Readers familiar with the Mississauga skyline will know that it has seen a boom in condo construction. The country’s sixth largest city grew by more than 8,000 people from 2011 to 2016, according to the Census. And forecasts expect its population of more than 800,000 will continue to grow.
Part of the Region of Peel, Mississauga, along with the municipality’s second largest city, Brampton, is welcoming new businesses and amenities to accommodate the population growth. A new light rail line, the 18km-long Hurontario LRT, will connect the two cities to existing transit options in the Great Toronto Area and serve the Region of Peel. Metrolinx and Infrastructure Ontario awarded the contract in October 2019 for the project’s design, build, finance, operation and maintenance for a 30-year term. The line is scheduled to open in fall 2024.
However, before the LRT construction starts, Mississauga needs new water and wastewater infrastructure installed in its city centre to meet the needs of its growing population. Schedule is critical for this work and the location is a densely-populated area with a large shopping mall, high-rise residential buildings, and not least of all, city hall, among other major facilities.
The Burnhamthorpe Water Project concerns more than 13km of water and wastewater infrastructure, and Hatch is providing engineering and construction administration services. Finding space for all the necessary components presented an early and complex design challenge.
The team aptly refers to this as “threading the needle” (see info box page 22). Construction comprises three separate contracts and for larger diameter installations the project is using tunnelling and trenchless methods. Technicore Underground is performing the work on C1 and C2, worth CAD 118M (USD 85M). Total project cost is approximately CAD 160M (USD 120M).
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By GlobalDataTechnicore was responsible for all utility relocations as part of its scope of the project, including an existing watermain, traffic lights and wires, hydro, fibre optic cables and street lighting. Hatch fast-tracked the design to the tendering stage, to help the project stay on schedule and complete within the timeline for the LRT.
“That didn’t afford Hatch enough time to do relocations,” explains Mike MacFarlane, project manager, Technicore. “That’s why it was passed to us. They had a contingency allowance for relocations to be done by the contractor, and we had to build this into our schedule.
“We jumped on the relocations as soon as we were awarded the project. That was a challenging task in itself, and for the most part everything went smoothly.”
He adds, “whether the region takes it on prior to tendering a project or leaves it to the contractor, I don’t see advantages or disadvantages to doing it either way.”
The contracted project work includes but is not limited to:
- 3,600m of 1500mm-diameter watermain constructed by tunnelling (two pass installation) along Burnhamthorpe Road;
- 2,400m of 1200mm-diameter sanitary sewer constructed by microtunnelling (single pass) along Centre View Drive, Duke of York Boulevard and Burnhamthorpe Road;
- 200m of 600mm-diameter watermain constructed by microtunnelling (two pass) along Duke of York Boulevard; and,
- 1.7km of 600mm-diameter open cut construction by subcontractor Memme.
Technicore used two companies for pipes. Ewing supplied the 1,200mmdiameter sanitary pipe and 1,200mmdiameter steel casing pipe. Decast supplied the 1,500mm-diameter watermain and the 600mm-diameter watermain. All drives were located fully within competent Georgian Bay Shale, with minimal to no groundwater.
Shaft Speeds
The 1,500mm-diameter watermain requires six valve chambers located along the alignment where it interconnects with other existing watermains. These are doubling as shafts for tunnelling. Initially the design specified rectangular-shaped chambers of varying sizes to accommodate the connections and make use of the limited space available on the street.
Technicore and the design team worked on revising the size and shape of the tunnel shafts/valve chambers. Four of the shafts became circular with 14.1m diameters and another one of 12.1m diameter, also circular. The new design allowed Technicore to pour the chamber walls right up against the edges of the shaft excavation or the support of excavation.
By doing so, no additional costs were incurred by the owner and the construction duration for each of the shafts/ chambers shortened. MacFarlane estimates it saves as much as five months for the project. “No one likes construction in their backyard, and when it has to happen you want it done and over with as quickly as possible,” he says.
For the 14.1m diameter shafts, they used 1m diameter caissons for the entire length of the shaft eliminating alternative support in the shale, “while providing watertight working conditions perfect for a long project in Canada,” says MacFarlane.
These are predrilled with 2.5m diameter holes to aid in the shaft excavation. “Two weeks of predrilling will save four weeks of excavation time,” he adds.
