We diverted five lanes of traffic and shifted Eglinton Avenue, one of the busiest streets in Scarborough, as we made way for something big. Digging a 10.7-metre diameter tunnel, Diggy Scardust is the largest tunnel boring machine (TBM) ever used on a project in Canada.

Diggy began tunnelling south at Sheppard and McCowan, and we just finished constructing the spot where Diggy will end its journey at Eglinton and Midland Avenues. The 24-metre-deep extraction shaft has a 25-metre by 15-metre opening.

Since last spring, our crews have been removing soil, shoring and scraping. The shaft was created using excavators as specialized machines installed stabilizing supports, and finally a concrete slab was poured to make the floor. As each level was excavated, tieback machines drilled into the walls of the shaft, keeping the surrounding earth stable. A tieback is a structural rod installed into rock and soil at between 15 and 45 degrees to transfer the load and create additional support for the retaining walls. Overall, 240 tiebacks were installed to support Diggy’s extraction shaft.

Once the final level was excavated, the site had to be dewatered and completely dried to create stable conditions to pour a concrete floor slab. Almost 1,200 cubic metres of concrete were poured, putting the finishing touch on this important piece of the project.

The construction of the TBM extraction shaft is an important part of tunnelling work to build the three-stop, 7.8-kilometre Scarborough Subway Extension project, which is expected to boost ridership by 105,000 people per day and give 38,000 people walkable access to transit.