(The following is one in a series of cases I have investigated that illustrate the different forensic engineering methods I use to investigate the cause of failures and accidents that result in civil litigation. The methods are described in some detail)
The investigation of the wall collapse is reported under the following main headings with several sub-headings:
- The case (a description of the collapsed gabion wall, the legal/technical issues, and my client)
- Forensic engineering investigation of the failure and the methods used
- Cause (of the collapse)
- Post mortem (an engineering “rule of thumb” might have prevented the collapse)
Description of collapse
The gabion wall was on the shore of a harbour in eastern Canada. The wall was 10 feet high and more than 100 feet long. There were short wing walls to the main wall aligned shoreward. A “gabion” is a wire basket about 3 feet by 3 feet in section and 10 feet long filled with course stone several inches in size.
The wall was being constructed to reclaim land on the seaward side of a quite large townhouse property. The wall fell over just before construction was complete. It was rebuilt before I was retained.
At issue was the cause of the wall’s failure. This was in connection with a claim of damages against the designer and his insurance company.
I was asked by the plaintiff, a property manager who was acting on behalf of a contractor, to determine the cause of the collapse.
Forensic engineering investigation
My forensic engineering investigation relied on the following methods. The methods are described in more detail below:
- Examining the site of the rebuilt wall
- Studying photographs taken of the collapsed wall
- Studying a design sketch of the wall
- Interviewing two workers who were on the wall at the time it failed, including one who slid down with the wall on a piece of construction equipment as it fell over
- Interviewing the design engineer
- Reviewing design principles for coastal and marine structures
- Reviewing weather and sea conditions at the time of the failure
Description of methods of forensic engineering investigation
1. Examining the site of the rebuilt wall
This initial site visit and visual assessment is standard in an engineering investigation and an important initial task by a forensic engineer (Ref. 1). Drawings and photographs are fine but picking up a concrete impression is important. It’s well recognized that, “A picture is worth a 1,000 words”. However, a visual assessment is invaluable. This is so even if the collapsed structure has been rebuilt as was the case with the gabion wall.
I was able to see how the toe of the gabion wall was constructed where it was exposed to the scour and erosive forces of wave action in the harbour.
I also saw the location of the townhouse with respect to the wall. The contractor had expressed concern that construction of the wall as designed would undermine the townhouse. A simple rule of thumb ruled this out.
2. Studying photographs taken of the collapsed wall
Photographs are important, and sometimes all we have. They are particularly important when detailed photographs are taken during construction. They are also important when taken of the failed structure that is subsequently removed before the forensic engineer gets there. The latter was the case in this instance.
The photographs showed the actual wall construction and that it failed in a quite classic manner – it just tipped, tilted, leaned over along most of its length. The exception was where the wall was tied in and anchored to the wing wall at one end. It remained upright there.
3. Studying a design sketch of the wall
It goes without saying that a professional engineer investigating a failure would want to know how the failed structure was designed and intended to be built. This is a standard task in a forensic investigaion.
The sketch showed how the design engineer originally wanted the base of the wall constructed and the toe of the wall protected against scour and erosion. Simple rules of thumb suggested the base design was adequate. The toe protection was less so.
4. Interviewing workers
Interviewing workers is a standard task in a forensic investigation. The interviews sometimes provide quite valuable information on conditions at the moment of failure.
I interviewed two workers who were on the wall at the time it failed, including one, an equipment operator, who slid down with the wall on a piece of construction equipment as the wall collapsed.
In engineering analysis we speak at times about a “trigger” in a failure. All conditions are present – or nearly so, for a structure, a wall, an earth slope, etc., to collapse. The trigger pushes the structure over the edge in a sense. Sometimes there is heavy rain – the trigger, just before a landslide.
The construction equipment just back of the gabion wall at the time was the trigger in this case, an extra surcharge/weight on the wall.
5. Interviewing the design engineer
We always want to talk with the design engineer when investigating a failure but often don’t have the opportunity during the investigative stage. This lack of opportunity is particularly the case when the design engineer is the defendant in a civil action.
In this case, however, the design engineer was quite professional in agreeing to talk with me. His design was okay in the short term. It turned out that a change he approved during construction caused the problem.
The change involved reducing the width of the base from about six feet – 2/3 the height of the wall, to three feet – 1/3 the height of the wall. The change was made because the contractor said he couldn’t build a six foot base. He also expressed concern that the townhouse would be undermined. Consideration of a simple rule of thumb would have raised an alarm that the wall would not be stable with a three foot base. Another rule would have demonstrated that the townhouse was not endangered.
6. Reviewing design principles for coastal and marine structures
Reviewing the design prinicples applicable to a situation is standard fare in a forensic engineeing investigaion and I did this. I was particularly interested in the requirements for protecting the toe of the wall against scour and erosion due to sea conditions.
7. Reviewing weather and sea conditions at the time of the failure
This is also standard fare during a forensic investigation and in this case it tied in with reviewing the design principles mentioned above. Sea and weather conditions were calm at the time of the wall collapse.
I concluded, based on the evidence, that the wall failed because of a change in the design of the wall during construction. The principle defect was that the base of the 10 foot wall was not wide enough at three feet. I also found that the toe of the wall was not well protected against wave action in the harbour.
There is a rule of thumb in the design of conventional gabion retaining walls that the width of the base of the wall must be about 2/3 the height of the wall – about 6.5 feet in this case, not 3.0 feet as agreed during construction. A design engineer starts off with this conventional wall geometry and then checks that the rule of thumb holds in the particular case.
There are lots of rules of thumbs in engineering, They expedite matters but must always be checked. And they should always be referenced when the pressure is on to change things during construction.
The matter was settled out of court.
1. “Technical” visual site assessments: Valuable, low cost, forensic engineering method. My blog posted on this site, September 4, 2012