The City of Greater Sudbury’s unique sewer system includes miles of cost-effective, engineered hard rock tunnels.
Greater Sudbury Water and Wastewater Services has picked up a trick or two from the mining industry, substituting hard-rock tunnels for traditional trunk sewers along 13 miles of its system. Unique in the world, the system has proved to be both cost-effective and durable, requiring little to no maintenance over more than a half-century.
Greater Sudbury, Ontario, is located about 250 miles north of Toronto. The area has become famous for its mining operations, harvesting large deposits of nickel, copper and precious metals from the Canadian Shield. While resources continue to make a valuable contribution to the city, the economy is diversifying across a broad range of endeavors.
Sewer construction in the city had been historically problematic. The hilly terrain is underscored by bedrock, often located very close to the surface. That means excavating for sewer construction can be difficult, and lift stations are frequently required to bridge elevations.
“We struggle with our topography and soil,” says Wendi Mannerow, water/wastewater engineer with Water and Wastewater Services, city of Greater Sudbury. “We can go from sandy to swampy to gravel to solid bedrock depending on what part of town you’re in, and sometimes depending on which side of the street you’re on.”
The city has 12 distinct wastewater systems, some constructed originally by mining companies to support the housing built for employees. The regional towns were amalgamated in 2001. While a more holistic approach is being taken to city planning, wastewater is still conveyed to 10 treatment facilities and two lagoons, employing gravity and 69 lift stations.
The oldest pipes date back to the early 1900s. They vary from about 8 to 51 inches in diameter and are made of a range of materials. The oldest are vitrified clay with other pipes made of concrete and asbestos cement. PVC is used for most new installations.
“The rock tunnel sewer system is located inside the original city of Sudbury,” says Mannerow. “Construction of the first length of about 5 miles — the Main Tunnel — was started in 1961 and completed in 1962. It was a huge decision for them to try this method of conveying wastewater, but the municipal engineers of the time recognized the opportunity that they had with the bedrock. They already had access to local mining contractors who could use traditional drill and blast methods to build the tunnels.”
Contractors worked at a rate of about 100 feet per week, removing broken rock by train. These were delivered to two of the six shafts drilled from the surface, which were designated for haulage.
The rough rock tunnels measure about 5 feet wide by 7 feet high. The tops of the tunnels are arched, while the broken rock base is lined with concrete in an invert the shape of a shallow “V.” Designed for a maximum flow of about two-thirds the height of the tunnel, the notched floor allows sewage to flow more freely during low-flow periods. Although concrete grouting is used to seal the tunnel against minor infiltration, no additional liner is required in solid rock.
“The tunnels avoid the need for lift stations,” Mannerow says. “They’re located between 75 and 100 feet deep and you just need to drop the tunnel to the right degree to employ gravity. Wastewater is simply dropped into the tunnel from collector pipes through drop shafts.”
The first tunnel provided value for the money, requiring no maintenance and performing as designed. A second tunnel followed in 1967. The Lockerby Tunnel was designed to connect the sewage system from the Lockerby area of the city to the Main Tunnel. The Minnow Lake Tunnel was the third rock sewage tunnel built. Completed in 1974, it stretched 3 miles and eliminated nine lift stations along the way.
In 2001, the city conducted an environmental assessment for an additional sewer system expansion to permit new commercial and residential development.
“We were facing a potential development freeze without increased wastewater service,” Mannerow says. “We were experiencing lack of capacity, I&I, environmental spills, basement backups and other issues more and more often. In our environmental assessment, the socioeconomic costs of traditional sewer construction and upgrades through the city were astronomical.”
The preferred alternative was construction of the 4-mile South End Rock Tunnel. Unlike typical sewer construction, building the rock tunnel caused no traffic disruptions — only occasional ground vibration resulting from blasts.
“To say the old tunnel served the city well is an understatement,” Mannerow says. “The previous tunnels had served us so well that we were using the same design parameters and the same construction techniques.”
