Small footprint, big impact

Toronto’s Fairbank and Silverthorn communities are located in a low- lying area prone to chronic and devastating basement flooding. “The entire catchment area is almost bowl shaped and mostly serviced by combined sewers” explains Simon Hopton, director, design & construction – major infrastructure, City of Toronto, “and that’s why this area has a history of basement flooding problems—when it rains it all funnels into this area, overloading the combined sewer system.”

Lowering the first section of the EPBM into the shaft. All photos courtesy EBC Bessac Joint Venture.

The flooding is also due, in part, to overloaded combined sewer systems and overloaded surface drainage systems.

The City of Toronto has implemented an extensive Basement Flooding Protection Program and has established 67 study areas to examine these issues across the city. This project is focused on one of the most heavily affected areas. A study was conducted in 2011 to identify solutions and the recommendation, among other solutions, is a new storm tunnel along with new local storm sewers and other subsurface infrastructure.

This new storm system will collect, store and convey stormwater from the neighborhoods to a new outfall at the nearby Black Creek, a tributary of the Humber River. Once in place, the system will reduce the risk of basement flooding, protecting more than 4,600 properties across 140ha (345 acres), as well as reducing incidents of combined sewer overflows polluting local waterways.

“We actually had two one-in- 100-year storm events locally during the design process,” says John Kinnear, project manager with Jacobs, the firm tasked with designing the system. “Photos and reports showed significant flooding at these locations where we have now built shafts that will convey the water to the tunnel. It was quite interesting, witnessing firsthand the exact problem that we’re here to try to fix.”

Phase one is currently under construction for the Fairbank Silverthorn Storm Trunk Sewer System. This is being delivered through a design-bid-build method and the owner awarded the C$184 million ($135m) construction contract in 2021.

It’s the city’s largest basement flooding protection project so far and completion is scheduled for 2025. Phase two is currently open for bid and concerns 25km of storm sewers, storm leads and other improvements to be completed in 2027.

Hopton emphasizes that while this project is large, it is only one part the city’s wider basement flooding protection program, which has invested more than C$1 billion on construction and associated activities supporting engineering, design studies and flow monitoring among other work within the last five years.

With the frequency of major storm events on the rise, combined with the city’s fast-paced growth, the TBM launch in 2023 marked an important milestone for this program as the contractor started the 2.4km- (1.5mi-) long drive for the storm tunnel. There are also 10 microtunnel drives totaling nearly 2km (1.2mi) and 20 shafts among other subsurface infrastructure to build.

Good neighbors

This 2.4km-long tunnel is being excavated in soft ground and will have a 4.5m (14.8ft) inner diameter. It connects to four drop shafts, the deepest of which exceeds 40m (131ft), and those connect to a secondary network of 1.8m- (6ft-) diameter sewers excavated by microtunneling. All this work

has been bid in one contract, which is being performed by a joint venture of EBC/Bessac, with Ward & Burke as subcontractor for the microtunneling and shaft construction.

The Fairbank and Silverthorn neighborhoods of Toronto are well established with many apartment buildings and single-family homes making up the area—though there are local businesses and main roads as well.

“We worked to minimize the amount of infrastructure that we were having to put on the main roads,” Kinnear says, explaining the neighborhood has recently had underground construction on its doorstep for Toronto’s new Eglinton Crosstown light rail project. “We were very cognizant that the residents are immediately adjacent to that site and have had a lot of construction and we’re coming in to do even more.”

The design team needed to mitigate the impact of having two major tunneling projects under construction concurrently in the same general area. “Some of that is quite subtle—making sure our truck routes are coming from the south not the north so we’re not putting traffic in the same place.”

However, other decisions were more deliberate such as minimizing the infrastructure that would be built as well as locating it in smaller streets to avoid further disruption on the city’s main roads. “Unfortunately, these local streets are narrow,” Kinnear says. “Most are single lane in either direction with parking.”

This means the available workspace for underground construction is extremely limited. It also influenced both the tunnel and TBM design. To stay within the public right of way, passing under streets and avoiding private property, the project features something of an unique alignment.

“We have very tight curves on the main tunnel, which is a little bit unusual for a hydraulic project,” says Bernard Catalano, Bessac tunneling business development manager for North America.

“You need an unusual TBM and a specific segment design that can pass through this alignment.”

