Toronto is one of the many cities throughout North America and the world to have a combined sewer system. The same pipes carry both stormwater and sanitary waste. In Toronto’s case, runoff is not only from heavy rains but from snowmelt as well.

When the combined sewers overflow, outfalls are many and various: some are into the Don River, which flows through the city, some into the inner harbour, some directly into Lake Ontario. The result is a severely polluted waterfront. Currently, some 31 combined stormwater sewers have their overflows along the lake and river. Bathers are advised not to swim on the waterfront for 48 hours after heavy rain – not that those are the only occasions on which raw sewage is released.

But it is not only the waterfront that is polluted. A series of natural ravines cut through the city. They have for the most part been left undeveloped and act as parks and wild spaces. They form, in effect, an urban forest that runs through most of the city. Nearly 20% of the city is ravine, leading Toronto to be described as ‘a city within a park’. Streams run through the ravines – and outfall pipes empty into the streams. As well as recreational spaces the ravines have a – generally unspoken – function as perennial open sewers.

Simon Hopton is Design and Construction director for Major Infrastructure Engineering & Construction Services for the city of Toronto. “Water quality in the Lower Don River, Central Waterfront and Taylor-Massey Creek [one of the largest of the ravine streams] is degraded in part due to combined sewer overflows (CSOs) and stormwater runoff,” he says. “In 1987, the International Joint Commission [International as between the US and Canada] identified Toronto’s waterfront as one of 43 polluted ‘Areas of Concern’ in the Great Lakes Basin, largely because of impaired water quality and sediment conditions in the Don River and the Inner Harbour.”

DON RIVER AND CENTRAL WATERFRONT WET WEATHER FLOW SYSTEM

All of these problems are what the Don River and Central Waterfront (DR&CW) Wet Weather Flow System scheme is designed to remedy. The plan is to keep CSOs out of the city’s waterways by upgrading the capacity to capture, transport and treat outflow.

The entire scheme has many facets, from small-scale legislation – discouraging householders from connecting rainwater drains to foul water drains – to tunnels and reclamation. Five connected initiatives, totalling approximately US$2.38bn, form the major engineering elements of the scheme.

Once fully constructed, the project will include a 22km tunnel system consisting of three integrated tunnels: the Coxwell Bypass Tunnel (10.6km), the Taylor- Massey Creek Tunnel (6km), and the Inner Harbour West Tunnel (5.6km). Between them they will gather in those 31 outfall flows. There will be twelve wet-weather flow storage shafts along the tunnels, seven off-line storage tanks, and 27 connection points to receive stormwater and combined sewer overflows. Real time control will regulate flows throughout the system.

A new integrated pumping system is scheduled to be built. The two existing pumping stations are aging and need replacement; one has been in operation since 1919, the other since the 1970s. Excavated spoil from the tunnels will be used for the Ashbridges Bay Landform Project, which will create parks and play spaces and also provide erosion and sediment control at the site of a new high-rate sewage treatment facility. This is designed specifically to provide treatment of the higher volumes of CSOs intercepted by the new tunnel system. A UV disinfectant system will replace the current chlorine-based one; the treatment plant outfall will be through a fourth new tunnel of 7m diameter and 3.5km in length, with 50 risers to discharge the treated and disinfected effluent into Lake Ontario – this will be larger, extending further into the lake, and deeper than the existing 1940s outfall. It will be bored through the shale bedrock, well below the lakebed sediment that surrounds the current version. Tunnelling operations for that will be supported from an onshore shaft and the risers will be drilled from barges.

In addition to all that, a new channel will be cut to re-direct the exit of the Don River to something nearer its original course; its shoreline, currently a chemically polluted post-industrial wasteland, is to be re-natured and greened. Rather than concrete, the banks will be a ‘green spillway’ that will allow the Don to spill over its banks to form wetlands of an area roughly the same size as, and nearly adjacent to, the downtown core.

DESIGN CONSIDERATIONS

Evidently, it is no small project. Snowmelt and stormwater capture are key to the entire clean-up solution. Storage, to prevent overflow, has therefore been a major element of the design. Some 505,000m3 of storage is required on the Coxwell Bypass and Inner Harbour West system; the calculation is based on storm events from 1991 which was regarded as a typical year. Vertical storage – using in-line and off-line tanks – and horizontal storage – in the tunnels themselves by making them of oversize diameter – formed part of the overall consideration.

Eventually a hybrid strategy was chosen in which 74% of the storage volume is provided within the Coxwell tunnel itself; the rest is in five shafts, 50m deep by 20m diameter, set along the tunnel length and descending from it, and in further off-line tanks to the north of the tunnel system. A dozen smaller drop shafts and vent/adit tunnels will also help stormwater management. The solution gives robust hydraulic design. It also reduces surface disruption, and costs, during construction.

THE COXWELL

The first stage of the system, and its spine, is the Coxwell bypass tunnel. It is a 6.3m (finished inner diameter) tunnel, lined with precast tunnel segments, at a depth of approximately 50-60m below ground. Starting at Coxwell Ravine Park, to the north of the city, it runs south then east, a distance of 10.6km, to the Ashbridges Bay Wastewater Treatment Plant.

As well as transport and storage, it runs parallel to, and will provide redundancy for the existing Coxwell Sanitary Trunk Sewer – which has failed in the past – and will eventually accept a series of connections to intercept storm and CSOs from the other parts of the DR&CW scheme. The contractor is a joint venture of Jay Dee Canada, Michels Canada, and C&M McNally Tunnel Constructors (North Tunnel Constructors ULC), for project owner the City of Toronto. Lead engineers are Black & Veatch and RV Anderson .

