Mid Halton Outfall Effluent Tunnel

The Mid-Halton Outfall Tunnel project focused on excavating two deep shafts and a 6.3 km rock-bored tunnel to carry treated effluent from the Wastewater Treatment Plant (WWTP) into Lake Ontario.

The Mid-Halton Wastewater Treatment Plant, located in Oakville, Ontario, serves the densely populated and rapidly growing Greater Toronto Area. To meet rising demand and continue protecting Lake Ontario’s water, it became necessary to increase the plant’s capacity. The tunnel comprised two reaches: the onshore reach spanned 4.1 km from a 63 m-deep shaft at the WWTP to a 54 m-deep intermediate shaft at Coronation Park, while the 2.2 km offshore reach extended from the Coronation Park shaft to the tunnel terminus beneath Lake Ontario. The final 300 m of the offshore reach was completed with a diffuser array consisting of 18 tunnel risers with diffuser port caps.
A tunnel boring machine with a 3.6 m excavation diameter drilled primarily through layers of shale and limestone. The tunnel was lined with cast-in-place concrete, providing a minimum internal diameter of 2.6 m. Additionally, a micro-hydro facility was installed within the WWTP shaft as part of the permanent shaft works.
 
Construction began in mid-July 2014 and was scheduled for completion within 39 months, with an approximate contract value of CAD 79 million.