Sanitary trunk sewer rehabilitation project protects Toronto recreational water way
The Humber River has long played a starring role in the rousing story of Ontario’s capital, Toronto. Bordering one of the world’s great cosmopolitan cities, this scenic body of water is a popular venue for cultural activities and recreational pursuits from kayaking to hiking. In its midst, an aging sewer—circa late 1950s—could trigger disastrous consequences.
Many understand that years of deterioration may be visible only after careful examination. Such was the case in 2012 when a detailed camera inspection and assessment program confirmed the need to rehabilitate several sanitary trunk sewers and maintenance holes in Toronto.
Toronto Water, the utility that conducted the review, developed a long-term trunk sewer rehabilitation program, and in 2013, the Humber Sanitary Trunk Sewer (STS) was identified as a priority rehabilitation project. The function of this massive underground pipe is to collect sanitary wastewater from different sewers and convey it to the Humber wastewater treatment facility. The rehabilitation project became part of the city of Toronto’s council-approved 2016 Capital Works Program.
This particular section of sewer topped the list, not merely because of severe corrosion in the pipes and the high potential risk of a sewer break, but also because of its location adjacent to and crossing the Humber River. If the Humber STS was left as is, residents would not be the only ones whose safety could be compromised.
Recognized as a Canadian Heritage River, the Humber River and its ecosystem nurture a broad spectrum of wildlife. In addition to serving as a migratory path for butterflies and birds, the river system is home to an array of mammals, including deer, beavers and raccoons, as well as turtles, frogs and other amphibians. Trout, bass and salmon are among its dozens of finned residents. Wildflowers bloom freely from spring to fall, at which time Mother Nature unfurls a multicolored show of foliage.
Following an extensive bidding process, which identified contractors with slip-lining, tunneling and other relevant construction experience, the contract was awarded to Southland Renda of Canada.
Before construction commenced, plans for an access path called for tree removal and pruning along the STS. Tree removal was reviewed and approved by the Toronto and Region Conservation Authority, Ministry of Natural Resources and City – Parks, Forestry & Recreation Division, then scheduled around the nesting period of migratory birds.
Rather than replace the sewer pipes, new linings were installed inside the original pipe by using a process known as slip-lining. This proactive initiative was an ecologically less invasive procedure and is projected to extend the life of the STS by 50 to 100 years.
The Humber STS is 1,500 to 1,650 mm in diameter, so the pipe was used to slipline the sewer. Prapan Dave, manager of Toronto’s Engineering & Construction Services, said the particular pipe used was recommended by the consultant for its corrosion resistance, strength and ease of use.
Installation was accomplished by first removing the crown of the host pipe, an existing concrete reinforced pipe.
“We saw-cut and hoisted the existing trunk to spring line, which gave us access to lower 20-ft joints into the invert,” said Joe Vera, project manager of Southland Holdings LLC. “We then installed a custom-built ring, which was machined to the internal diameter of the bell end of the Hobas joint, into the bell end of the joint. Subsequently, we placed our excavator bucket up against a 1-in. plate welded to the ring, and upon retraction of the arm, the joint would be forced upstream. This process continued until the liner reached the next upstream manhole.”
Dave said the contractor was able to push up to a 400-meter length of pipe in one shot in live sewer flow conditions without requiring temporary bypass. The push procedure was timed according to flow. The contractor utilized the optimum flow conditions from 8:30 a.m. to 3:30 p.m. to float the liner into place. During this window of time, the flow was between 550 liters and 850 liters per second.
A total of 2,450 meters of 1,400- and 1,500-mm pipe was inserted, and the annular space was grouted.
“Hobas has many diameters which were developed just for slip-lining, as they can fit into the existing sewer while optimizing the finished inside diameter of the sewer which maintains flow capacity,” said Erin Boudreaux, marketing manager for Hobas Pipe.
To facilitate the grouting procedure, bulkheads were installed on the upstream and downstream sides of each shaft.
“We installed two 2-in. PVC risers into the bulkheads, extending approximately 3 ft into the annular space,” Vera said. “These were fitted with 90-degree bends and extended to the ground surface. On the upstream side of the run, these served as injection ports. On the downstream side, they served as confirmation ports. When we observed grout spilling from these ports, it confirmed the annulus had been filled. To drain water from the annular space prior and during grouting, we installed two 1-in. PVC drain ports on the downstream end of the run.”
Using cellular grout, which Vera described as “lighter than water,” only one lift was necessary. An efficient material for slipline projects, this type of grout reduces buoyancy during annular space grouting and also lessens the chance of damage.
“Grouting was completed from upstream to downstream, and from crown to invert,” Vera said. “The grout is 44 lb per cu ft and will provide an average 28-day compressive strength of 300 lb per sq in. Hobas was extremely efficient in providing guidance for lateral tie-ins and repair work. Every time I requested clarification, I had an
Finding storage during the rehabilitation process was not a problem. Southland Renda of Canada worked with the Toronto and Region Conservation Authority to obtain additional areas to stockpile pipes as needed.
Upon completion of the sewer rehabilitation, slated for fall 2018, all areas affected by work will be restored. Trees and shrubs will be replanted as approved by the Toronto and Region Conservation Authority, Ministry of Natural Resources and City – Parks, Forestry & Recreation Division.
During construction, the city learned that Hobas pipe offers a certain advantage because it can be directly buried.
“The thickness for direct-buried Hobas pipe may increase, but once properly installed, there is no long-term concern for corrosion,” Dave said. “For large-diameter sanitary sewer projects, the city of Toronto will consider installing Hobas pipe in the future.”