Construction begins on headwalls for Scarborough Subway Extension

TORONTO—Construction crews have started work on the headwalls for the Scarborough Subway Extension in Scarborough, Ont.

Headwalls are underground support structures made up of a series of concrete columns, called piles, that create a watertight wall around the area of future subway stations and emergency exit buildings, indicates a Metrolinx blog.

To prepare for excavation, piles will be drilled into the ground using large drill rigs, which are usually over 33 metres high. About 19 piles will be required for each headwall.

Crews will build headwalls at 12 sites along the project route and all of them need to be complete before the project’s tunnel boring machine Diggy Scardust arrives at each location.

At each future station and emergency exit building site, crews will be installing a pair of headwalls to mark where the buildings will start and end, states the blog, adding after the tunnel boring machine bores through these headwalls and tunnelling is completed, the successful stations, rail and systems contractor will use the headwalls as frames to complete building the future stations and emergency exit buildings.

The headwalls need to go deeper than the future tunnels to ensure water and soil don’t disrupt the area during tunnelling or construction.

Each pile will be drilled roughly 40 metres underground. To complete this work, a single drill rig will be used at most sites. It will take crews about four to six months to build each headwall and the work will also involve utility relocations.