Trenchless on a Large Scale The Mid Halton Outfall Project
Halton Region is located in the province of Ontario, west of the city of Toronto. Halton has been named one of the fastest growing communities in Canada with growth expected to continue through the year 2031.
To accommodate growth, an increasing volume of wastewater is being directed to the Mid Halton Wastewater Treatment Plant (WWTP) in Oakville, Ontario. Halton is expanding the WWTP in stepwise phases with growth. As part of the current WWTP expansion phase, Halton was required to construct a new effluent outfall. To reduce the impacts of construction to adjacent residents, businesses and the environment, the project Environmental Assessment recommended the new outfall be constructed entirely by tunnelling. While the Region of Halton has long been a proponent of trenchless construction methods including tunnelling, the scale of this project in terms of diameter and length made it a significant challenge. The internal diameter for the outfall is 2.6 meters (8.5 feet). The alignment length for the outfall is 6.3 kilometers (3.9 miles) including 4.2 kilometers (2.6 miles) of onshore tunnel and 2.1 kilometers (1.3 miles) of offshore tunnel.
This paper will outline several of the challenges that were addressed during design and construction of this unique project. Some of the challenges discussed include: tunnelling at significant depths (50-60m/165-200ft) below ground surface in horizontally-bedded shale bedrock known to have high in-situ horizontal stresses, mining a 4.2km reach of tunnel downhill and without an intermediate shaft, and mining a blind offshore tunnel to intercept 18 risers which had been pre-drilled into the lakebed.