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How does a contractor building Ottawa’s Confederation Line light rail transit (LRT) system wind up building part of the city’s massive combined sewage storage tunnel (CSST) project? It’s all a matter of scheduling as the city works to complete two mega-projects simultaneously.
The construction of Ottawa’s LRT is adding 12.5 kilometres to the city’s public transit infrastructure. Meanwhile, construction of the 6.2-kilometre CSST project will deal a significant blow to combined sewer overflows. The $232-million project will store up to 43,000 cubic metres of combined sewer overflow during severe wet weather. The projects overlap at the CSST’s Site 1.
"We numbered the sites in order from the west, but this is actually the last site where we’ll extract our tunnel boring machine," says Steven Courtland, CSST program manager for the City of Ottawa.
The CSST work on Site 1 needed to be completed first because much of it was located underneath or adjacent to track infrastructure. The scope of the work included:
An expanded LeBreton Flats diversion chamber, which also regulates sewer flow, located underneath Booth Street just south of the Booth Street bridge, which was built to cross the new LRT line.
The existing chamber was doubled in size and represents a main conjunction point, with four major sewer pipes feeding in and three pipes exiting.
The extraction chamber for the tunnel boring machine
Open cut construction of the first 165 metres of the CSST system, a 2,400-millimetre combined sewer pipe along the south side of the right-of-way of the rail line.
A new 3,000-millimetre storm pipe 112 metres long, which crosses underneath the rail line.
The project drawings were provided by the City of Ottawa and presented to LRT contractor The Rideau Transit Group (RTG). RTG’s subsidiary responsible for construction is Ottawa Light Rapid Transit-Constructors (OLRT-C), a joint venture between EllisDon, SNC-Lavalin Inc., Dragados Canada and ACS Infrastructure.
The overalll CSST tunnel construction project was awarded to a joint venture between Dragados Canada and Tomlinson.
However, OLRT-C sub-contracted its portion of the CSST work to Taggart Construction, an Ottawa company specializing in engineering and infrastructure construction, including water works, environmental projects, roads and bridges.
"It was a cash allowance project," says Courtland. "It was separate from the LRT, but administered by RTG. Essentially, they tendered it and sent it out for bids. Taggart was the low bidder."
Work has been carefully staged to allow both projects to move forward efficiently, says Luke Foley, project engineer for the City of Ottawa.
Taggart’s work began in early 2015 with the excavation and expansion of the four-storey-deep diversion chamber and excavation of the extraction chamber.
"The extraction shaft is a secant wall," says Foley.
"It features 50 massive concrete columns a metre in diameter, sunk into the ground to form a circle that’s 10 metres in diameter inside. You form the shaft by excavating the dirt from the inside."
The storm and sewer pipes were installed in early 2016. The Booth Street bridge and associated LRT line were completed by OLRT-C in the fall of 2016.
Taggart recently vacated the site so that RTG could build a large retaining wall next to the extraction shaft. Taggart’s remaining work consists largely of excavating and removing a layer of rock at the bottom of the extraction shaft.
"Once the tunnel boring machine is extracted, the shaft will be lined and then converted into a maintenance access hole for the CSST," says Foley.
Portions of the Confederation Line will be completed this summer with full service of the entire line scheduled for 2018. Commissioning of the CSST is scheduled for mid-2020.
In the meantime, Courtland notes that CSST construction will have to coordinate with another major Ottawa construction project as the tunnel boring machine passes beneath it on its approach to Site 1.
"It’s the new Ottawa Central Library," he says. "But that’s another story."
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