and don’t require maintenance or repair beyond what you would undertake for any reasonably maintained structure.

Public infrastructure initially installed by a builder or developer (e.g., waterlines, sewer lines, roadways, sidewalks) will continue to require maintenance or repair by their private owners like in the case of condominiums or governments if the utilities are public at some point. Essentially, they are like any other piece of public infrastructure in that repair or maintenance that maintains the original design doesn’t present any additional risks beyond infrastructure installed elsewhere.

Under Alberta Regulation 34/2020, developers are required to hire a qualified professional engineer to undertake the detailed engineering work to prepare the recommendations for building on a site with undermined land. In addition, an independent, third-party qualified professional engineer must review the recommendations prepared by the first engineer.

In Alberta, the Engineering and Geoscience Professions Act outlines professional engineers’ responsibility to the public for professional and skilled conduct. This applies to everything from the design of safe bridges to drinking water systems, dams, aircraft, vehicles, buildings and hundreds of other systems or structures the public relies on and trusts daily. All professional engineering work must be conducted to accepted professional standards. In fact, undermining mitigations are one of very few areas in Alberta that are required by legislation to have external third-party review as a part of the process. If the responsible professionals have been found not to have followed reasonable professional engineering standards, they can be held liable. Gross negligence related errors are rare, and such an error would be much more difficult to have occur given the extraordinary measure of the external, independent third-party review.

Initially, one regulation addressed the Town of Canmore’s ability to review development applications within the designated area of Three Sisters Mountain Village. This regulation (AB 113/97) was only applicable to the Town of Canmore. Another regulation (Alberta Regulation 114/1997) outlined the process and requirements of developing on undermined land from 1997 to 2020.

In 2017, TSMV identified the need to update the regulations and associated undermining guidelines used for design and engineering purposes as a part of the Province’s review of the Municipal Government Act. We did this because the guidelines and regulations no longer represented the advanced state of engineering practice that has benefited from more than 20 years of experience and learning related to undermined land. The Town of Canmore supported this review, and subsequently, TSMV, the Town of Canmore, and the Province worked diligently to clarify and update conditions and processes found in Alberta Regulation 114/1997, including the timing, nature and scope of the required studies. The undermining guidelines were completely re-written to reflect current state of the engineering practice and to align with the scope and timing of the studies identified in the 2020 regulation. This new regulation was approved and is called Alberta Regulation 34/2020.

Undermining is well documented in Canmore. The Province of Alberta records receipt of an undermining technical report within the designated area of Three Sisters after it is reviewed by a third-party professional engineer. Once the Province records receipt of the report, the developer or builder is required under law to have the undermining report registered on the title of each building, unit or apartment. This way, each future owner is made aware of the existence of undermining and the report is a standard part of disclosure in every real estate transaction as properties change ownerships over time.

Just like a lawyer or real estate agent, every buyer is responsible to review the title of the property being purchased for easements, rights-of-way, judgements, caveats, architectural controls, condo bylaws or other legal documents recorded on title. The undermining report is placed on title for the public and any purchaser to see. A copy of the report is also provided to the Town of Canmore for reference and convenience.

Registration of the undermining report on title doesn’t occur until the report is acknowledged by the province. When construction occurs prior to registration, the undermining report is also used to document the work undertaken onsite related to undermining.

Undermining exists in several areas in Canmore and is well documented. Under the AB113/97 undermining regulation, the Town of Canmore is excluded from considering undermining in any planning, land use, ASP, development permit, subdivision or building permit process. However, the new 2020 regulation clarifies that the Town can ask for a copy of the undermining report for various development applications to determine that the undermining work has been completed. If the undermining work has not been undertaken, the Town can refuse the development application on the basis it is an incomplete application. The Town of Canmore can only refuse an application if the process for developing on undermining lands described in the regulation, such as third-party engineering approval, is ignored.

When the Resort Centre golf course was first approved, avoidance of some undermined land was the preferred mitigation due to cost considerations, not technical ones. The unfinished golf course was mainly placed in its location due to now outdated and ineffective thoughts about layering uses near wildlife corridors. Additionally, the golf course was also initially proposed as a buffer to address human-wildlife conflict concerns. Science has shown that human use (especially off-leash dogs) is the main and ongoing deterrent to wildlife use of the protected corridors. Fencing to control access into the corridors is now the primary recommendation, instead of a buffer.

For more than 20 years, engineering best practices and technology innovation have led to an immense amount of data and technical proficiency in evaluating and mitigating undermined areas within the designated lands of Three Sisters. Extensive field research has confirmed the high level of accuracy of historical mine maps. The advancement of data analysis tools we have today—including 3D models, simulations and evaluations — bring significant confidence to addressing undermining, which is then evaluated by qualified professional engineers.