Depths range from 16m to 24m and overburden of 2m to 6m at most. For support they shotcreted the overburden and used bolts and mesh for the shale. For the sanitary sewer, the process was similar with deeper, 22m to 30m shafts and smaller 7.1m diameters.
For the shaft identified as mts-1a Technicore used an LB44 rotary drilling rig with a custom-built 5m diameter bit, supporting the 5m of overburden with a 5.5m diameter rib and lagging shaft. “We then drilled a 2.5m pilot bore in the center of the shaft to act as a guide for the 5m bit,” MacFarlane explains. “Using this, we sunk a 5m diameter shaft that is 26m deep in approximately two weeks.”
For a project with immense schedule pressures, the savings in construction duration associated with the changes made to make the shafts and chambers a common size and shape was “invaluable,” says the project team.
Well-Equipped
Thus far the project has had no problems with the ground conditions and everything has been as anticipated. However, during tunnelling construction for the Cooksville Creek Trunk Sanitary Sewer (CCTSS) in 2009 to 2011 the TBM encountered a buried bedrock valley near Burnhamthorpe Road.
Bedrock valleys are localized incisions in the bedrock, which are thought to have formed by meltwater during glacial retreat. These features often contain saturated deposits of coarse-grained glacial outwash materials including cobbles and boulders.
On the CCTSS project intersecting the buried valley meant groundwater inflows and flowing ground conditions that put above ground infrastructure, including an arterial road, at significant risk. As well as increasing the project’s cost and schedule. With the potential for the 1,500mm-diameter watermain alignment to be in or near the vicinity of the buried valley, the team undertook extensive geotechnical investigations.
These did not conclusively verify the presence of the valley, but could not categorically rule out the possibility.
The project team put in a contingency allowance for hitting the valley, and mandated the use of a fully-shielded TBM for this drive, an initial support system consisting of full circumference steel ribs and wood lagging where the risk of encountering the buried valley was highest, and advance-probing in this area.
Technicore proposed mining the drive using a fully shielded TBM with a screw auger, which could be converted to an EPBM—in lieu of the requirement for advance probing. Region of Peel agreed, and in the end the drive did not encounter the buried valley. For the 1500mm-diameter watermain Technicore will use three different machines. The first, for a drive of 800m, is a refurbished spider TBM MacFarlane describes as an older, rudimentary main beam, which has been used on projects across the GTA. The second is a Technicore shielded TBM with a 120-inch diameter. This has completed about 2km of tunnel for the watermain. A third machine, which is set to launch imminently at the time of writing, is a 105-inch diameter Technicore shielded TBM.
Technicore is using two machines for the sanitary sewer, a refurbished MTBM originally manufactured with a 51-inch diameter, which Technicore skinned up and added a new head for a 60-inch cut diameter. The second machine is a Herrenknecht AVD1200.
The longest drive for the 1500mmdiameter watermain was about 1.2km and the longest drive for the microtunnels was 422m, which included several curves. Both of these drives have been completed successfully. By early March, the project will have four different TBMs and MTBMs in the ground.
Good Neighbours
With such a tight timeline to install the watermains double shifts would be required, which is less than ideal in an area with high-rise residential towers and other important amenities. The city has allowed Technicore to tunnel, install the lining and build the chambers on a 24-hours basis. MacFarlane reports all of the tunnelling has been done on two, 12-hour shifts, Monday through Friday.
Microtunnelling is dependent on location, either doing one 12-hour shift or two. Peel mandated the project team must try to keep constituents happy.
“We’ve done everything that we could,” he says. That includes installing a 30ft-high wall made of sound absorption panels in one particular site to reduce noise from the slurry separation plant. Noise monitors have been installed in various locations near active sites, including a nearby day care located about 20ft from a shaft location.
“We went in and met with the director, and we installed additional monitoring devices within the children’s play area where they would be napping. We were even willing to work around their nap schedule,” he explains. “After the first week of working at that site she called us and said, ‘why don’t you just try working through nap schedule? Because we really don’t hear you guys working.’ We were able to work through their nap and not have a single complaint.”
Technicore also opted to power the TBMs and MTBMs from a completely different site by running power cable through an existing tunnel to another location in order to keep the peace with the neighbours. “It’s costly, but we’re willing to do it,” MacFarlane says.
He adds, “rather than always having a confrontation with the community, if you can be light-hearted with them, and be actively out there communicating with them, I think that makes a big difference.”
