Stantec microtunneling

Markham, Ontario Incorporates Microtunneling in its York Downs Sanitary Sewer Connection

Microtunnels can provide many advantages for a project. Stantec’s work on the York Downs Sanitary Sewer Connection project — in Markham, Ontario, Canada — incorporated the use of a microtunnel, which benefited the project in a few ways.

First, let’s provide some background. A group of real estate developers — including Minto Communities, Metropia, and Kylemore — is developing a section of land along Markham’s 16th Avenue into a new residential community. This project required the sewage collection system to connect into the 16th Avenue portion of the York Durham Sanitary Sewerage System (YDSS), a 2,100-mm diameter sanitary trunk sewer.

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We needed to design a sanitary connection. After discussing options for the project with our clients and other stakeholders, we recommended the use of a slurry microtunnel machine under the surrounding Bruce Creek, which would connect to an existing maintenance hole for the YDSS.

Now that it’s complete, this new infrastructure provides sanitary sewer capacity to service Markham’s Union Village and Angus Glen South Village communities. It will also bring in additional capacity for future residential and employment lands in northern Markham.

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But let’s go back to the beginning and examine some of the more intriguing elements of this project, including our use of a microtunnel.

Studying Multiple Options

Before developing the design, we evaluated five options for connecting into the YDSS. We knew that the recommended connection would need to be feasible, constructable, and maintainable.

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We received input from a variety of stakeholders, including the City of Markham, York Region, and Toronto and Region Conservation Authority. Then, we narrowed our focus to two alternatives and did a preliminary design of both, followed by an evaluation matrix criteria analysis, which included risk and cost.
One alternative involved a direct connection to the YDSS. During the preliminary design stage, however, we established that this option involved the potential for significant dewatering requirements, as well as the need for inline bypass. We would have had to build a massive hole shaft in the ground around an existing sewer and bypass it.

All stakeholders, including our clients, agreed on the other alternative. The team decided to use a slurry microtunnel machine under Bruce Creek, as this option best met the criteria for cost, least construction risk, and maintainability. Going under the creek made the most sense.

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Benefits of Using Microtunneling

For this project, our microtunnel consisted of two 1,200-mm single pass microtunnel drives — a short 55-m run, and a longer 282-m run. The microtunnel provided several benefits, including precision for the end of the tunnel, where we needed to hit an exact target. We appreciated the level of accuracy and control that came with using a microtunnel.

The microtunnel also allowed us to tunnel under a significant wetland and environmentally sensitive area with no surface impacts. The lack of surface disruption, especially compared to open-cut construction, is another great perk of microtunnels. Thankfully, it let us confine soil removal to just the tunnel itself.
Using a microtunnel also meant we could avoid dewatering the aquifers, which would have added unnecessarily complexity and risk to the project, especially since the area contained an artesian aquifer, the Thorncliffe Aquifer. We wanted to avoid any significant dewatering, as it could impact surrounding infrastructure and houses. The project’s contractor, Erritt Construction, used sealed shaft technology, which also prevents any requirements for dewatering.

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Stantec microtunneling

Being Aware of Endangered Species

Tunneling under a significant wetland meant that our team needed to get the proper environmental approvals and avoid disturbing fish that are endangered. We had to make sure that our tunneling took place within a certain window — from July to November — because the creek has been identified as a habitat for redside dace, a colorful minnow that feeds on flying insects by leaping out of the water. This minnow, which has been considered endangered in Ontario for over a decade, helps to manage insect populations.

The team couldn’t start using the microtunnel boring machine until July 1, when the redside dace finished reproducing and our window opened. This timing window added some complexity to the project. If the team missed that timing window, it could have postponed the project for an entire year, as we would have to wait until the following July.

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Stantec microtunneling

Challenging Ground Conditions

Our project also faced another challenge for the design and construction teams, as the area included tricky ground conditions. The soils in the area are influenced by a high groundwater table and potential artesian conditions. The soils were soft clay, which we thought could also contain cobbles and boulders.

We also found a possible concern at the connection maintenance hole of the YDSS. To mitigate risk, we recommended the use of either jet grouting or ground freeze technology to help make the connection. In the end, we were able to avoid any issues for that part of the project.

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A Smooth Project

Overall, the project went smoothly. We enjoyed working with the team members at Erritt Construction, as they brought thorough knowledge of microtunneling and how to manage challenging ground conditions.
Now, construction is finished, and it will be great to see people move into the new houses with their families and enjoy their homes for many years to come.

The project team was able to satisfy the many stakeholders involved in this project. Time was of the essence — and we accomplished our work within the crucial project window. We’re proud that we achieved consensus on the final design of the tunnel, and we’re very pleased with microtunnel option.

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Neil Harvey is a senior project manager with Stantec’s water team, specializes in trenchless technologies, conveyance, and large diameter infrastructure. Hiva Mahdavi is Stantec’s regional conveyance leader in its Canada East region, supporting her team with larger tunnel and trenchless projects.

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