The South Halton Water and Wastewater Master Plan identified the need for a trunk sanitary sewer that could convey existing and future flows to the Mid-Halton Sewage Treatment Plant in Oakville. The existing sewage pump station and forcemain conveying sewage from Milton was at its maximum capacity. The Boyne Trunk Sewer was commissioned to replace the existing system.
In 2008, Halton retained Dillon to complete the environmental assessment (EA) and subsequent design and construction phases for the large diameter (up to 2400 mm) gravity sewer.
The EA was completed for a section between the Boyne Sewage Pumping Station (at the Waste Management Site on Regional Road 25) to the intersection of Dundas Street and Third Line, about 8 km away along the pipe alignment. Two alternatives were recommended with the second preferred as it was shorter and traversed primarily open fields.
Several environmental and construction concerns were addressed:
- A comprehensive environmental management plan was developed and mitigation by way of tunnelling through the area addressed concerns of the sewer crossing through the Glenorchy Conservation Area
- A geotechnical analysis was completed for future rail-based transit adjacent to Highway 407 ETR
- The sewer was moved away from, and access chambers appropriately placed, around active areas of the future Town of Oakville Community Park
- Comprehensive environmental studies and extensive negotiations with the Halton Conservation Authority were required to secure approval for the preferred alignment. Additional discussions were held with the Ministry of the Environment and Climate Change regarding potential bobolink habitat along the sewer tunnel alignment and potential impact during construction.
The final recommended configuration of the sewer included approximately 1700m of 2100mm diameter sewer, and approximately 5400m of 2400mm diameter sewer. The maximum depth of the sewer to invert was anticipated to be as much as 26m deep.
Dillon provided specialized expertise in the areas of natural environment, social environment, land use, agriculture, hydrogeology and groundwater, and wastewater planning and design.
Design and Tendering
Upon completion of the EA, the Region authorized the completion of comprehensive geotechnical investigations, preliminary design, detailed design and preparation of tender documents.
The alignment passed beneath a number of environmentally significant areas. These restrictions, along with the significant depth of portions of the sewer (>25m), meant that trenchless construction methods were preferred for a significant portion of the alignment.
Two months before tendering, the client asked that Dillon design an additional 1250m of 1500mm sanitary sewer at the north limit of the project along Regional Road 25 from the Boyne Sewage Pumping Station to Brittania Road. With insufficient time to obtain temporary construction easements to allow conventional open cut construction for this work, it was tendered as a trenchless construction component of the project.
Due to environmental and property access restrictions, it was determined that portions of this sewer would be installed in a tunnel advanced using a tunnel boring machine (TBM), in casings installed using jack-and-bore/hand mining technology, and with conventional open cut construction. Three TBMs were used concurrently on the project by two different contractors.
To reduce the potential for infiltration and exfiltration, watermain quality concrete pipe (AWWA C301 and/or AWWA C302) was used.
The design included a detailed hydraulic analysis, appropriate placement of manholes so as not to encroach on the environmentally sensitive areas, coordinating geotechnical investigations, a geotechnical baseline report, hydrogeologic study archaeological studies, land acquisition, HCA and MTO permits, Highway 407ETR permit, permission to enter agreements, review of adjacent regional development work (Oakville Hospital section of trunk sewer) and providing support to Halton during construction and tendering.
A carbon footprint analysis was also completed to compare the emissions from the existing pumping system to the construction of the new sewer. Analysis showed that this project was carbon negative in that the carbon emissions created during construction would quickly be balanced by the reduction of carbon emissions associated with the continued use of the pumping station.
In order to meet an aggressive schedule, contractors working on the project engaged four separate trenchless crews and two open cut crews. To meet the tight schedule and to accelerate the tunnelling operation, crews worked 24 hours a day during key times in the construction. Some construction issues that were encountered and overcome included:
- Very harsh and prolonged winter of 2015 – contractors engaged additional crews in the spring, working up to 24 hours per day to recoup the schedule.
- The shale through which the tunnel was excavated contained high in-situ lateral stresses. The tunnel remained open for a minimum of 100 days before grouting began.
- More firmly bedded shale (less easily excavated) than anticipated. Blasting of the shale layer was employed for most of the southerly open cut section.
- 2014 late start - the effect of environmental timing windows on construction was magnified on the project. Nest searches and land clearing occurred early in the project to avoid disturbing any potential nesting sites, and a temporary crossing over a regulated watercourse was installed during late winter to provide access to portions of the site isolated by the watercourse.
- A revision to the pipe alignment at MH8 was proposed to reduce the time required to re-launch the TBM. The minor grade adjustments were estimated to have saved more than 2 weeks on the overall project schedule. These adjustments were specific to the Contractor’s chosen TBM and were customized to address the land requirements along with the project hydraulics.
The project was commissioned in 2016, with occupancy of new developments in Milton to follow.
This project was recognized with the following awards: