Sinkholes, like the one that undermined the house in Falmouth, Nova Scotia last week, are a serious issue for subdivision and lot developers. As reported in the newspapers, the risk of sinkholes forming in an area can be known from published maps. (Refs 1, 2) And the cavities in the ground associated with sinkholes can be found – before construction.
Sinkholes start their life as roofed-over cavities at some depth in the limestone, gypsum and salt deposits beneath an area. The cavities get bigger and the roof gets thinner as rock like this dissolves in the ground water. In a sense, the cavities “migrate” to the ground surface and eventually break through as sinkholes. The fairly precise size, location and depth of cavities can be determined beforehand.
The technology for locating cavities in rock is well developed and has been around a long time. It’s called ground penetrating radar, GPR for short, and it’s well known to experienced geotechnical engineers. I used GPR years ago to locate cavities beneath an airport runway on South Andros Island in the Bahamas. The same technique is used for locating unmarked graves.
It sounds technical but all it involves is sending radio waves into the ground and analysing what is reflected back. There’s nothing too exciting in the data that comes from uniform soil or intact rock. But lots of excitement in the data from a cavity, a well-recognized anomaly to the GPR operator – and a potential sinkhole beneath a building. The remote sensing, non-intrusive technique is not unlike CAT scans and MRIs in medicine.
The type of potential sinkhole I was looking for at the airport is called a banana hole. So named because banana plants grow in the sinkholes once the cavities break through the surface. A banana hole/sinkhole can be seen in the runway by an inbound pilot but not the roofed-over cavity just before it breaks through the surface.
Geo engineers investigate the adequacy of the foundation soil and rock conditions beneath a site where someone wants to build something – like a house, for example, or a bridge, dam, or airport runway. The conditions would not be adequate if there was a cavity beneath the building site.
GPR would not normally be used on a building site in the Atlantic provinces, nor in Canada for that matter. But it certainly should if deposits of limestone, gypsum and salt are noted when the engineer checks the published geology of the site – an important and standard task in a geo investigation. These types of rock with their inherent risk are not so common but they do exist as noted in the news report on the Falmouth sinkhole.
If there’s a risk and the technology is available to quantify it, and it’s not used, then it seems to me it’s a potentially litigious matter.
References
- Falmouth: Sinkhole wrecks family’s house, pg. 1, The Chronicle Herald, Halifax, Tuesday, September 5, 2017
- Geological conditions: Sinkholes not unusual in N.S.: Scientists, pg. 1, The Chronicle Herald, Halifax, Wednesday, September 6, 2017