Coal was the primary resource that gave Nanaimo its start in the 1800s. It was mined in various locations around Nanaimo from 1852 until the 1950's.
Coal is found in sedimentary rock along underground seams varying in depth and thickness. The process of removing coal from these seams created open spaces (or mine tunnels) which miners temporarily reinforced using timbers or other means. Once a seam of coal was removed and mining activity ceased, the remaining open space would be abandoned and allowed to collapse through a process called subsidence.
Subsidence involves material dropping from the ceiling of the open space onto the floor and, over a period of time, causes the location of the open space to shift upwards towards the surface. Subsidence can occur shortly after a mine is abandoned or it can take many years. The deeper the mine is, the lower the impact of subsidence on the surface; the shallower the mine, the greater the impact of subsidence on the surface.
In Nanaimo there are a few known incidents of subsidence caused by shallow mines. The recent remediation work on Pine Street is an example of an open space that was detected before it reached the surface. The attention the Pine Street remediation work generated has highlighted the issue of old coal mines in Nanaimo.
Questions and Answers:
The following questions and answers have been prepared to help residents of Nanaimo better understand how they may be affected.
1) What is subsidence?
a. Subsidence occurs when the ground slowly or suddenly lowers in elevation. It can be localized or spread over a larger area. The possibility of subsidence is a concern for the City of Nanaimo, and potentially, some residents.
2) What is the maximum amount of subsidence that could occur?
a. The coal seams ranged in thickness from one to several metres; however, the subsidence can be more or less than this amount. The actual subsidence that can occur is a complex function of how the coal was extracted, the size and orientation of the mine and the nature and amount of rock/soil between the mine and the ground surface.
3) What is being done about the risk of Coal Mine related subsidence in Nanaimo?
a. The City and the Ministry of Energy and Mines are working together to better identify the areas of the City with the greatest potential for subsidence. The intent is to produce a map showing the magnitude of ultimate settlement over areas that were mined. What is unknown and will remain unknown when the study is complete is the current status of the open spaces. Some mines will have collapsed and caused surface subsidence long ago; others have not but may do so in the future. Once the study is complete, the next step is to assess the information and decide if action is warranted.
4) Why is the City undertaking a risk assessment for subsidence?
a. With the open space that was discovered at Pine Street and Victoria Road it was apparent that adjacent open spaces could eventually reach the surface. Although there were no voids found that would suggest an immediate concern for the road; if nothing is done it could become an issue in the future.
b. The cost of remediating mine voids can be considerably lower if dealt with proactively. The risk assessment will help the City determine which areas are a concern for City roads and utilities and will help guide future decisions about how subsidence is dealt with.
5) How do I find out if my property is over a coal mine?
a. Property owners can arrange to meet with a staff member at the City's Service and Resource Centre building at 411 Dunsmuir Street. Alternatively, local book stores and the Nanaimo museum www.nanaimomuseum.ca offer a range of information on the subject of coal mines.
6) I know my building is over a coal mine, what do I do?
a. Determining the risk for any individual building is a complex undertaking. If property owners wish to obtain information on the ground beneath their property, there are qualified local geotechnical engineers that will be able to provide assistance. It is not possible for City staff to tell property owners what the risks are.
7) I am interested in getting a building permit and my house is over a coal mine. What is required?
a. There may be a requirement for property owners to undertake a geotechnical investigation prior to building. It is best if property owners meet with City staff to discuss the requirements.
8) Who do I call if I notice something that concerns me?
a. If property owners see something that concerns them on a public road they can call the City of Nanaimo Public Works Emergencies 24hr number at 250-758-5222.
b. For mine related safety issues the Ministry of Energy and Mines can be contacted at 250-952-0732.
Coal Mine Presentation to Council