I posted an item recently about an accident that injured four people when they were struck by flying pieces of metal from a ride in Yarmouth, Nova Scotia. I thought, based on what I read in the paper, that it was an accident that might be adequately investigated on site by means of a simple visual assessment of exposed surfaces.
(See, “Flying objects, injured people, and forensic engineering investigation”, published August 3, 2012)
A visual assessment doesn’t sound very technical but it is always carried out at the start of even the most complex investigations, and sometimes it is all that is necessary for the simpler ones. It can be thought of as “calibrating” the forensic engineer to the site (Ref. Sowers). If little else is necessary by way of investigation then it can be quite a low cost investigation. If more is necessary then it helps ensure a thorough, reliable, cost effective forensic engineering investigation.
A visual assessment is an essential part of the main steps in the failure investigation process:
- Acquisition of data
- Analysis of data
- Formulation of opinion
A visual assessment or examination involves simply looking – by whatever means, at what you can see in the exposed surfaces at the scene of a failure or an accident. This as opposed to taking things apart and looking below the surface – an intrusive examination.
Forensic engineers sometimes go right up to an object and look at it with a hand held lens – I examined the fibres in a crack in concrete one time that told me when the crack was formed. Other times we simply walk the site and look at the failed structure or an accident scene from a few feet off. We take some measurements and a few photographs, and make sketches and notes.
We can also look at an object with binoculars (see below), and also with the telescopic lens on a camera – and study the exposed surfaces later as recorded by the camera.
A visual assessment, in a sense, can also mean examining the images of surfaces in photographs including photographs in documents provided by counsel (see below). But, you can’t beat being “at the scene” and getting a feel for the situation with a site visit and a good look, a good poke around.
In the earlier article, I explained why I thought the visual assessment approach was possible and valuable in Yarmouth and gave an example from my own pracitice. Of course, as soon as I published my article I thought of other good examples of visual assessments I’ve carried out. Some of these are described below:
1. Flying Metal- In Yarmouth I noted that the exposed surfaces of different units on the ride (chairs?) would lend themselves nicely to a visual assessment of what caused the metal to come loose. Having a number of chairs the same on the ride, presumably some with the metal still intact, would allow a valuable before-after assessment.
2. Falling Ice- I investigated the cause of ice falling and injuring a person solely on the basis of visual assessments.This involved an examination from the ground surface of the exposed exterior surfaces of the lower level of the structure and the upper levels from a distance using binoculars. I looked at some photographs that recorded the position of the ice on the ground after the accident. I was also able to carry out a before-after assessment in this case somewhat similar to what might be possible in Yarmouth. It helped to look at how ice formed on other structures during the winter. Application of some basic principles on how ice forms and melts coupled with observations from the visual assessment identified the cause.
3. Retaining Wall Failure- A quite high gabion retaining wall failed on the shore of Bedford Basin, Nova Scotia (a gabion is a wire basket filled with rock). A part of the wall just fell over onto the shore during the backfilling stage. Another part remained upright where it was connected to an anchor of sorts that prevented the wall falling over.
A site visit and visual assessment, comparison of the collapsed and upright sections of wall – the before-after comparison that forensic engineers like so much, and application of a well known rule-of-thumb in retaining wall design quickly identified the cause of the wall failure.
4. Tank Collapse - I was asked to establish why a fuel oil tank collapsed into a trench by examining photographs only and reading the documents – a visual assessment by remote. I was instructed not to visit the site not even to drive along the road adjacent the site.
I examined the site as portrayed in the photographs, studied sketches prepared by other professional engineers – during a visual assessment, checked rainfall records, applied some very basic scientific soil mechanics principles and established the cause.
Explaining the scientific principles and the mechanism of failure was a bit of a challenge but I was pleased with the result and the clients said they understood.
5. Soil-Steel Bridge Failure - I investigated the reason a large corrugated steel culvert suddenly collapsed injuring a car driver. The culvert carried a highway over a stream. A culvert more than 10 feet in diameter in north America – in this case spanning or bridging about 22 feet, is defined as a bridge.
I was retained two years after the accident and after the collapsed bridge had been removed and a new bridge constructed. Several different engineering investigations were carried out including detailed examination of photographs taken at the scene on the day of the failure – a visual assessment by remote.
The examination established the cause quite quickly but couldn’t completely discount a hypothesis of failure put forward by the defence. Until a detailed topograhic survey provided additional evidence that, coupled with data from the photographs, clearly showed why the hypothisis was not valid.
This was a costly forensic investigation because of the different modes of possible failure that had to be investigated and discounted and the fact the debris had been removed by the time I got there. At the end of the day it was the visual examination of the exposed surfaces in the photographs taken at the scene and the results of a simple topographic survey that established the cause.
6. Fatal Step Ladder Accident
A simple visual examination of the scene of a fatal step ladder accident established that a re-enactment of the accident using stunt men was the best way to determine if a flaw in the ladder was the cause of the accident. The first forensic method was inexpensive, the follow-up forensic method not so much by a long shot. Fortunately, the case was resolved without proceeding to the costly and risky re-enactment.
7. Flooding Problems - I see quite a few engineering failures involving poor drainage and the flooding of land and the basements of structures. Investigation relys on different methods, most notably establishing how the structure has been designed and constructed and the site topography – the lay of the land. All investigation starts with a visual examination of the scene.
One of the most valuable investigations is an examination of the structure and the land before, during and after heavy rain – a visual assessment of before and after runoff and flooding conditions at the site.
In one instance, an initial visual examination that occurred during heavy rain suggested that flooding was not in fact occurring as thought by the client, contrary to the findings of very expensive investigations by others. And in addition, another slightly different visual examination that could have been carried out years earlier would have established one way or the other if flooding was occurring – very unlikely, and saved the client many 1,000s of dollars.
Visual examinations don’t sound very technical but this simple method reduces forensic engineering investigative costs even when more complex investigations are seen to be necessary subsequently. And simple visual examinations of exposed surfaces sometimes demonstrate that the complex investigations aren’t necessary – and weren’t necessary in some cases.
- Sowers, George F., Introductory Soil Mechanics and Foundations: Geotechnical Engineering, 4th ed., MacMillan Publishing 1979