A 2011 video inspection of a storm sewer under Highway 174 did not suggest it was in danger of imminent collapse, but raised enough concern that a visual and physical inspection was warranted, the city heard today.
B.M. Ross and Associates were tasked with providing a review of the Sept. 4 storm sewer pipe collapse that caused a large sinkhole on Highway 174 to form and swallow a car.
The engineering consultants said a CCTV video inspection of the section conducted a year before the pipe collapsed showed it was in need of replacement, but said the video did not imply there was imminent danger.
However, they said the video showed enough to warrant further assessment.
"In our opinion, the appropriate immediate response would have been a visual inspection with the goal of better understanding the condition of the pipe, followed by whatever response the visual inspection determined was warranted," they wrote in their report presented to the city Friday.
"The immediate need for a more robust assessment of the pipe was not understood, identified or communicated," the report said.
City's records incomplete, inaccurate
The engineers found the pipe had greater than normal risks because of its age, size, location and material.
Part of the problem was that city records on the storm drain were incomplete. While the city thought the entirety of the storm sewer pipe in question was galvanized steel. As it turned out, the section that failed was non-galvanized "tunnel liner plate" dating back to about 1975.
The report also shed more light on events that occurred on the day the sinkhole formed.
Contractor work likely influenced sinkhole timing
From Aug 31 to Sept. 4 a contractor was inside the pipes installing lights and removing debris and rocks using a mini-excavator and skid-steer loader. When rain began to fall on the day of Sept. 4, work stopped as the depth of the water flowing in the pipe began to rise.
Shortly after, the sinkhole formed and Juan Unger drove his car into the hole as he was travelling east on his commute home from work. Unger managed to climb out with the help of two witnesses.
The engineers said while it's possible the debris removal was a coincidence, the more likely scenario is that "vibrations from the excavator and loader, or even changes in the flow path of the water, as a result of debris removal, influenced the timing of the collapse."
The group made five recommendations:
- Expand the definition of a "high-risk storm sewer" to include consideration of the probability of failure as well as consequences of failure.
- Examine all high-risk pipes as soon as possible and have these examinations reviewed by qualified personnel.
- Assess the quality of the city's storm sewer inventory and improve data accordingly.
- Where feasible, physical inspections should be used to supplement CCTV inspections.
- Revise procedures for scoping capital projects to always include a discussion on the consequences of not proceeding quickly.
The city said it agrees with — and is following up on — all of the recommendations.BMR Final Report Sinkhole