As a tunneling contractor and a TBM manufacturer, Bessac was up for that challenge. The project specified an EPBM to cope with the soft ground conditions under high groundwater pressure, a firm clay with sand lenses and multiple boulders. The JV knew it would need to procure a new machine and held an open tender for the TBM. Using a Bessac-designed and manufactured machine presented more advantages than the competition, Catalano confirms.

The EPBM with three cans

His colleague, Romain Abbad, deputy project director for the JV, has been with Bessac for 15 years, working on tunnel projects around the world. “From the beginning, the main risk of the project was the capability of the machine to take these very tight curves,” he says, explaining several of the

tunnel’s curves have a narrow radius, and one of around 120m (390ft), and as a consequence, the TBM and segmental ring design accommodated 90m to allow for additional tolerances.

“We had to think about the whole machine, not just the TBM shield, but also all the back-up gantries and all the segment carriers that have to follow along.”

Along with two copy cutters mounted on the cutting wheels, the solution is a TBM shield divided into three different cans or sections, and each measures fewer than 4m (13ft) in length. The front can has an active articulation and the back can has a passive articulation, allowing the machine to navigate the alignment’s tight curves.

Microtunneling work on a residential street.

However, these separate cans created another issue as there wasn’t enough space for the airlock.

“We had to create an intermediate chamber where we could put all the motorization, but we didn’t have enough space to put the air lock inside there too. We modified the design to move back the airlock through the length of the cans,” he explains.

Abbad says among all the projects he’s worked on around the world, he’s not seen any with these types of S-shaped curves that would require a TBM such as this. “We’re talking 120m to the right, and then going 120m on the left, with only 7m of straight line between the two curves. But we did it and we are very proud of that. We are inside our tolerances of 75mm. Our maximum deviation is 50mm from the beginning of the TBM drive.”

Tight working space

As with many tunneling projects located in cities, working among a residential area leaves few options available for staging worksites.

The project is borrowing space from, among other locations, a high school parking lot, local baseball field and a nearby church. Eagle-eye readers may even spot hoardings extremely close to residents’ front doors.

“We knew from the beginning that it was a very tight space for construction,” says Camilo Quintero, principal tunnel engineer with Jacobs and design manager for the tunnels and shafts. “There was no opportunity to provide the contractor with more space and limit the impact on the public. The contractor has done a very good job with the equipment.”

Further complicating the space challenges, the original launching method with a 35m (115ft) long tail tunnel for the TBM’s muck skip operation had to be scrapped and a new plan developed last minute, which required, among other things, modifying the TBM and finding any solutions to expedite the process.

This had a particular impact on muck removal—until the TBM was fully assembled and in its final configuration, there was limited space on site and in the 12.5m (41ft) diameter shaft. To cope with this, the team developed a logistics cycle in the shaft and tail tunnel to operate two locos, working in phases, to deliver segments and lift out the muck skips.

“It’s not an innovative technique if it’s planned from day one, but we had to develop all of this in three months,” Abbad explains.

“We started excavation with a muck pump, and once we had enough space to put in the back up we removed it to use the gantries and put in the conveyor.”

Once the TBM had been fully launched and assembled, and had taken the first few curves, he reports tunnelling operations began to resemble more of a more standard project. At the time of writing, the trunk tunnel excavation is more than a third of the way through the drive, achieving a daily rate of 10 rings in 24 hours.

Adits

The project includes four adits connecting the four drop shafts to the main tunnel. Due to their short length ranging from 4–12m (13–40ft) and small section of less than 5m (16ft) in diameter, the contractor needed to hand mine the adits through the soft ground geology, which couldn’t be efficiently treated from the surface level due to the high clay content.

Working with the engineer of record and the owner, the team suggested changing the original square shape to round to optimize the excavated section, reducing it to less than 3m (10ft) diameter.

With 3.5 bars of water pressure during excavation and to mitigate any large face instability, the project team implemented a 139mm
(5in) OD canopy tube system at 10-degree spacing on the upper half of the excavated section.

The concrete lining is made of microtunneling jacked pipes with a dual purpose—securing the hand mining operation and providing a permanent lining.

The adits are constructed before the TBM passes through the area. The next stage will consist of bracing the internal structure of the tunnel in order to partially remove three lining rings and connect to the adit. Abbad explains, “this method also allows us to hand over the compounds earlier, as the works are not linked with the main tunnel excavation, minimizing the impact on the neighborhood.”

