A new tunnel project will give Canada’s capital city greater capacity for preventing CSOs.
Some projects protect waterways. Some preserve a city and nation’s history.
As the capital city of Canada, Ottawa will play a central role in the country’s 150th anniversary this year. It’s fitting that the city is also ramping up its efforts to knock out combined sewer overflows.
A combined sewage storage tunnel (CSST) is being built to retain as many as 11.3 million gallons of sewer overflow for treatment. According to design parameters, it will reduce CSOs to an average of no more than two per year. For the city’s engineers, a commitment to getting the right design also helped the city achieve greater efficiency in its existing wastewater system.
While the city has made significant progress against CSOs in recent years, overflows to the Ottawa River continue to be a problem. The project is driven by the city’s Ottawa River Action Plan and provincial safe water regulations. The importance of water quality in the city’s Rideau Canal is also significant, as it is a UNESCO World Heritage Site and the oldest continuously operated canal system in North America.
“CSOs have been part of the Ottawa sewer and wastewater infrastructure since the construction of the system,” says Steven Courtland, CSST program manager. “Initially, 100 percent of sewage passed to the Ottawa River untreated, but we reduced that by 100 percent of dry weather flow and 97 percent of overall volume when we commissioned the interceptor-outfall sewer, CSO regulators and treatment plant in 1961.”
Since then, much of the combined sewer system has been separated. In today’s dollars, the city has already invested more than $750 million in that effort.
Today, 3.4 square miles of the city are still served by combined sewers.
“Of that remaining area, 2.5 square miles will remain combined,” says Louis Julien, senior engineer of Water Resources. “Part of the reason is simply the difficulty of separating that section. Another is the fact that the topography tilts toward the Rideau Canal. Because it’s a UNESCO heritage site, it’s out of bounds to our stormwater. The nearest suitable outlet for that stormwater would be more than a mile away at the Ottawa River, so a combined system remains the most efficient solution for that area.”
Operation of the city’s infrastructure, including water, wastewater, stormwater, roads and solid waste collection are the responsibility of the recently consolidated Public Works and Environmental Services Department, which serves 900,000 citizens. Maintaining an efficient sewer system is part of the effort to control CSOs.
The city rates the condition of its sewer system as “fair to good,” although its SCADA system is probably due for an upgrade, says Courtland.
Pipe materials include asbestos cement, concrete, brick, cast iron, steel and PVC with pipe diameters ranging from 4 to 118 inches in diameter.
Many of the pipes are dug to the depth of the limestone rock underpinning the city. When older sewer pipes supported by soil are replaced, they’re also lowered into bedrock to provide an improved level of service. While the city performs routine maintenance with in-house crews, it outsources major construction and cured-in-place pipe rehabilitation projects.
The city plots GIS coordinates using ArcGIS by Esri, and employs InfoWorks by Innovyze and PCSWMM by Computational Hydraulics International to analyze and model its wastewater infrastructure.
Temporary storage was always the top-of-mind solution for reduction of the remaining combined flow, whether that storage took the form of a tunnel, tank or chamber. In order for that plan to go forward, however, the city realized it first needed to rehabilitate the CSO regulators that direct flow to the interceptor system and treatment plant, or to the Ottawa River. It allocated $25 million to the project, which included major upgrades to five CSO regulators, and capability for continuous, real-time adjustment of gate positions at three of those regulators.
“The city implemented the real-time control system by early 2011,” says Courtland. “The surprise was that, as a result of implementing those controls, it improved the city’s capture rate and reduced the overflow volumes by about 65 percent and the frequency of overflows by 25 percent.”
There’s a good reason for that. Julien explains that the large interceptor constructed in the 1960s collects wastewater from seven major contributing sewers with combined or formerly combined areas. Each was allocated a portion of the interceptor’s total capacity.
“If any one of the seven contributing pipes reached its allotment during a rainstorm or heavy snow melt it raised a mechanical float,” he says. “Under those conditions, it was assumed that the interceptor was at full capacity, with all contributing sewers reaching their allotment simultan-eously and that flow was discharged to the river. In other words, we were over-controlling and shunting water to the Ottawa River when the interceptors weren’t actually at capacity. The mechanical regulators were rehabbed with a real-time control system to better make use of the interceptor’s capacity by having the outfalls talk to each other.”
