Daily Commercial News and Construction Record
Cracks in the concrete liner of a 60-metre section of sewer line in east Toronto has Toronto Water staff working overtime to find a fix to prevent the line from collapsing, which would cause sewage to spill into several emergency line outlets into the Don River and a tributary, the Taylor-Massey Creek.
A proposed remedy calls for the construction of at least a 600-metre bypass section around the existing line, which would allow crews to empty the faulty section and do repairs in the 2.7-metre diameter line. The bypass project would cost $20-30 million and take 18 months to complete, says Lou Di Gironimo, general manager, Toronto Water.
Di Gironimo calls the problem 60-metre section a “level 5 condition,” meaning the repair is of the utmost urgency.
Called the Coxwell Avenue Trunk Sewer, it is encased in steel with steel ribbing and wood lags. A robotic camera has produced images showing that the inside 56-centimetre-thick concrete liner has cracks and deformation.
The 40-metre deep line handles six cubic metres of flow per second, “a significant amount,” he says, adding it is a major line serving 750,000 city residents.
There is no telling when or if the line could collapse but the good news is that there is no “significant groundwater infiltration,” he points out. In a nutshell, the integrity of the sewer’s outer steel liner probably has not been compromised.
Toronto Water is closely monitoring the site through level sensors installed along the sewer line to measure flow changes that would signify a collapse.
If the line should break, contingency plans call for opening safety outfalls to allow for a controlled discharge into the river to prevent surface settlement problems that could damage homes, properties and the infrastructure.
Di Gironimo says Toronto Water hopes to put out a proposal for the construction of the bypass once a geotechnical report on the subsurface and groundwater conditions is completed this spring.
“Anybody that’s doing construction will want to see that information first (before they bid).”
Damage to the concrete liner was discovered by a robotic camera during an environmental assessment being conducted on the line. The assessment, which commenced in 2006, is part of Toronto Water’s look into possibly twinning the line so that one line can be turned off when repairs to the other are required.
The chief engineering consultant for the work, MMM Group, retained Andrews Infrastructure, which used a “floating camera” to obtain some images showing cracks but heavy sewage flow surcharge prevented the camera from completing the inspection.
Andrews then retained D.M. Robichaud Associates Limited to complete the inspection, using the latest in robotics technology. It provided images of cracking and deformation of the liner along a 60-metre length, says Di Gironimo.
Over the years, inspecting the inside of the liner has been no easy feat because of the line’s depth and the distance between access shafts to the line, says Di Gironimo.
“For many years we couldn’t get camera equipment in because the line was too deep and the spacing (between access shafts) was too far. No one had cabling camera technology long enough to run through such a large sewer with significant flows.”
Construction on the sewer line started in the late 1950s, with completion by 1960. Based on research at the time, if the line broke it could cause some “shifting at the surface” that could affect homes along the route.
When the emergency bypass is completed, Toronto Water will continue to work on a plan to twin the entire sewer line over its five-kilometre length.