Metrolinx completes tunnels under 21-lane highway

12 February 2021

Greater Ontario transportation agency Metrolinx has announced the successful excavation of twin tunnels less than three metres beneath 21 lanes of live highway – all achieved with no disruption to traffic.

Constructing the tunnels at the junction of highways 401 and 409 – one of North America’s busiest highway sections – is part of the GO Rail Expansion programme to create two additional tracks to increase capacity on the Kitchener line.

At 11m high and 180m-long, each horseshoe-shaped tunnel was excavated working from both ends toward the middle. Two-part sequential excavation removed first the heading then the bench, with robotically-applied shotcrete used for temporary support allied with mesh and lattice girders. A waterproofing sheet membrane was positioned prior to the installation of the final in-situ-poured concrete lining.

When digging reached the mid-point of each tunnel, breakthrough was achieved by using a blowtorch to cut through an 18mm-thick steel wall. The steel had formed part of a sheet-pile box driven at the mid-point of the tunnels prior to the start of mining, then excavated to create a working area below the highway. This would allow the installation of a pipe-umbrella support created by the auger boring and jacking of 12m-long, 813mm-diameter steel pipes.

To facilitate excavation, work began at the east portal of tunnel two to allow construction at the west portal of tunnel one to be completed safely. Spoil was taken away by trucks, which on tunnel two numbered 3,900, carrying away 35,000m3 of excavated material.
Work on the tunnels began in October 2019 and is expected to be completed by 2022.