Now that excavation on Tunnel One is complete on a task to burrow an important rail tunnel below and through Toronto’s busy Highway 401 and 409 area, the crews are moving a liner machine through to ensure a smooth mold inside the new subterranean passage. We take you into the depths to show you how this liner construction works, as well as introduce you to the unique and mighty machine.
Digging through rock, packed dirt and debris can be a rough job.
But there’s a machine that can smooth out the lingering bumps along the way.
Now that excavation is complete on Tunnel One of the Highway 401 and 409 Rail Tunnel Project – work that includes digging two tunnels to accommodate additional tracks that will help increase GO Transit rail service on the Kitchener Corridor – the construction crews have cleaned the subterranean passage in preparation of liner construction.
The liner construction is a process of ensuring the tunnel is smooth in order to install track in the future. However, before the liner machine enters the tunnel, a few things need to happen first.
A waterproofing membrane wraps around the entire tunnel to prevent water from seeping through. The waterproofing membrane is rubber/plastic that is installed in sheets that are welded – heat sealed and rolled – to create a single impermeable membrane against water inflow into the tunnel.
At the same time, a permanent base – or invert – will be constructed to allow the liner machine to move through easily. This permanent base will ultimately support future tracks and rail service.
(Concrete is poured on top of the layers of membrane to create an even surface. (Metrolinx video).)
The cured concrete floor becomes the new permanent base and is ready for the tunnel liner to move through.
How did the tunnel liner get to the site? The liner machine was assembled in multiple steps over a period of four weeks.
The liner machine is assembled on-site and operated by Toronto Tunnel Partners. It provides a simple advantage – the team has control over the entire tunnel liner construction process. In turn, the quality of the work will reduce issues during on-going maintenance and increases longevity of the tunnels.
A special concrete mix – including reinforced polyethylene fibres – are delivered by a regular concrete truck and pumped through a network of pipes to the tunnel liner machine.
During liner construction, the panels on the machine expand outwards, pressing against the tunnel. Concrete is then pumped in and cured for 48 hours.
After the concrete has had enough time to cure, the panels on the machine retract and the unit moves to the next section, covering approximately eight metres per section and the process is repeated until the entire tunnel is smooth.
See – for experts used to doing this kind of tunneling work, it’s not so rough to get rid of those rough spots.
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Story by Teresa Ko, Metrolinx senior advisor