Shovels in hand, people contracted by the city will soon perform a complex and costly rescue deep under the streets of Toronto.
They won’t be freeing a raccoon from a sewer. Instead, it’s a “micro-tunnelling boring machine” stuck underground for nine months after accidentally chewing into buried construction waste.
If this was a Pixar film, the digger would have a cute name, a backstory rife with challenges, and a harrowing account of how its big break — tunnelling a storm sewer to help protect homes from basement flooding — went so awry.
Not only did it veer off course, it got stuck and dangerously destabilized the earth above it.
The city is just hoping to get the device above ground, finish the tunnel and learn from a $9-million reminder that the soil under Toronto is rife with unseen construction material, as well as utility cables, creeks and more.
The boring machine — cylindrical, about five metres long with a 1.5-metre-diameter “cutting wheel head” — was lowered into the ground a year ago at Old Mill Drive at Riverside Drive for a 282-metre grind route to just north of Bloor Street.
Being installed right behind it was a storm sewer to divert excess rainfall from homes in the Old Mill area, one of several flood-proofing projects being undertaken by the city as aging infrastructure is overwhelmed by increasingly intense storms.
The digger, tethered by cables, stayed mostly on course, chewing its way under the Line 2 subway tunnel to seven metres short of its destination when, on June 9 it hit “tie-backs” — steel wires left from foundation work on nearby condo buildings.
“The cutter head started to chew into that wire,” says Mika Raisanen, a director in the city’s engineering and construction services department.
“And it just became a became a bunch of spaghetti and got wrapped up in the machine. And the machine couldn’t advance anymore.”
Sensors on the digger that are used to guide it raised the alarm for Earth Boring Co., the subcontractor doing the work. The sewer pipe is smaller than the digger, which couldn’t just be pulled back out from the entry hole.
Emergency plans began to form.“We had to rethink and refocus on how we’re going to achieve this,” rescue, says Raisanen.
A two-metre-diameter pipe was put down the exit hole. Workers shovelled and used a hydraulic jack inside the pipe, extending it forward to protect them until they hit the wire-snarled cutter face.
But around it they found groundwater and soft, destabilized earth that triggered a sinkhole above, a situation a city report calls “a matter of extreme urgency, as there was a significant health and safety hazard to the public.”
Extrication efforts were put on hold while a ground stabilization firm was called in. That work is mostly done, Raisanen says, setting the stage for rescue of the boring machine, finally, around early April.
Workers will keep digging and move pipe forward until it surrounds the boring machine. In a Pixar film, the plucky device would help free itself and rocket to the surface amid relief and cheers.
In reality, workers will dismantle it and carry up the pieces.
The sole-sourced emergency rescue contract with primary contractor Clearway Construction Ltd. totals about $9 million — an unwelcome surprise for a city so far unable to fill massive budget gaps triggered by pandemic costs and revenue losses.
Raisanen says the condo developer did nothing wrong, it had city permission to bury the tie-backs used in early stages of parking garage construction. The city map warning of such hazards failed to show the wires.
In a metropolis bursting with condo development, “the trend is that there’s more and more utilities being installed,” says Raisanen, “so certainly we need to take a close look at where there are potential conflicts such as tie-backs,” and get them all charted.
The storm sewer will be completed, so the digger’s work will not be in vain.
The trapped tiller doesn’t have a name, Raisanen says, unlike its giant, more glamorous cousins that are rewarded for digging huge subway tunnels with crowdsourced monikers such as Diggy Scardust.
“No nickname,” he says with a chuckle, “but it will see the light of day.”
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