Adit construction as seen from the surface

Prepared for soft ground

The trunk tunnel excavation passes under a subway line while the microtunnel excavation crosses over a light rail tunnel. “During the design we analyzed potential impacts on these lines due to the deformation of the ground,” Quintero says, adding that the project passed all of its testing.

There is also extensive monitoring and instrumentation in place and the teams are working closely with the local transit agency’s staff and consultants. There are five monitoring points, installed up to 35m (115ft) deep, Dave reports.

“They are just about one meter away from the tunnel so that any settlement or ground movement can be captured earlier.”

More than 50 monitors have also been installed within the rail corridor to check the differential settlement between the tracks and other ground movement. For the contractor, the soft ground conditions also presented the potential risk for clogging due to the high content of clay in the geological formation.

This had an impact on the machine’s design, including for cutting wheel components on the injection additives. Accurately tracking the boring parameters is also paramount and the team has installed a TBM data acquisition system (CAP). The firm’s software collects data and tracks parameters from sensors installed on the TBM. Through automatic analysis, it provides a calculation in real time of the levels of risk of clogging. “I can have a sense of the risk of clogging and react a little bit earlier,” Abbad explains. This supervisory set-up has reduced the risk of clogging thus far on the project.

Adit construction required hand mining

Connecting the Black Creek outfall

The final component of the project will be tying into the outfall at Black Creek. Hopton highlights the discharge point is a very sensitive area. The City of Toronto has existing infrastructure there, including three sewers and a storm tank, and the design effectively needed to weave through the structures without causing too much disruption.

Another issue is that where the system will be discharging, portions of Black Creek are manmade, constructed out of cast-in-place concrete. Prapan Dave, manager, design & construction – major infrastructure, City of Toronto, explains there is also very limited capacity in the creek, which is prone to overflowing. “We have a restriction not to discharge into this creek at a rate that is more than 8m3 per second. That’s why this tunnel is large, because it also stores the stormwater flow and only discharges at a controlled rate.

“Otherwise, if we discharge at a higher rate, we’ll just be transferring the problem causing more flooding in downstream areas.”

Microtunneling

Another major part of the Fairbank Silverthorn Storm Trunk Sewer System includes smaller diameter sewers. There are four main microtunnel drives of 1.8m internal diameter, with some comprising several shorter drives. These have been spaced over a roughly 12-month period from October 2022 to November 2023.

For this portion of the work, 20 shafts in total have been excavated for both permanent structures and temporary facilities for launching and retrieving
the MTBM. There is even less space available for the project’s microtunnelling operations, explains Sean Enright, surface project manager with EBC.

“It’s a very heavily congested residential area,” he says. “It’s very narrow. We’re installing compounds in intersections of a street. There’s really no space between the houses.”

Working in an environment with so many residents in close proximity, the team had systems in place to mitigate noise, dust and other disruptions from construction. However, one of the key efforts to reduce impact on the neighborhood came from changing the shaft excavation methodology.

As the Fairbank Silverthorn Storm Trunk Sewer project is a gravity system, shaft depth varied from 9–46m (30–150ft) depending on where the microtunneling drives connect to the main trunk tunnel. The original design for the primary support of excavation for the main tunnel shafts and deep shafts called for secant pile walls, for which vibrations could be disruptive to the surrounding residents.

Subcontractor Ward & Burke suggested sinking caisson shafts as an alternative. “It’s good for everyone involved,” Enright explains. “It’s top-down construction where the shaft literally sinks down to its final formation level, and they can do it without dewatering, which is another benefit.”

Even when the excavation encountered water in the shaft, crews only needed to use a clamshell bucket with a crane to remove the material. Once the excavation reaches its depth, a base is poured with tremie concrete.

He explains that sinking the caisson shaft also resulted in a more rigid system, “which is very helpful when you need to push against it to propel the TBM and the MTBM at launch.”

It also presented a stronger temporary shaft that’s safer for the workers as no one is present in the shaft during excavation, and offered other advantages for reducing the potential for issues related to dewatering, vibration and settlement.

As for the microtunneling drives, everything went well, he reports, adding “They could have been more challenging if we didn’t do the sinking caissons because it’s a flood zone. There is a lot of water as the ground isn’t fantastic in some of these areas, and with the methods that we did and the machinery we used we were able to do all the work without any settlement.”

All the microtunnel drives are completed, as well as all the shaft excavations. Enright’s team is working on the secondary lining for permanent shafts as they become available, and the project continues to move forward.