Volume 12 Week 5

Wednesday, Oct. 25


 

Posted Oct. 25

Posted Sept. 21

Posted July 20

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Orléans Ward
Bob Monette

Beacon Hill,
Cyrville Ward
Tim Tierney



 

 

 


(Posted 5:30 p.m., Dec. 7)
Series of failures led to Hwy. 174 sinkhole, report confirms
By Fred Sherwin
Orléans Online

A massive sink hole that closed the eastbound lanes of Hwy. 174 for two weeks was caused by a series of failures dating back to when the stormwater collector pipe was first installed under the highway in the late '70s.

According to a report prepared by the engineering firm B.M. Ross & Associates for city council, the root cause of the sinkhole was a 48-metre section of the pipe that was made out of non-glavanized liner plate.

What is not clear is why the section of pipe wasn't galvanized before it was installed. According to the City of Ottawa's infrastructure asset database, the stormwater sewer pipe is listed as a Corrugated Steel Pipe (CSP). No mention is made of the 48-metre section of non-galvanized liner plate.

The original drawing of the pipe as proposed in 1972, indicated that approximately 55 metres of the pipe was not CSP, but rather "10 gauge, hot-dipped galvanized and asphalt coated liner plate".

An as-built drawing submitted to the former City of Gloucester after the pipe was installed only identified the addition of 15.2 metres of CSP on the south end of the pipe. There was nothing on the drawing to indicate that a liner plate had been installed as part of the project.

The liner plate wasn't discovered until a closed circuit television inspection was done of the pipe in August 2011, which was also when inspectors discovered major areas of corrosion in the liner plate itself.

Ultimately, it was this section of the pipe that failed due to corrosion resulting from it not having been galvanized. Non-galvanized steel corrodes much more quickly than galvanized steel.

The fact that the liner plate wasn't galvanized was not discovered until the section was removed and sent for testing where they not only found out that it wasn't galvanized, they also discovered that it wasn't asphalt coated as was indicated in the original drawing.

After establishing that the collapsed section of pipe was not treated properly for its intended use, the investigating consultant focused on the events which led from the initial inspection of the pipe by closed circuit TV in August 2011 to it inevitable collapse.

In particular, the consultant suggests in his report that the closed circuit inspection showed enough damage that it should have been followed up by a physical inspection. He also suggests that a physical inspection may have resulted in a quicker timeline for repairs.

"An 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," the consultants state in their report. "In our opinion, the immediate need for a more robust assessment of the pipe was not understood, identified or communicated."

As it was, the closed circuit inspection resulted in city staff identifying the pipe was in need of rehabilitation and they immediately started the process to reline the existing liner plate.

Unfortunately, it would take a whole year to approve the required work, send it out to tender and hire a company to do the job, which is another failure of the system which allowed the pipe to reach the point of collapse.

The final factor which the consultants say "likely" precipitated the collapse on Sept. 4, was the work being done in the pipe on both Aug. 31 and the day it happened.

The contractor had been inside the pipe on both days installing lights and removing rocks and debris. Workers were using a mini-excavator to remove rocks and other debris from the pipe to prepare it for the new liner.

They left the pipe at approximately 4 p.m. after it started raining. The collapse occurred less than an hour later. In the consultant's opinion, "vibrations from the excavator and loader, or changes in the flow path of the water, as a result of the debris being moved influenced the timing of the collapse".

Once the pipe collapsed it created a sinkhole which grew at a fairly rapid rate and engulfed a car belonging to J.P Unger who accidentally drove in it.

The rest, as they say, is history. The eastbound lanes of the highway had to be closed for two weeks creating traffic chaos for commuters, and it ended up costing $5 million to replace the pipe and fix the roadway compared to the $1.7 million it would have cost just to reline the pipe.

To prevent similar catastrophes from happening in the future the consultants made five recommendations.

1) The current definition of a high-risk storm sewer asset should be expanded, to include consideration of the probability of failure as well as the consequences of failure.

2) Storm sewer assets designated as high risk — other than those that were assessed following the event in question — should be examined as soon as possible and reviewed by persons qualified to assess the condition and judge the need for further action.

3) An attempt be made to assess the quality of the information in the City’s storm sewer asset inventory. Where there are weaknesses related to the inventory’s source materials or as determined from observations, an effort to improve the data should be made.

4) With full consideration of safety issues, and where feasible, physical inspections be used to supplement CCTV inspections for high risk assets.

5) Procedures for scoping capital projects should always include a discussion of the consequences of not proceeding quickly.

In response to the report senior staff are already implementing all five recommendations.

The report and the city's infrastructure asset management procedures an protocols will be discussed by city council at a special meeting in January.

(This story was made possible thanks to the generous support of our local business partners.)

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