I was surprised at the news last week about the evacuation of 14 homes in Sechelt, on the Sunshine Coast near Vancouver because of sink holes and unstable ground.
What’s wrong with the picture is why such unstable ground was developed and built on in the first place if the risk was known. Sink holes as large as an estimated 15 feet across and 3 to 4 feet deep in one news picture if typical are an obvious sign of unstable ground.
A geotechnical investigation – a well developed applied science – had been carried out and the risk identified according to news reports.
Sink holes not unusual and easy to investigate
Sink-hole-prone ground is not unusual in nature. We got unstable ground like this in the Atlantic provinces. There’s lots of sinkholes in the Bahamas where they’re called banana holes because banana plants grow in them.
A neighbourhood of 14 homes is about the size of my neighbourhood, and compact and the ground easy to investigation.
Geotechnical investigation identifies ground that is susceptible to sink holes like this. The ground can be natural or due to the works of man. It also identifies the different layers of soil and rock beneath the ground – the stratigraphy in geology – and the areal extent of the different layers. It determines the physical properties of the materials forming the layers and their susceptibility to conditions of interest – sink hole development in this case. Finally, it checks out the depth to the ground water and where it’s flowing.
Was the risk of building a house high or low?
Did the geotechnical work really conclude that the risk was 10% – possibly a low number to some? Or did the work miss something which seems likely as evident by the evacuation? How was the 10% calculated? A probability analysis was not mentioned in the news.
It occurs to me that I wouldn’t build a house in an area where there was a 1 in 10 chance of my house being undermined by a sink hole.
Easy to investigate and improve the ground
Ground terrain like this can be improved. It’s called ground improvement in engineering and is a well developed technique. But it can be expensive involving lots of geotechnical work and construction work.
These kinds of investigative and improvement techniques are so well developed and understood in engineering that it’s motherhood. The ground may be complex but finding this out and doing something about it is fairly straightforward. Not building on the ground is one solution.
Simple, preliminary investigation
There’s a list of geotechnical techniques in the Appendix. They are roughly in the order they might be carried out. You can repeat some depending on what you’re finding.
One of the least expensive at the beginning of an investigation is a good walk-over survey of the ground. This would be accompanied by a study – before, during or after the walk-over, or all three – of published topographic and geologic maps of the area and published aerial photographs. New and old sink holes like those reported would be seen in a good walk-over. An engineer experienced in terrain analysis could also pick out big sink holes on aerial photographs even those taken from 1,000s of feet high.
Such a walk-over and study are standard procedures in geotechnical investigation. They’re cast in stone.
Video and stills taken from drones 10s and 100s of feet high have been available for a few years now. They have been invaluable in my work. Sink holes and unusual features on the ground show up well in aerial video.
Before drone photography I hired a small plane and had the pilot fly low over a site as I took photographs of the ground.
These preliminary techniques would be standard at the beginning of a geotechnical investigation of a site like the one evacuated near Vancouver. The results would indicate if more expensive investigative work was justified like that mentioned above and listed in the Appendix and how to plan and do it.
Investigation and/or use of findings questioned
Of course, good and thorough geotechnical investigation must be followed up with good use and implementation of the findings. The evacuation would seem to call into question the investigation and/or the use of the findings. What’s wrong here is that something went wrong and shouldn’t have.
There are many techniques that could be employed during a geotechnical investigation. I’ve used all of them in my consulting engineering practice at different times over the years:
- Terrain analysis using published aerial photographs from high flying aircraft – 1,000s of feet high
- Walk-over surveys and examination of the terrain on foot
- Studying published topographic, superficial (soil) geology and bedrock geology maps of the area
- Terrain analysis using published Lidar mapping of the area
- Terrain analysis using video and stills taken for the purpose from low flying drones 10s and 100s of feet high
- Studying contour and topographic maps prepared for the area
- Carrying out and studying a geophysical survey of the area
- Carrying out and studying ground penetrating radar (GPR) surveys of the area
- Drilling boreholes, measuring the thickness of the different layers of soil and rock, testing the physical properties of the soil in-situ and sampling the soil and rock for laboratory testing
- Testing the physical properties of the soil and rock in a laboratory
- Analyzing the stability of the ground using the data on the different layers of soil and rock obtained during the geotechnical investigation