Monday, December 10, 2012
There's a troubling contradiction in that B.M. Ross report on the Highway 174 sink hole. (See what happens when you release important documents late on a Friday? Journalists who were forced to rush their stories and limit their questions to "high level" have the whole weekend to go back for a second look at the details.)
Among the consultant's findings:
- Based on the August, 2011 CCTV video alone, "it would not be possible to determine when the pipe might collapse."
- "...the video image does not indicate well that almost two square metres of material had disappeared from the side of the pipe."
- The video showed the pipe was still "circular...considering the condition of the pipe an emergency was not warranted" despite the severe corrosion that was clearly visible on the video.
B.M Ross describes the pipe as a "soil metal structure," which is to say the soil and the pipe act as two, interdependent parts of the same machine. The forces created by the loads above the pipe (such as road surface and traffic) are transferred to the surrounding soil. The stability of the entire structure depends on something called "ring compression," or the way the circular pipe interacts with the soil around it. "Loss of soil results in loss of stability. When combined with the loss of pipe strength through corrosion, there is definite potential for failure."
Anyone who's seen the CCTV footage has also seen the large voids in the clay behind the holes in the pipe. In fact, B.M. Ross suggests by the time the collapse occurred one year after the video was taken, those voids were even larger: "Following the failure, one of the Contractor's personnel who was working in the structure commented that the eroded pockets behind the pipe appeared to be larger than indicated by the image in the 2011 CCTV inspection."
B.M Ross draws the following conclusions:
- "Consideration should have been given to ensuring that the soil erosion outside the pipe was not a concern and that further erosion did not occur,"
- "The significance of the deterioration in terms of the structural integrity of the pipe was not known. The potential for soil erosion to continue and further weaken the structure was apparently not understood.
- "Those who viewed the video did not appreciate the importance of protecting the native soil on the exterior of the pipe from erosion."
- "We believe that individuals with experience in design and/or inspection of larger steel structures would have reacted differently."
So here we have a video which was reviewed by staff in three different offices the very day it was recorded. They clearly recognized there was an issue, because the very next day the City changed the work order from a routine headwall replacement to a complete re-lining. (Why it took the contractor a little over a year to get started is another question that deserved to be asked -- and answered -- last week.) But did they recognize the right issue? It seems none of the City's professionals understood what I found out showing the video to the first engineering professor who would talk to me: Soil erosion leads to structural failure, sooner rather than later.
So while B.M. Ross notes on one hand that the timing of the collapse couldn't have been predicted by looking at those rusted gaps, he also concludes "although those who saw the CCTV video recognized the need for rehabilitation, they did not translate what they observed into potential instability that needed to be immediately investigated further and acted on." So it's not so much what that video didn't show; it's what was shown, but wasn't seen.