Following the success of the real-time control project, engineers overhauled the storage project design. The idea of building CSO storage tanks at the end of each outfall was eliminated in favor of a design where all outfalls could share common storage volume for later treatment at the Robert O. Pickard Environmental Centre.
Ottawa project manager Randy Dempsey saw the project through its design phase. The final configuration involves the construction of two intersecting tunnels: a 2.5-mile east-west tunnel through the city’s downtown core, and a 1.25-mile north-south tunnel, which joins up to existing wastewater infrastructure, just behind the Supreme Court building.
The tunnel construction project bid was awarded to a joint venture between Dragados Canada and Tomlinson. The total project budget is $232 million, with the city picking up about $108 million and the provincial and federal governments splitting the remainder at $62 million each.
“We also factored in additional volume for possible future climate change and additional development in the downtown core,” says Julien. “The north-south tunnel was added as a dual-purpose feature. It will double as a flood protection outlet for an at-risk area.”
A tunnel boring machine will carve two 10-foot-diameter tunnels out of limestone rock formations for a total length of about 4 miles. The tunnels will range from a depth of 40 to 100 feet below the surface, allowing gravity to take the wastewater to the treatment facility. The project will also include the construction of four odor control facilities.
The boring machine will be lowered into an entry shaft, where the rotating cutter will chip away at the rock while conveyors transport the material toward the back of the machine. Tunnel “muck” will be carried back to the launch shaft via electric train for reuse in other construction projects.
Sealing the tunnels
Limestone is porous, so precast concrete liners made up of five segments will be placed in rings behind the machine. Each segment will be gasketed and the outer perimeter will be sealed with grout. That’s less to keep wastewater in as it is to ensure groundwater from above doesn’t seep in during construction, which could dewater and destabilize sensitive overburden clay soils.
“We’re currently excavating the borer launch shaft for the north-south tunnel,” Courtland says.
Thinking ahead, the city constructed the last portion of the project first — the exit tunnel of the east-west shaft. The project was handled by the contractor building the city’s light rail system because the exit point to the municipal sewer system is located at the flank of one of the newly built light rail stations.
“At the north end of the first tunnel the boring machine will emerge from a cliff face behind the Supreme Court of Canada building,” Courtland says. “It will then be used for the east-west tunnel.” (See sidebar).
Construction of both tunnels should be completed by late 2019, with commissioning scheduled for mid-2020.
“In 2015, our total capture rate was 99.8 percent, with only 0.2 percent untreated overflow,” Julien says. “Our wet weather capture rate is 95 percent. We have reason to believe that figure is competitive with any city in the world. When the tunnel is completed, we’ll be doing even better.”
Sensitive negotiations part of tunnel design
As Canada’s capital, a large part of the city is managed by the National Capital Commission, a federal agency that oversees federally designated lands. Designing and building the city’s new combined sewage storage tunnel has involved intricate negotiations with the NCC.
The access shaft required to launch the tunnel boring machine on the east-west tunnel is being excavated in the NCC-managed Stanley Park and will emerge in
the city’s LeBreton Flats neighborhood, also overseen by the commission. Construction of a significant access shaft and diversion chamber will take place in Confederation Park, a national historic site.
The north-south tunnel will pass under both the city’s Parliamentary Precinct and its Judiciary Precinct. The Judiciary Precinct is home to the Supreme Court of Canada, which sits high above the Ottawa River, with its back to an imposing cliff.
“The tunnel will connect with an existing storm sewer outlet tunnel,” says Steven Courtland, CSST program manager. “The boring machine will actually emerge from the cliff face next to an existing outfall and allow us to replace and aesthetically improve it with something more low-profile.”
As the borer emerges from the cliff, tunnel construction crews will load it onto a truck and transport it across the city to begin work on the east-west tunnel site.
“Operating in Ottawa, we’re always mindful of the fact that this is Canada’s capital,” Courtland says. “After many years of negotiations with various departments
of the federal government, we have agreements in place to make sure this project runs smoothly.”