Among the ornate buildings of Ottawa’s Parliament Hill, just to the west and set back from busy city streets by an expanse of lawn, Canada’s Supreme Court building, it is said, provides a dignified setting worthy of the country’s highest tribunal.
Perched on a bluff over the Ottawa River boaters this summer were privilege to more than just the view of the federal government’s storied edifices. Just behind the court is the reception site for a 3.71m diameter TBM creating Ottawa’s new Combined Sewage Storage Tunnel (CSST).
Into the bright sunlight the Herrenknecht TBM emerged from the rocky cliffs in mid August where the contractor JV of Dragados/Tomlinson began to dismantle and mobilise it to start the next drive.
The City of Ottawa is building the CAD 232.3M (USD 178.3M) CSST to reduce the frequency of sewage overflows from entering the river during storms. Underground construction includes roughly 6km of tunnels divided into two drives, a north-south tunnel (NST) and an intersecting east-west tunnel (EWT).
The project design team consists of Stantec Consulting in conjunction with CH2M. Subconsultants to Stantec are Golder Associates (geotechnical consultant) and McMillen Jacobs Associates (part of the team’s quality and risk review panel).
Staging from the start
Figure 1 shows a map of the two tunnels and various access shafts, drop shafts and diversion chambers spread across the city. Almost every site is located on either city, provincial, or federal property and that includes a lot of park spaces and vacant parts of the city.
“It’s not a coincidence that we are using a lot these public properties,” says City of Ottawa’s Steve Courtland, program manager CSST. “We designed this system around available spaces.”
Tunnelling started in December 2017 with the NST drive from Site 10 to the smaller Site 6 at the base of the Supreme Court. On the EWT where the TBM will be extracted at Site 1 the area is currently occupied by construction work for the city’s new light rail.
Shafts have been constructed through the overburden into the bedrock, and the contractor has been using secant pile shafts to create a waterproof structure at most locations across the city.
There are clays that overlap the rock in multiple locations around Ottawa, and some are considered sensitive, that is unconsolidated, says Colin Goodwin, Stantec’s deputy contract administrator for the CSST, and elsewhere there is glacial till.
“The tunnel is entirely in bedrock—a combination of limestone primarily and then there is shale in some other areas,” he says. The depth of the tunnels from the surface is approximately 10-31m, and most shafts are around 20-30m deep.
“The rock cover changes throughout the alignment depending on the bedrock’s surface but tunnels themselves are on a pretty set slope,” Goodwin explains.
Previously built
Above and below the tunnel drives there are numerous concerns for existing infrastructure. Goodwin splits these into several groups; there are structures like large buildings above ground that are in areas where there is sensitive, unconsolidated clay.
“During design we were concerned about depressurising the clay, leading to consolidation and potentially settlement of buildings.”
For this reason the design specified TBM excavation with a pre-cast segmental lining to reduce the potential of groundwater drawdown. The steel fibre reinforced segments are six pieces to a ring, 3m id and manufactured locally by Power Precast Solutions.
“The single pass TBM is the primary mitigation measure,” he says, adding, “in terms of infrastructure there’s a large program of geotechnical monitoring—everything from groundwater level monitoring, surface points installed on buildings, tilt meters, and seismographs.”
Underground, and below and above the new tunnels, there are existing sewer interceptors and the city’s new LRT tunnel, which is still actively in construction—though not tunnelling.
“In that case during design we looked at rock redistribution analysis, what would happen when you have the amount of cover we have between the two tunnels,” explains Goodwin, “and virtually there’s no impact from that.”
Inspections of the LRT were carried out prior to passing below it, which the NST drive did successfully with about 4.5m of cover.
“We pass under the LRT twice, and we also go underneath the Rideau River and the Rideau Canal,” Courtland explains.
The EWT tunnel will cross under the LRT with around 8m of cover. Another 4m of cover is expected under the Rideau River, which snakes through the city reaching a confluence with the Ottawa River in the east, near the TBM launch shaft for the next drive west. Goodwin explains there is significant cover over the Rideau Canal.
