Routine engineering investigation – the fix – ensures sinkholes don’t undermine our buildings – the problem.
***
That was quite a picture in the newspaper a few days ago of a sinkhole that appeared last August in Oxford, Nova Scotia. (Ref. 1) And the pictures of the sinkholes in British Columbia a few weeks ago (Ref. 2) and before that – back to Nova Scotia – some time ago in Falmouth..
Striking pictures but sad too when homes are lost, and scary at the thought of potential injuries and death.
But it’s time to stop. There’s no need for the formation of sinkholes to surprise anyone nor pose a risk.. Nor be a “…a money pit”. (Ref. 1) There would be no news and no pictures if experienced engineers were involved before these areas were developed. And every chance some of these areas would have been undeveloped. Engineers would carry out a standard geotechnical investigation of the ground at a site proposed for construction. Simple as that.
The news story mentioned a geophysical survey. That’s an engineering technique that’s been around a long time. The fact that this is sinkhole country – Karst terrain – has been known a long time too.
Karst is an irregular limestone area with sinkholes, underground streams and caverns. (Ref. 3)
Remote sensing geophysical surveys – sort of like MRIs in medicine – detect features in the ground of interest to land owners. Features like voids or conditions conducive to the formation of voids. Sinkholes start as voids in the ground.
Limestone bedrock, gypsum and salt dissolve in the water in the ground to form voids, caverns and underground streams. The voids get larger with time and eventually the top, or roof of the void, appears at the ground surface – a sinkhole. The voids are said to migrate to the ground surface.
The most recent picture shows red soil around the perimeter of the sinkhole. This is glacial till – soil deposited by glaciers 1,000s of years ago. The soil is heavy and would cause the top of a void to break and appear at the ground surface sooner rather than later.
Investigating undeveloped Karst country for sinkholes
What would an engineer do if asked to investigate the foundation conditions at a proposed construction site? For example, the site of a building, or any of the structures in an urban area, or a residential subdivision or a strip mall. This would be an engineer experienced in geotechnical or geological work.
They’re not always asked – and it appears not always in Karst country – but what would he do if he were?
He would first check.the published geology maps and aerial photographs of the area available to all of us. It’s called terrain analysis in engineering. He would see in the maps that a large area of Nova Scotia – many square kilometres – is underlain by Karst terrain. He would also see in the photographs evidence of large sinkholes like the one in the picture. He would tell you that the area is susceptible to the formation of sinkholes. He would also tell you that he can’t predict where the sinkholes will appear in a large area.
But tell him approximately where you want to construct a subdivision or strip mall and he’ll give you a pretty good idea of the risk of sinkhole development in a small area like that. Not the location of all future sinkholes but the location of some, and the risk of others. He would do this after carrying out a geophysical survey. The survey could be expensive for an area proposed for a subdivision or strip mall.
I did a geophysical survey of an airport runway on South Andros Island in the Bahamas one time. I was looking for voids that might form sinkholes. A runway is not unlike a residential street or a strip mall. You can be sure I ran a lot of closely spaced survey lines down the runway looking for voids.
Tell the engineer precisely where you want to build a house, a multistory building, a bridge, a road, etc., a tiny area, and he will tell you if and where sinkholes will form and undermine your structure. He would do a geophysical survey. This wouldn’t be that expensive for a single structure and a good investment considering construction costs. You might consider relocating the structure after the survey..
If the risk of sinkholes forming is low and you still want to build there then he would drill boreholes at the location of your structure and any features of interest found during the geophysical survey. Boreholes retrieve samples of bedrock like limestone or indicate when the borehole passed through a void. It’s called ground proofing in engineering work that relies on non invasive, non destructive geophysical surveying and terrain analysis.
Summarizing, this is what the experienced engineer might do depending on what you need:
- Terrain analysis of a large area of Karst country – square kilometres in size – using published maps and photographs
- Geophysical survey of a small area possibly with a few boreholes
- Geophysical survey of a tiny area – a proposed construction site – plus some boreholes .
It wouldn’t take a “money pit” of money to investigate for sinkhole-forming voids if you know the precise location of your proposed structure.
Investigating developed Karst country for sinkholes
If you are concerned about the stability of a developed area, particularly foundation stability, then, depending on the size of the area and the preciseness of the information wanted, the engineer would go through the above steps. The emphasis would be on Step #2 if you are concerned about a small area. Or Step #3 if you are concerned about a single structure and wanted precise information.
***
The approach for developed or undeveloped land would be much the same – that of a fairly routine engineering investigation by an experienced person. It wouldn’t be so expensive for a single structure, a tiny area – a few 1,000s of dollars rather than many 10,000s. But more, of course, for a larger area, particularly if a lot of precise information was wanted.
It wouldn’t be so newsworthy either particularly if undeveloped land was being investigated. The reporter’s eyes would glaze over at the thought of covering an engineering story like that.
That’s where we need to get to in Karst country – investigate before at reasonable cost not after when newsworthy sinkholes and problems develop.
References
- The Chronicle Herald, Thursday, March 14, 2019 page A3
- What’s wrong with this (sinkhole) picture near Vancouver. Posted February 20, 2019
- Merriam-Webster dictionary, March, 2019