A massive new tunnel is being carved out 40 metres below Toronto. This $380 million infrastructure project is not part of a new transit line, but an enormous new storm sewer being built as the largest flooding prevention project in the city's history.

Construction of the Fairbank Silverthorn Storm Trunk Sewer System project surpassed a key milestone on Monday, when a tunnel boring machine was launched to drill out the new three-kilometre, 4.5-metre diameter tunnel that will collect, store and move stormwater from the Fairbank-Silverthorn area to Black Creek.

Monday's milestone saw the first section of an enormous 270-tonne tunnel boring machine lowered into a 40-metre-deep shaft inside Fairbank Memorial Park near Dufferin and Eglinton.

Launch shaft at Fairbank Memorial. Photo via City of Toronto.

The project has come a long way since construction began in 2021, and the launch of a tunnel boring machine brings the trunk sewer a step closer to its projected completion in 2026.

The tunnel boring machine that will carve out the new storm sewer. Photo by Laurent Barranco.

Deputy Mayor and Chair of the Infrastructure and Environment Committee, Jennifer McKelvie, called Monday's achievement "particularly important as we see the increased frequency of impactful storms that can damage people's homes and cherished belongings."

Spanning across four different city wards, the new tunnel will relieve flooding risks for 12,500 people living in 4,645 homes, and eliminate 40 million litres of annual combined sewer backups that pollute Black Creek and other waterways in this area of the city.

At peak flow, the massive sewer will move 9,500 litres of stormwater per second to Black Creek, which, for a bit of an extreme reference, is approximately 1/300th the flow rate of Niagara Falls.

However, don't expect a raging torrent of water to turn Black Creek into fast-moving rapids, as the City states that "the new sewer will hold and store stormwater and release it gradually into Black Creek at a controlled rate."

Construction of the new trunk sewer includes a series of smaller storm sewers, totalling 17 kilometres in length, which will collect and feed the tunnel from local catch basins. These basins will contain over 320 devices that control rainwater flow, cutting down on flooding risks in surrounding areas.

The City is footing the majority of the $380 million bill, contributing $307 million, with the remaining balance funded through the Government of Canada's Disaster Mitigation and Adaption Fund.