More challenging was crossing over a critical sewer built in the 1960s. “It’s the backbone sewer for Ottawa. It carries something like 70 per cent of the sewage from the city,” Goodwin explains. “There was only about 0.9m clearance between the two.
“Despite having done a lot of analysis through modelling during design, we still had obvious concerns passing over that to do with unknowns of how they excavated that original tunnel in the first place.”
The design team knew it was a drill and blast excavation but it had no information on whether it was unreinforced or if the original construction installed rock bolts.
“We came up with some pretty detailed emergency response plans in the event that something would happen impacting that sewer, both within the city’s emergency management team and wastewater collections as well as with the contractor.”
There was virtually no impact during the tunnel drive over the sewer. “We had live CCTV inside the interceptor sewer during the crossing,” Goodwin says. “We slowed the TBM down during that period; otherwise it went well.”
As the drive proceeded north after the crossing they did encounter existing rockbolts, and the TBM negotiated those just fine. They also encountered a second existing tunnel, which the team refers to as the cavern, around 200m or so from the termination of the NST.
“That was built at the same time as that interceptor that we crossed over,” Goodwin explains. “It was hand mined and left open below ground. So upon finding this we had to go and fill that cavern with high strength grout, around two weeks before our TBM proceeded to bore through the cavern.”
Access to the unlined and unreinforced cavern, measuring approximately 170m long with a 2m diameter, was very limited.
“You couldn’t enter the cavern physically, and it was located on federal property,” Courtland explains.
There are approvals required if they’d wanted to access the site to do the work, and there was limited time before the TBM would arrive.
He explains, “when we were preparing the cliff to receive the TBM we could get pretty close to the cavern and so we launched some exploratory bore holes to give us an idea.”
This is the portal located at the Ottawa River, behind the Supreme Court.
“They did horizontal drilling from the cliff face of about 20m into that existing cavern,” Goodwin explains.
These would also be used to pump grout into the space, as well as a number of other exploratory boreholes on the surface.
“We put CCTV inside, figured out what was going on, figured what it looked like,” Courtland adds. “We developed a plan to fill the cavern with grout via these exploratory boreholes and we put down a few new ones.”
The contractor pumped 500cu.m of high strength grout into the cavern, a job that took about two days.
“It was probably about a month and a half before we filled it, from when we became wise to it,” Goodwin explains. “We only knew there was an existing sewer within that cavern that had to be backfilled before we got in there with the TBM but it was unclear based on existing data whether that cavern was backfilled or left open.”
We kind of had a hunch, he says.
Court date
Site six isn’t a shaft, but more of a receiving pad about 16m long on the face of the cliff. The contractor started by removing loose overburden material, to square up the cliff face, and make the pad to receive the TBM. From there the contractor walked out the TBM after its breakthrough in August.
“In most of Parliament Hill, there’s the river below the government buildings but in our case at the Supreme Court there’s a bit of a parking lot before the river,” Courtland explains.
That, though small, gave the contractor enough space to receive the TBM. By early September the jv had mobilised the machine to the next launch site in the east.
Upon completion in 2020 the NST and EWT will intersect creating one storage system. This design allows the city to reduce the potential of flooding in residential areas with a history of flooding along the north to south tunnel.
The two tunnels will hold up to 43,000m3 of sewer overflow during major rainfalls, which will then be treated and returned safely to the Ottawa River.
In the rare event the system does reach capacity during a storm event it will overflow, at site 6, into the river rather than flooding residential basements.
“The Government of Canada recognises that modern, reliable water and sewer infrastructure is essential to developing healthy and sustainable communities,” says Catherine McKenna, minister of environment and climate change.
“By supporting major wastewater infrastructure initiatives, like Ottawa’s combined sewage storage tunnel, we are investing in green infrastructure that will help protect communities and the environment, benefitting Canadians for generations to come.”
