They seem mysterious and frightening – Sinkhole!, suddenly appearing in the ground and “swallowing” things at the surface. Like quicksand and all of a sudden there but, in another sense, not like quicksand which you maybe can see ahead of time and walk around. Swallowing things like entire cars as shown in the picture on page A4 of the Globe and Mail yesterday.
The picture was of the rear of a vehicle just showing above the paved surface of the Highway 174 off-ramp at Jeanne D’Arc Boulevard in Ottawa.
The hole was deep enough that almost the entire length of the vehicle was in it, wide enough to take the width of the vehicle, and the width of the lane long. A fair size hole to suddenly appear in the highway.
The traffic lane appears to be one of at least four lanes, two of which are at a lower level. The lower lanes appear in the upper, right corner of the picture. The lower lanes glisten with water.
We don’t expect this to happen in our highways, and particularly in the natural environment if the source of the problem lies there. Seemingly natural events like this destroy our fundamental assumptions about the safety of the world (Ref. Janoff-Bulman).
But events like this are not so mysterious and their occurrence can be fairly easily predicted, but, unfortunately, not so easily the timing and location of their occurrence.
A forensic engineer investigating the cause of this sinkhole might consider four elements to the problem even before leaving his office (I’m pretending I’m the engineer and just got the call and haven’t even seen the site, but know about structures in general):
- Conditions in the terrain where the highway off-ramp is located
- Foundation soil, bedrock and groundwater conditions in the highway subgrade,
- Roadbed design and construction, and,
- Road and infra structure maintenance
As soon as a professional engineer gets to the site and does a visual assessment (see posting, September 4, 2012) he might well refine this listing but I don’t think a lot. The visual assessment would draw the engineer’s attention to any unusual conditions in the terrain beyond the highway. Conditions like evidence of heavy rain and its effects, flooding, nearby construction works that have impacted the road, etc. The engineer is likely to have checked the recent weather reports for the area.
Seeing nothing unusual the engineer is certain to hyposthesize initially that the problem lies with the infra structure buried in the roadbed, service pipe work of some sort. Like storm drains and water supply distribution pipes.
He might form this hypothesis based on evidence such as the following:
- Finding no unusual conditions in the terrain where the highway is located.
- The reasonable assumption that the highway roadbed has been properly designed and constructed. We expect this of our highway departments.
- The rectangular shape of the hole in the road opening to the lower lanes on the right.
- The finite depth of the hole: 5 feet, 10 feet, 15 feet? Depths that approximate those at which service pipes are placed.
- What he sees in the hole on looking in.
The forensic engineer might reason that the location of the hole and its shape are typical of those that might form when a water main bursts. The escaping water seeks the easiest path down and out to the right eroding the roadbed as it flows.
Of course he would look in the hole once at the site. If he sees a burst pipe discharging water then he’s reasonable in assuming he’s solved the problem of what caused the hole to form.
If he doesn’t see a burst pipe then he will start to think of other possibilities while waiting for documents on the existence and location of buried infra structure in the roadbed. It’s possible there is a burst pipe but it’s buried deeper and not visible in the bottom of the hole.
One possibility is the types of natural soils and rocks forming the subgrade or foundation of the roadbed. If he has geotechnical engineering experience he would want to know if the area of the road is underlain by Karst terrain. This is a type of terrain formed on rock like limestone that dissolves in the presence of water forming different types of solution cavities – like sinkholes, for example. Or ‘roofed over’ sinkholes that are just about to break through like the banana holes in the Bahamas. There is Karst terrain in the Windsor area of Nova Scotia.
Karst terrain has a certain appearance at the surface and the forensic engineer would look for this while waiting for a copy of the geotechnical report for this section of the highway.
So, the forensic engieer would form different hypotheses during his investigation of the sinkhole – the hypotheses above and possibly others, investigate each in turn with site work and information from documents until he arrives at the cause of the sinkhole. The sinkhole wouldn’t be a mystery to him. It would be a problem with a solution that he would be confident he could find.
For certain, professional engineers with the Department of Highways in this area are working on the problem now. They would follow procedures like I’ve outlined above – consciously or unconsciously forming and revising hypotheses and checking them out.
The hole formed in the road and the vehicle drove into it on Tuesday. Based on my experience of these things, I’m certain they have already solved the problem. It can be that quick in forensic engineering.
- Janoff-Bulman, R., Shattered Assumptions: Towards a New Psychology of Trauma, The Free Press 1992
- Jorden, Eric E., “Technical” visual assessments: Valuable, low cost forensic method, posted September 4, 2012