Southland Mole of Canada tackles complex lakebed riser project with robotic demolition
In 2020, the City of Toronto embarked on the largest and most significant water quality improvement project in its history.
The Ashbridges Bay Treatment Plant Outfall project, part of the largest and most significant stormwater management program in the city’s history, sought to create a new fallout and watertight shaft to replace the existing 80-year-old system.
The project was designed to address shortcomings in the Ashbridges Bay Treatment Plant. Its outfall was where treated water from the city’s sewer system was released and dispersed into Lake Ontario. However, a shared pipe system carried both raw sewage and stormwater. The combined sewers occasionally released untreated wastewater into the waterways, increasing pollution. The outfall also had insufficient capacity and no longer met regulatory standards.
The contract for the tunnelling and concrete work was awarded to Southland Mole of Canada and Astaldi Canada Design and Construction Joint Venture. Southland Mole of Canada, an excavation company specializing in trenchless technologies and tunnelling configurations, joined the project to create the tunnel and break through the lakebed to the risers above. For such a complex project, they needed machines that didn’t require operators to stand in the danger zone.
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A new outfall was constructed as part of the project and required creating a new 16-metre onshore shaft that’s 85 metres deep and 3.5 km long with an internal diameter of 14 metres.
The project also involved excavating through the floor of the lake to locate 50 preinstalled risers, which convey treated and disinfected effluent from the Ashbridges Bay Treatment Plant into Lake Ontario.
Once the new outfall is constructed, all effluent flows from the plant will be directed to the new outfall up to the wet weather design capacity of 3,923 megalitres per day. With only 1 metre of hydraulic head available at peak design flows under the highest recorded lake water level conditions, the outfall components were designed to minimize head losses and allow the outfall to operate by gravity.
Southland Mole used a tunnel boring machine to mine the shaft. At the same time, another team on a barge positioned the risers and inserted them into the lakebed of Lake Ontario.
“The point of the project is to convey the treated wastewater, which has been processed through the wastewater treatment plant, back into the lake,” said Kevin Collins, Equipment Manager with Southland Mole. “Barges prepositioned the risers, so we just had to locate them by drilling up into the rock. Although the project is straightforward, it is not simple.”
The Southland Mole crew had to excavate upward through the new tunnel into the lakebed above to link the risers to the tunnelling system.
Collins and his crew needed to dig straight upward into the lakebed to locate the risers positioned between 1.5 to 3 metres above. The team on the barge used GPS to locate each riser, but the location wasn’t exact. This meant they needed to excavate more material than strictly necessary to find the risers. As the team worked, debris and water would fall directly onto any machine being used.
Robots recruited
The solution required enough hitting power to break through the concrete shell of the tunnel and into the Georgian Bay shale without succumbing to the pressure. They also required more precision. The risers were full of water and featured valves at the bottom of the pipe. Breaking the valve would result in a deluge of water falling into the tunnel. This not only endangered the machine, but also the operators. Fine control of the machine was crucial to guard against these dangers.
They attempted to use a large piece of equipment. However, while the larger machine broke the concrete, it lacked consistency and accuracy.
In the past, Southland Mole used Brokk remote-controlled demolition robots for tunnelling. They understood they would need a smaller, more compact piece of equipment.
That’s why they decided on a Brokk 120DII armed with a Breaker 155 attachment. The Brokk 120DII comes in a compact size of 76.2 cm wide, 2 metres long and 121 cm high, weighing about 1,225 kg. Powered by a compact and efficient diesel engine, the Brokk 120DII can run a full shift independent from any power source.
Excavating up to the risers required extensive overhead work. The Brokk 120DII boasts a vertical reach of 4.3 metres with the breaker attachment. Collins and his crew commissioned three outfall riser gantry platforms for the project. The first platform was manufactured to raise and lower with the Brokk 120DII loaded onto it, while the second and third platforms were only rated to transport personnel and equipment.
Fighting falling rock
The arm of the Brokk extended up to break through the concrete and into the lakebed above, was primarily composed of Georgian Bay shale.
However, the size of the shale wasn’t consistent. The way the shale layered, when it broke off, it would fall and break on top of the Brokk robot. So, the machine regularly battled falling rock and water while working up toward the risers. The crew ended up renting two Brokk 120DIIs. With the extreme conditions the machines were subjected to every day, the team had to stay on top of maintenance. As such, Collins and his crew worked closely with Brokk’s team on maintenance.
“We underestimated where Southland Mole’s team needed to position the machines,” said Mike Martin, Vice President of Operations for Brokk, Inc.
“They were directly in the line of fire when it came to the debris field, both for the shale and for falling water; and the arm was fully extended upward. We knew the machines would take a beating for this application. But it got to the point where we needed to send another machine up there as backup to ensure nonstop service.”
The Brokk team sent a Brokk 110 to the Ashbridges Bay Treatment Plant to ensure they could continue to work at full capacity.
“No matter what conditions our equipment is working in, we do everything we can to get the job done,” Martin said. Thankfully, the Brokk team got parts and equipment to the worksite in time to keep both Brokk 120DIIs up and running despite the demanding conditions.
“The Brokk robots were really the only equipment option to get this type of work done. To achieve the same power, we would have needed bigger machines that didn’t fit in the tunnel as well, and then we’d require handwork, which put our crew in the danger zone,” Collins said. “We would much rather have the machine itself be at risk than risking one of our crew members. While the machines took a beating from the shale and water, it was much safer and much more productive than any other option.”
The Southland Mole crew worked 24-hour days with 20 workers per 12-hour shift. The first platform that supported the Brokk carried three crew members with a second and third platform following behind.
The second platform carried the crew that completed rock bolts and mesh, while the third platform applied shotcrete to the mesh and rock before sealing the surface.
