Just when you thought there was hope the City would make a final decision regarding the planning and construction of the long-awaited Green Line LRT, a new tunnelling proposal has come along that could very well prove to be a game changer. According to a recent report to City Council by Green Line project manager Paul Giannelia, Calgary's poor soil conditions mean that a large, single-bore solution would best serve the 4-km underground portion of the $4.6-billion line, as opposed to the more traditional twin-bore tunnelling practice as seen in most subway systems around the world.

Single-Bore VS Twin-Bore, image via City of Calgary

Viewed above, the single-bore model would see the construction of one extra large tunnel, instead of two separate tunnels side-by-side, through which trains would travel in either direction one above the other. Passengers would enter and exit the platforms via a single entry shaft, placed to one side of the road above, as opposed to having a single entry point placed between the tracks to be accessed from the middle of the street. The main advantages of the twin-bore method appear to be three-fold; hinging upon lower construction costs, less disruption during construction (access points moved to side of road), and a more efficient layout once it is complete (station access not in the middle of the road).

Green Line LRT Map, showing tunnelled section/stations, image via City of Calgary

The Green Line LRT will run underground from 20 Ave to Macleod Trail, the 4-km subterranean route passing beneath the Bow River, the downtown core, light rail, heavy rail, and countless major power and water utilities. To put it lightly, this will be a massively disruptive, albeit highly transformative project for the city, one for which major decisions about its construction could have significant short- and long-term ramifications.

In essence, it is critical that the Green Line engineers and City Hall staff get this right.

Boring details get you excited? Let us know what you think of the Green Line's new digs in the comments section below!