Ehsan Alavi is the project manager. “Coxwell was the first contract in the Don River and Central Waterfront scheme,” he says. “We were awarded the project in February 2018; in August 2018 we received the notice to commence, we ordered the TBM from Lovsuns.” The timing of the TBM procurement coincided with the retaliatory tariff wars taking place between the US, Canada and China. This caused a delay in the delivery of the TBM” says Alavi.

“During this timeframe, we completed excavation of the starter tunnel, with a length of about 117 metres. The starter tunnel was utilised to fully assemble the TBM, with the vertical muck conveyor already assembled in the shaft and all the required tunnelling ancillaries in place.

“We received the machine in December 2019 and were able to launch in March 2020. Since then we have been mining continuously with the TBM.

“But tunnelling is just one part of this complicated project,” he says. “The scope of work is fairly extensive. There are five large shafts to be excavated: these are about 20m in diameter and on average 55m deep. All shafts will have cast-in-place base slabs and exterior walls with a set of cast-in-place interior baffle structures. The support of excavation for these shafts consists of interlocking secant piles in overburden and rock dowels and welded wire-mesh in the bedrock.

“In addition to these five major shafts we have 11 drop- and air-shafts which get connected to the main tunnel through a set of adit tunnels. The overall length of these adit tunnels is about 1.1km. For excavation of these adits, roadheader(s) will be used.

“The TBM is a dual-mode, open rock machine with the capability of mining in earth pressure balance with a pressure of up to six bar,” says Alavi. “The majority of the tunnel is excavated in the Georgian bay shale. In the geotechnical baseline report [GBR], a potential buried valley was identified that runs adjacent to the Don River. Areas of weathered shale and unconsolidated, saturated sands were outlined in the GBR with pressures up to six bar and as such, the contract required a TBM with the ability to mine in closed mode with an active support pressure of six bar.

“We successfully crossed this buried valley in the last quarter of 2020. We were able to mine through this zone in open mode without major issues by implementing proper mitigation measures, including extensive dewatering, plans for probe drilling and the potential switch of the TBM from open mode to EPB.

“Precast concrete tunnel lining consists of 300mm-thick fibre-reinforced universal ring assembly with six segments to a ring. We designed the segments to be six feet, 1.83m. This was to facilitate longer excavation cycles and improve our production. Segments are cast by CSI-Forterra JV at their plant in Whitby, Ontario, which is about an hour away from the project site.

“For muck removal we have a continuous horizontal conveyor and a vertical conveyor which takes the material to the surface at shaft IHES-2(B) for disposal.

“Of course, every project has its challenges, and I cannot say that it has all been a smooth ride on this project. Continuing to work through the COVID-19 pandemic has been definitely quite challenging for us. We have a great team of workers but when it comes to working through a pandemic, we had to learn how to undertake operations while taking into account COVID limitations.

“Maintaining social distancing in underground operations is extremely difficult. Shift turnarounds take longer. The access shaft has an elevator and we had to limit the number of crew members inside the elevator and limit the number of crew inside the dry houses, disinfect the work areas within the TBM and all the equipment every day. All these COVID provisions cause significant amounts of adverse impacts to the operation. However, these provisions were needed in order to safely continue with the operation.

“We did our best to manage it. We never had a huge number of employees out at the same time. We had a pre-shift questionnaire for the workers so that if they didn’t feel good, they didn’t show up at the site, and that helped a lot; but again, when some on a crew don’t show up the task gets prolonged and it impacts the overall operation.

“Another issue that we dealt with was the excavation of the second shaft, shaft LDS-3(B). This shaft is in a vacant railyard which had been in operation since the early 1900s, and there were the remains of many industrial applications around the site: oil batteries, above-ground fuel storage tanks, coal gasification and crude oil refineries. The environmental site assessment identified that we might encounter some contamination within the first 4.5m of the excavation. The shaft design was based on using secant piles around 15m deep as a temporary support of excavation for the upper part of the excavation, with the lower 3m of the piles embedded in the intact shale.

“When we started drilling the secant piles we encountered VOCs – Volatile Organic Compounds. Shaft excavation started in January 2019.” However, the excavation in shale could not be continued due to continuous VOCs and coal tar/oil seepage into the excavation. So the team developed an engineering solution to address this issue. This consisted of a new set of secant piles inside the already drilled piles with a depth of about 37m as there was no indication of contamination past this depth.

Delays caused by this issue in turn impacted the TBM operation. The TBM had been intended to break through into Shaft LDS-3(B) for maintenance. However, it arrived at this shaft prior to it having been fully excavated. In April 2021, NTC successfully reached the crown of the tunnel with the shaft excavation. The next stage of operation is to connect the shaft LDS-3(B) to the main tunnel by removing the crown segments and using this shaft as an access to support the operation for excavating the adit.

At the time of writing (August 2021), the TBM is between Shaft 3 (BB-1) and Shaft (NTTPT-1); and 6.5km out of 10.6km of the main tunnel has been excavated.

“Looking at the Coxwell project overall, we have had 16 project sites, five large shafts and 11 drop shaft locations, all distributed along 10.6km of prime inner city. Even the logistics of getting from one location to another through Toronto traffic is by itself a struggle; so, given the scope of work, and the complexity of the project, I believe we have met the project milestones thus far,” says Alavi.

The scheduled completion date for the Coxwell project is October 2023. That, as we have said, is but a part of the entire programme for Toronto’s wastewater; but when that is completed, CSOs into the Don River, Taylor Massey Creek, and the harbour will be virtually eliminated. Water quality will be significantly improved. There will be capacity for future population growth; and green spaces along the waterfront will replace post-industrial dereliction.