No. Canmore Town Council voted against the Resort Centre Area Structure Plan (ASP) in 2017, because they wanted to see applications for the Resort Centre (Three Sisters Village) and Smith Creek presented in a way that the concepts for the entire property can be considered more holistically. TSMV is currently working on a new ASP that makes the concept more clear for Three Sisters’ Village and Smith Creek lands.

Undermining in this area is well understood and documented. Over 50 per cent of the Resort Centre (now known as Three Sisters Village) is undermined. This is comparable to the proportion of land undermined in Stewart Creek and Three Sisters Ridge areas. Other areas have less (Smith Creek ASP area) and some have more (Cairns on the Bow). Three Sisters Village has some vertical mines on the south edge of the property; however, this type of mining represents less than five per cent of the land. While it is possible to develop, these areas will be avoided because the costs to mitigate the undermining in those areas for development are currently too high, however the lands have been addressed for public safety through a variety of methods, including fencing and backfilling.

The vast majority of the developable Smith Creek ASP has no undermining. Most of the mining that occurred in this area was well to the south of TSMV property. Investigation of the property within the Smith Creek ASP will still occur prior to development to confirm these records.

The depths of the mines vary. There are coal seams that come all the way to the surface and were worked as pits dug down from the surface (for example, the very popular and heavily used Quarry Lake is a reclaimed surface mining feature, and the park is undermined in many areas). There are also mines that are over 100 to 200 metres below the surface.

Undermining engineers undertake drilling programs to verify historical plans and understand the current conditions of the mines. They do enough drilling to confirm that the mine plan is a reliable representation of what exists. This work has found that the previous Canmore miners kept excellent records of their work, a tribute to their due diligence, and the historical maps to date have had a very high level of reliability as a result.

At all times, public safety is the primary concern. If there is uncertainty, appropriate work is done, including setbacks from building development where warranted, so that in the event there is a future surface depression formed, the hazard is very small. The undermining work is undertaken to bring the level of risk to the public presented by the potential hazards to the same levels as driving over a bridge, taking an elevator or developing near steep creeks in the Bow Valley.

There are tried and tested engineering practices and construction solutions for undermined lands around the globe. It is important to note that many undermined areas require little mitigation at all, because the mines are so far in the ground that they have stabilized or collapsed and so either there will be no effects seen at the surface or only minimal effects possible.

If an assessment of an undermined area determines a future underground tunnel collapse and subsequent ground settlement could still occur, an appropriate mitigation technique is chosen. Two things are taken into consideration 1) the unique undermined conditions of the site and 2) the type and location of development planned.

In development areas where roads and homes are involved, a technique called paste backfilling can be employed depending on the depth of the mines and what development is proposed above. This technique stabilizes the surface area by adding a bulk material that includes cementitious elements into the void of the former mine. This limits the potential for future collapse by essentially filling the void or tunnel with stable, solid materials which can permit construction and human use to occur safely on the surface.

Another approach is structural design with the buildings themselves. Sometimes strong, stiff concrete foundations can serve as part of the building design, or the structures are designed to be able to safely accommodate the stresses that could be created, just as buildings or structures can be designed to safely address earthquake movements of the ground. Another approach is avoidance. Avoidance is not selected because these areas cannot be safely developed. Avoidance occurs when the mitigation would not be cost effective for development or desirable or needed to develop in a location.

In the case of paste techniques used in appropriate locations, the mitigation is considered essentially permanent, as such techniques will far exceed the design lifespan of the structures or infrastructure constructed above. New or replacement buildings in future would require assessment under the regulations and so even redevelopment can occur in future with proper undermining assessment for the new structures or infrastructure.

Structural mitigations within a building or structure or infrastructure elements are designed to last the life of the structure or building or infrastructure itself, assuming like all manmade structures, reasonable maintenance as needed is undertaken.

Mine entrances are typically not targeted for development. Instead, they are made permanently inaccessible by closing them off, commonly with a thick concrete cap which is sometimes combined with burial. The surface is then covered up with soil so that it can be replanted. If the development plans call for a trail, or natural area, undermined lands nearby can be planted with dense vegetation to dissuade public access. These concrete caps have a life expectancy of over 100 years. Some portals (former mine entrances) can still be seen at the surface in place in Three Sisters, however, they are also sealed against intrusion with heavy metal doors, walls, and locks.

There are numerous artifacts scattered around the Town of Canmore, including rail cars and engines. Many of the mine entrances on TSMV, though sealed, remain in their original locations, but are generally recreated with actual access to the mines sealed off. There are some small artifacts found throughout the site (e.g., cables, wooden water pipes, small weirs, small machinery parts, timbers), but generally not historically significant objects. Many elements are removed during engineering investigations for public safety if they are deemed hazardous or relocated for display.

