In 1987 Toronto's waterfront was cited as one of 43 polluted areas of concern in the Great Lakes Basin, largely due to poor water quality conditions in the Don River and the Inner Harbour.
An environmental assessment found that a main source of water pollution is stormwater runoff and combined sewer overflows released from outfalls into the waterways after heavy rains or snowmelts.
The solution is the City of Toronto’s biggest tunnelling programme – the Don River and Central Waterfront (DRCW) project, which will reduce wet weather flow overflows into the Inner Harbour of Lake Ontario, the Lower Don River and Taylor- Massey Creek.
Two of the tunnels proposed for the 22km-long system cross buried bedrock valleys with reduced rock cover in several locations.
“The risk of encountering weathered bedrock, significant groundwater inflows or surficial soil deposits through these buried valleys was thought to be significant,” says Daniel Cressman, Toronto area tunnel lead for Black & Veatch’s Water Division, which, in association with R.V. Anderson Associates Limited, is undertaking detailed design and contract administration for the tunnel project.
He explains, “The typical approach in the Greater Toronto Area has been to excavate the Georgian Bay shale with an open faced main beam type rock TBM and install temporary support directly behind the main shield.”
The final lining, cast-in-place concrete, is typically installed a minimum of 100 days after excavation. This two-pass system allows stress relief and swelling or time-dependent deformation of the rock mass to take place prior to installation of the final lining.
“As an alternative, the design team looked at the use of a single shield rock TBM with a precast concrete tunnel lining to mitigate certain risks associated with the two-pass system,” he says.
The DRCW will be one of the first applications of precast concrete lining in Toronto’s Georgian Bay shale. Each stage of tunnel construction presents unique challenges.
The Coxwell Bypass Tunnel is significantly larger in diameter and longer in length than tunnels historically constructed in the Georgian Bay shale of Toronto. On the Taylor-Massey Tunnel, the soft ground tunnel will cross underneath an existing sanitary tunnel, with under 2m of cover, and will require TBM operation at face pressures exceeding 5 bar. On the Inner Harbour West Tunnel, the potential exists of encountering a buried bedrock valley that could result in non-uniform excavation conditions for this segment of the project.
On location
The new system includes 22km of tunnels ranging in diameter from 4.4m to 6.3m. Over the length of the tunnel, the geology and associated challenges change significantly, Cressman explains. Ground conditions include shale rock of the Georgian Bay Formation and soft ground soil consisting of glacial till, glaciolacustrine and glaciofluvial sand, silt and clay deposits.
The majority – 16 km – of the tunnels is anticipated to be located in the shale bedrock. “The shale bedrock has a high horizontalto- vertical stress ratio and is known to exhibit long-term swelling behaviour when excavated and exposed to fresh water,” he says. “The vertical profile of the rock tunnel requires tunnelling through locations with reduced rock cover and has the potential to encounter several buried valleys consisting of soft ground soils. The tunnel also crosses the historical alignment of the Don River (it was realigned in the late 1800s) in several locations.
The remaining 6km of tunnel is anticipated to be in soil. The soft ground section of tunnel is expected to be excavated through glacial till layers separated by interstadial deposits of sand, silt and clays. The glacial till layers are known to present hard and abrasive tunnelling conditions, while the interstadial deposits, present below the water table, have the potential to run into the tunnel face if not properly controlled during excavation.
“Another challenge is that the land use changes significantly along the tunnel alignment,” Cressman says. “The tunnel moves from an industrial area currently undergoing rapid development to the high-density residential areas of downtown Toronto and the Lower Don River and then transitions to parkland in the upstream portions of the alignment along the Don River. The presence of diverse stakeholder groups has created unique challenges related to stakeholder engagement and coordinating and obtaining project permits and approvals.”
The tunnel alignment has been selected to follow the Inner Harbour of Lake Ontario, the Don River and Taylor- Massey Creek to facilitate connection of the existing outfalls to the tunnel system (instead of discharging to the various bodies of water as is currently happening).
The system will convey flow by gravity to a new pumping station at Ashbridges Bay Wastewater Treatment Plant. The depth of the pumping station is governed by the hydraulic conditions and the availability of screens to accommodate sediment removal. “At the upstream end, the new tunnel must be able to divert flow from the existing Coxwell Sanitary Trunk Sewer into the tunnel,” Cressman says.
Within these criteria, the tunnel alignment and depth have been determined by considering a number of factors including cost, geotechnical risk, utility conflicts, constructability concerns and the hydraulic performance of the system.
The tunnel slopes were designed to maximize sediment transport characteristics through the tunnels and move sediment down the tunnel system to the new pumping station. The established slope places the tunnels at a relatively consistent and significant depth of approximately 50m. The vertical alignment of the tunnel was selected to reduce the number of buried valley crossings and ensure that utility conflicts were avoided.
The new tunnel crosses an existing water supply tunnel, which was constructed in 1929, in two locations. “The alignment was selected in coordination with a finite element analysis that was performed to ensure the risk of damage to the water tunnel had been mitigated to a very low level,” Cressman says. “All these factors were considered while trying to minimize the long-term operation and maintenance cost associated with pumping from a significant depth.”
Tunnels and Shafts
The Coxwell Bypass Tunnel and Inner Harbour West Tunnel will be excavated through shale bedrock of the Georgian Bay Formation. They are proposed to be constructed with a single shield rock TBM and lined with precast concrete tunnel lining.
“This method was chosen to mitigate risks while crossing buried bedrock valleys, including the risk of encountering weathered bedrock or incurring significant ground water inflows or surficial soil deposits through buried valleys,” Cressman explains.
The Taylor-Massey Tunnel will be excavated through Quaternary (surficial) deposits and will require use of a pressurized face TBM, either EPB or slurry. The tunnel will be lined with a precast concrete tunnel lining. He says, the use of a pressurized face TBM will eliminate the need for active dewatering and mitigate the risk of ground loss and surface settlement.
The proposed shaft excavation and support methods consist of impermeable support through the surficial soils to mitigate the use and impact of dewatering. The proposed impermeable support consists of either secant piles or slurry walls, depending on the depth of shaft.
“The impermeable soil support is to be installed a minimum of 3m below the soil rock interface and into good quality rock to cut off the ground water,” he says. “Below the impermeable support, the use of welded wire mesh with rock bolts is proposed as the temporary rock support. The final structure of the shaft will use the excavation support lining as the outside form of the structure, which will eliminate the need for fill and provide the diameter required for hydraulic performance.”
The project is currently scheduled to be procured in five stages: three separate stages of tunnel construction, one for wet weather flow connection structures and one to construct offline tanks and prevent flow surcharging north of the tunnel alignment. The first stage of construction, the Coxwell Bypass Tunnel, is scheduled start prequalification in early 2017, with tender documents released to prequalified bidders in late 2017. The subsequent stages will be scheduled with available funding.
