Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

No-Dig North 2023 Canadian Project of the Year New Installation: Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

In the summer of 2013, Calgary made national headlines following heavy floods in the region due in part to heavy rainfall on the melting snowpack.

In June 2013, Calgary experienced the largest flood in its modern history. It had a devastating impact on the built, economic, natural and social environments. The full impact of the June 2013 flood to City’s infrastructure encompassed more than 200 projects needing repairs or restoration work originally estimated at approximately $445 million.

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According to data from the City, the Bow River flows peaked at 2,400 m3 per second, which is eight times its regular flow rate; the Elbow River inflow peaked at 1,240 m3 per second, which is 12 times its regular flow rate; and the outflow below the Glenmore Dam was 700 m3 per second about seven times its normal rate. The floods were the largest seen in Calgary since 1902.

One of the outcomes of the flooding was City leaders coming together to take a hard look at where it could improve the flood resiliency across the city. To date, $1 billion has been invested to increase flood resiliency. One of the areas the City looked at was in the Sunnyside neighborhood.

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“Sunnyside is located within the 1:100-year Bow River floodplain. When the flooding happened, there were a number of houses and properties that were damaged,” says Shawn Pillai, P.Eng. PMP, City of Calgary. “That led the city to initiate a drainage study [with Associated Engineering] that looked at number of options.”

The Northwest Inner City Drainage Study – Sunnyside Review examined the drainage system in the Sunnyside neighbourhood and included several recommendations to improve the drainage and alleviate the impact of flooding,” says Nadeer Lalji, vice president and general manager Alberta South at Associated Engineering.

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It was Associated Engineering’s recommendation that the City initiate the construction of a gravity siphon to help convey stormwater away from the Upper Plateau – an area of higher elevation – directly to the Bow River, bypassing Sunnyside. The rationale, Pillai notes, is that the gravity siphon would not require a pumping mechanism or electricity, making its operation more reliable during an emergency.

“We are managing peak flows at 20 m3 per second. We looked at pumping, as we had elsewhere in that study area, but at that magnitude of flow it just wasn’t feasible given the space available for pumping infrastructure, the city’s capacity for maintenance and the need for certain redundant pumping systems,” says Lalji. “It wasn’t particularly sustainable given that this is stormwater, and this infrastructure would sit idle for a lot of the year.”

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NWIC Upper Plateau separation storm trunk tunnel profile

Stormwater Conveyance in 2013

At the time of the 2013 flood, Sunnyside was protected from the Bow River by an earthen berm. The berm also helped mitigate storm water runoff generated in Sunnyside from entering the river. While protected from the river by the berm, Sunnyside was susceptible to flooding from stormwater coming from the Upper Plateau.

Water from the Upper Plateau was conveyed away from the area via concrete storm trunk sewers that converge – joining a system that also collects storm water from Sunnyside – and travels under the neighbourhood and outlets at the river.

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When the flood occurred, the Upper Plateau storm sewers surcharged and already exasperated system and flooded Sunnyside. This was coupled with the Bow River’s record high levels overflowing the earthen berm.

“The goal of this project was to collect all the water from the Upper Plateau and bring it to one location where the new larger gravity siphon tunnel will convey it to an outfall on the Bow River,” says Pillai.

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The City issued an RFP for the installation of the new sewer and Associated Engineering was issued the contract to move forward as the prime consultant. Associated Engineering tapped McMillen Jacobs & Associates (now Delve Underground) and Thurber Engineering to engineer a solution that made this possible. Delve Underground handled the shaft and tunnel design, and Thurber handled the geotechnical on the project. The designs were completed in 2021.

Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

Why Go Trenchless?

“In terms of the question, ‘Was trenchless best for this project?’ I don’t think there was a choice,” says Jason Lueke, Phd., P.Eng., national discipline leader trenchless technologies at Associated Engineering. “You have the depth and the ground conditions. And then if you take a look at the alignment, you are going underneath a little mountain, you have the LRT on top of that. Any excavation down that street would have closed it off entirely because you would have been going from property line to property line with the excavation. It just wasn’t feasible.”

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Speaking to the ground conditions, Lalji notes that the siphon construction starts in low strength but competent bedrock, but as work progressed toward the river, the conditions changed to the Alluvial formation with overburden consisting of sands, gravels, silts and more loose and less cohesive materials. Adding to the challenge is an alignment that is mostly below the Bow River water table. “We knew we needed an installation that changed methodologies at that change in geological conditions – but that wasn’t particularly easy to pull off because we were going through a densely built neighbourhood,” Lalji says. “Or we needed equipment that could provide a pressurized face and manage those looser more mixed soil conditions.”

To meet the expected flows, it was determined that a smooth-walled pipe, with a 3.4 m ID, was the optimal solution. The new 756-m long sewer originates at a launch shaft at 10 Street NW and 8 Avenue NW, travels under a light rail line and greenspace and then under 7 Street NW. The reception shaft is just past the intersection of Memorial Drive NW at the Bow River, and the new outfall structure was built above that shaft.

“The tunnel design has a flow of 11.2 to 20.2 m3 per second,” says Pillai. “The reason why one is higher than the other is that the [flow] rate is dependent on the Bow River’s elevation. If the river is higher, the flow is more resistant. If the river is lower, then it will be 20 m3 per second.”

The tender for the installation was proposed as either a segmental liner tunnelling project or an installation via microtunnelling. Two bids were received for the project, and both went with the microtunnelling alternative.

“When we reached the tender, we received two submissions, and both were for microtunnelling,” says Pillai. “We did a risk analysis and consulted with manufacturers, and all said it was feasible, so we decided to go with that.”

Winning the bid was Whissel Contracting Ltd as the prime contractor and Ward and Burke Microtunneling Ltd. as the trenchless subcontractor.

“Credit goes to the city of Calgary which selects contractors, especially for projects like this, not just on low price but a mix of qualifications, relevant experience and low price,” says Lalji. “We helped them with those criteria, and for this project it was 60 per cent qualifications and 40 per cent price. This gave the city value in a contractor who could hit the ground running.”

Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

Trenchless Works Commence

In June 2022, Ward and Burke began construction on the project with construction of the 13 m concrete caisson launch shaft, which also serves as a connection point for the storm sewers serving the Upper Plateau.

For the project, Ward and Burke used its Herrenknecht AVN2500 machine, which was up skinned to accommodate the 3.4 m ID pipe, and VMT guidance system to steer the closed faced microtunnel boring machine (MTBM) along its 756 m route.

“This pipe has a 4 m OD and 3.4 m ID, which is not a normal size and none of the Calgary pipe manufacturers had the pipe,” says Pillai. “Ward & Burke turned to Langley Concrete Group in Chilliwack, British Columbia to source the pipe.”

Langley Concrete Group has supplied concrete microtunnelling pipe for many of Ward and Burke’s microtunneling projects in Western Canada. For this project it supplied 230 sections of 3.3 m long concrete pipe. Ward and Burke had the pipe molds designed in Europe and shipped to Langley to make the pipe to their spec.

Because the work took place primarily in a residential area and close to the homes, Pillai notes that there were noise and vibration monitoring requirements, as well as restrictions on working hours. So, while tunnelling through bedrock, Ward and Burke maintained shorter 12-hour shifts. However, as tunnelling progressed to the gravel sections, Ward and Burke proposed and were approved to run 24-hour shifts to maintain face pressure on the machine.

“When the tunnel reached the gravel space, the contractor proposed shifting to a 24-hour operation and I believe that was the right choice,” says Pillai. “We supported it and that is one of the reasons the project was completed in a timely manner.”

Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

Exemptions for the project

According to Pillai, Ward and Burke were able to tunnel approximately 10 m a day at the beginning when it ran the 12-hour shifts and achieved a maximum of 18 m a day during its longer 24-hour operations.
When it comes to advancing the industry, Lalji notes that this project – especially for Western Canada – is a milestone that expands the horizons for the microtunnelling industry, the magnitude of infrastructure they work in and the types of solutions they provide.

“Microtunneling is a big part of the underground infrastructure community, and locally their bread and butter is 1 to 1.5 m,” he says. “This expands the horizons of owners, engineers and contractors that this type of pressurized face solution can be effective in a larger diameter opens some doors for the industry in the future.”

Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project

Lueke adds, “Typically pipes of this size going into the ground – based on Edmonton and Western Canadian [installation] history – is that anything over 2.7 m was a segmental liner type tunnel. Now we have this microtunnel at 3.4 m. We aren’t increasing the capabilities, but we are increasing the awareness of microtunnelling in these larger diameters.”

Tunnelling work was complete in December 2022 and turned over to the city in February 2023. As of the completion in 2022, the Northwest Inner City Microtunnel for the Upper Plateau Sewer Separation Project was the largest diameter microtunnel completed in North America.

Owner: City of Calgary
Engineers: Associated Engineering (lead), Delve Underground (tunnel design) and Thurber Engineering (geotechnical)
Contractors: Whissel Contracting Ltd. (prime) Ward & Burke Microtunnelling Ltd. (trenchless), Englobe Corp. (noise and ground vibration monitoring) and Maidment Land Surveys Ltd (surveying)
Products/Suppliers: Langley Concrete Group, Herrenknecht, VMT
Value of Trenchless Project: $43 million

Mike Kezdi is the managing editor of Trenchless Technology Canada.