Like every engineering analysis for every building built in North America under modern building codes, no one can be 100 per cent confident in every single variable considered for design. Any analysis must estimate the impacts of natural variability (e.g., earthquake magnitude potential, soil settlements, materials used, snow loads, live loads, etc.). Undermining engineers apply conservatism or safety factors to the analysis based on professional assessment (i.e. buffers around potential hazards are larger than estimated impact area).

These localized historical failures (one undertaken prior to implementation of the 1997 regulations), while unfortunate, have helped provide engineers with data they can use to increase knowledge, test predictions and ultimately increase confidence. Undermining engineering has been a continual learning and ongoing improvement process. While no construction can be deemed zero risk, undermining engineering has advanced to a state where the possibility of an occurrence is not different than any other structural engineering process, like driving under a bridge or taking an elevator.

The Dyrgas Gate sink hole occurred in May of 2000. It is not a failure of mitigation or investigation. Previous design engineers and the third-party engineers knew there was an air shaft (known as B14) in a very small area, and several boreholes were drilled to find its exact location. The shaft had been backfilled to a significant depth by miners, and so the exact location could not be determined, but its approximate location was known. Knowing the shaft had been there, but unable to determine its precise location, engineers replaced the soils, and laid a geogrid mesh over a much larger large area, well beyond the mapped location of the shaft to prevent future subsidence from presenting a risk to public use. In addition, building development was separated from the potential air shaft on all sides, so there was no risk to structures or residents within the buildings. The buildings are structurally sound and conform to building codes. All buildings in this area have the undermining report registered against their title, and the airshaft and its mitigation were fully disclosed to the public through the registration on title.

The intention of this approach was that if something did happen, the undermining engineers would know exactly where the shaft was, no-one would get hurt as they could not fall into the shaft, and no structures would be damaged. At that point, long term mitigation could be implemented. The investigation, engineering, and mitigation worked exactly as intended, and the resultant repair work was limited to a very small area containing a local pathway. The ground was replaced, and further mitigation was undertaken as the shaft location was now known, and the pathway and park were repaired.

There has been extensive methane gas testing conducted. Results have found that these mines are not releasing the concentrations of methane gas that would be considered a hazard at surface. Many of the mines have also been flooded with groundwater, which impedes ignition of the coal seams below the water table. While there are examples of coal mines catching fire around the world, generally that occurs in mines located in thermal coal seams, and not the higher grade of coal found in Canmore.

Ground water flows have been heavily influenced by the mines. Due to the lateral extent of the mines and the interconnections between them, there is very little opportunity for water pressure to build up. The water can flow between and along mines easily, and the surface deposits are largely gravel, which is very well drained. Generally, water flows south to north toward the river, but exactly what happens underground is quite difficult to map. However, the detailed mapping of groundwater flows is not considered to be an unusual factor for development of Three Sisters given the surficial gravels that provide a high level of drainage, as most of the lands beneath Three Sisters are quite permeable.

In that location under the Three Sisters Parkway, there are two known and mapped shafts that were mitigated by placing large concrete caps over the shafts after they had been backfilled by the undermining engineer at the time. A significant watermain nearby failed for reasons unrelated to undermining, leading to a large amount of pressurized water causing soil erosion beneath the asphalt of the Parkway. The asphalt of the Parkway and shallow soils were still frozen from the winter, hiding the localized lack of support caused by the soil erosion beneath the asphalt. The frozen surface and the severely leaking watermain combined to create a void under the shoulder of the Parkway, however this was not an undermining sinkhole. The road was shut down while the area was investigated by engineers to find the watermain leak and determine the extent of the damage. It was determined that the erosion caused by the significant water leak impacted some of the soils supporting the cap over the north shaft and it appeared the concrete cap itself had been damaged in the past, potentially impairing its ability to continue capping the north shaft effectively. The decision was made by TSMV to remove much of the old cap, remove the soils weakened by the watermain leak and construct a new, larger cap over the north shaft to prevent a re-occurrence in future. Concrete was also injected into areas of the two shafts to fill in gaps where soil erosion had likely occurred from the water leak. After the reinforced concrete cap was constructed, the water main and the Parkway were repaired. There have been no further issues since 2004, and the undermining mitigation measures have continued to perform as designed.

The Town of Canmore placed snow fencing around a reported subsidence in the disc golf course as a precaution. During this time, a photo was posted on Facebook was shared claiming there was a sink hole on the Three Sisters Disc Golf Course. Subsequent review of undermining reports for the area suggests that it was not undermining related. The subsidence is in an area where storm pipes converge as part of the areas stormwater management system. There was evidence that the storm pipe was damaged and contributed to settlement of the surrounding soils. There was no risk to the public. Further investigation and repair was undertaken by the Town of Canmore.