Construction began in 2005 and was completed in 2010. It featured three access shafts and nine drop shafts, eliminating six existing lift stations. The project won numerous awards, including the 2010 Ontario Public Works Association Public Works Project of the Year and the 2011 American Public Works Association Project of the Year Award–Environment ($25 million to $75 million).
Mannerow notes that sections of the rock tunnels were inspected in 1972 and 1980, with excellent results.
“However, when we say the tunnels require minor maintenance work, we don’t mean that no work is required,” she says. For example, a more detailed inspection of the Minnow Lake and Main Tunnels in 1997 and 1998 revealed minor debris and a little groundwater intrusion. Some drop shafts revealed solids buildup while sludge buildup in some tunnel sections demonstrated slightly reduced flow.
“Those later inspections involved sending people down fully covered and armed with video cameras and air packs,” says Mannerow. “Labor regulations for confined space access are far stricter today and robotics are now the way to go for detailed tunnel inspections.” (See sidebar.)
With rising energy costs, the tunnel system also provides convenient temporary wastewater storage, as waste flows are slowed down during the day so that the city can take advantage of nighttime electric rates to pump and process the bulk of the sewage. Tunnel monitor devices allow the utility to check the depth of wastewater to prevent overfilling the tunnels.
The city is currently planning a further extension to the rock tunnel, this time a little more than 2,000 feet with one drop shaft at a cost of about $8 million.
“In 2007, the north bank of the city’s Junction Creek experienced a slope failure, causing failure of a portion of the Gatchell Outfall Sewer,” Mannerow says. “We completed temporary emergency repairs, but the sewer is still out of alignment horizontally and vertically. We were considering replacing the sewer in traditional fashion but the costs just to stabilize the banks would be in the millions of dollars. Following an environmental assessment, we’re again looking at a rock tunnel as the recommended solution. This leg will twin the last section of the original tunnel at the downstream end of the system where we collect the flow from all the sewers. If we want to divert some of the wastewater to the new tunnel to relieve the strain on the old tunnels, we’ll have an opportunity to do that.”
In the conceptual planning stage, construction is scheduled for completion in 2019.
Nick Benkovich, director of water/wastewater with the city, says that, as far as he knows, the city’s rock tunnel sewage system is unique in the world.
“We take it for granted that these tunnels are part of our infrastructure,” he says. “It’s only when you begin talking with colleagues in other districts that you recognize it as unique. It’s a good fit for Sudbury because the bedrock is amenable to tunneling and we have local contractors with tunneling expertise. From my perspective, you trade off the capital cost of installation of the rock for the reduced maintenance over the life cycle of the tunnel. Over the years, there’s been a huge cost advantage to the community.”
The city of Greater Sudbury’s rock tunnel sewer system has proved an excellent capital investment, offering a low-maintenance conveyance for wastewater. While inspections over the years show that the tunnel system is robust, some maintenance will eventually be required.
A 2012 zoom camera inspection of the oldest section of the tunnel system, built in 1962, showed some significant groundwater infiltration that may eventually be rehabilitated using cured-in-place pipe lining.
The most recent inspection occurred in 2014. It included visual inspections through drop shafts and an automated inspection overseen by D.M. Robichaud Associates Ltd. of Toronto, using cameras supplied by RedZone Robotics. It’s not a casual endeavor. Even lifting the lids of the sewer drop shafts to access the tunnels requires the use of a crane. Flow volumes through the tunnel can’t be controlled, so the surveys are also weather dependent.
“The robot is tethered, so they got to the contracted point of just under 2 kilometers surveyed,” says Wendi Mannerow, water/wastewater engineer with Water and Wastewater Services.
“The structure looks good but there’s a little bit of rock rubble on the floor and a little bit of buildup that could use cleaning. While the maintenance required is minor, simply going into the tunnels with people and equipment is always a major endeavor that requires significant planning for confined-space entry.”
Mannerow says that a more thorough survey will be completed before any maintenance work occurs. The use of robotic assistants for maintenance will also be considered.
“We’re saving up money in our capital budget to do a more thorough shaft-to-shaft survey that will show us exactly what needs to be done.”