Locale : North America (English)
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Arial view of the construction site

3D modeling provides a "passenger relief valve"

Toronto’s PATH system is the world’s largest pedestrian tunnel network, connecting more than 50 office towers through 27 kilometers (17 miles) of shopping arcades.

The new Northwest PATH Tunnel will provide a faster and safer means of crossing a very busy intersection, reduce congestion, improve pedestrian flows.


The revitalization of Toronto’s Union Station, a leading example of Beaux Arts architecture, has gained attention from the press and public. More people pass through Union Station every day than through Toronto’s international airport, and that number is expected to double by 2020.

Toronto’s PATH system is the world’s largest pedestrian tunnel network, connecting more than 50 office towers through 27 kilometers (17 miles) of shopping arcades. The PATH tunnel connection on the east side of Union Station is severely congested, prompting the desire for a tunnel on the west side to provide a “passenger relief valve.”

To create such a tunnel, 25 different Toronto utilities owned by 12 utility owners would need to be relocated, including water mains, sewers, gas mains, fiber optics, underground hydro transmission circuits, underground primary and secondary hydro distribution circuits, overhead and underground street lighting, traffic control systems, and steam heating networks.

All of Canada’s major financial institutions depend on these power and fiber-optic connections. In addition, the job site is adjacent to large construction projects and surrounded by heritage buildings. Designing a solution was reportedly described as trying to push a brick through a plate of noodles without touching them.


HMM, a joint venture of Mott MacDonald and Hatch, was retained in association with NORR Limited as the prime consultant providing both design and construction administration for the project.

In addition to the tunnel structure, the project includes the following:

  • Two steel/glass surface buildings for stair/elevator access
  • Road restoration and landscaping
  • Demolition and rebuilding of a fan shaft for the subway system
  • Full architectural and mechanical/electrical system installations integrated into Union Station’s control systems

We developed complex 3D models that laid out multiple tunnel alignments and utility relocation options, allowing the client to choose from a variety of custom solutions.

One major challenge was to relocate a sewer whose existing profile went directly through the proposed tunnel. We designed a siphon made of 6-millimeter-thick stainless steel under the tunnel, directly between the new tunnel’s floor slab and the existing subway’s roof slab.

Because there was not enough space to accommodate a circular sewer, we designed transitions from a circular cross-section to an equivalent rectangular cross-section and back again, with only 50 mm clearance between the siphon and the tunnel structure.


The new Northwest PATH Tunnel provides a faster and safer means of crossing a very busy intersection, reduces congestion, improves pedestrian flows and travel times through the PATH network, and improves access to the buildings and businesses in the network.

By clarifying the challenges involved, and the impact on a variety of stakeholders, the 3D model eased the process of gaining approval for the work from 12 utility companies, each with different standards, operating procedures, physical clearance requirements, approval processes, and financial concerns.

To produce the contract drawings for construction, detailed plans, elevations, and cross-sections were generated directly from the 3D model, saving time compared to traditional methods of 2D CAD.

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