Toronto has been building subway tunnels in pairs for decades. One tube per direction, two smaller machines, two smaller holes, and a method nobody had much reason to argue with. The extension now being cut under Scarborough throws that out and runs both directions of track through a single tube 35 feet across.<\/p>\n
That one decision is why the machine doing the cutting is the largest ever put to work on a Canadian transit job. It is also why the thing got famous, because a hole that big attracts attention, and in October 2021 Metrolinx handed the naming rights to the internet. The public came back with Diggy Scardust.<\/p>\n
Metrolinx’s most recent public update, in early June, put the machine past the four-kilometre mark of a 6.9-kilometre drive. That is about 2.5 miles of 4.3, or roughly 58 percent, with close to 2,000 precast concrete liner rings sitting behind it.<\/p>\n
What that update skipped is the more interesting number. Diggy launched in January 2023 and was specified to advance 33 to 49 feet a day. Three and a half years later it is a little over halfway home, and eight of those months were spent not moving at all.<\/p>\n
Diggy is a mixed-face Earth Pressure Balance shield built by Herrenknecht in Schwanau, Germany. Strabag, which holds the design-build-finance contract for the bored section, picked the supplier it had already bought more than 20 machines from.<\/p>\n
The cutting diameter is 11,910 mm, a shade over 39 feet. Behind the shield it erects a precast concrete liner with a 10.7-metre internal diameter, so the finished tunnel is 35 feet clear. The gap between those two numbers is the ring itself plus the grout.<\/p>\n
The rest of the spec sheet comes from a case-history paper written by four Metrolinx engineers for the Tunnelling Association of Canada<\/a>. The shield runs 71 metres, about 233 feet. The cutterhead carries 54 disc cutters, 19 inches across each. Sixteen electric motors make 5,600 kW, roughly 7,510 horsepower, and 48 hydraulic rams push the whole assembly forward with 103,440 kilonewtons of thrust, which is 23.2 million pounds.<\/p>\n Weight depends on what you are counting, and the two published figures do not match. Metrolinx has said the fully assembled machine weighs 2,050 metric tons, about 2,260 US tons, and likes to compare that to 42 GO train cars. The engineering paper lists the TBM itself at 1,500 metric tons. Both are correct. One includes the trailing gantries, the other does not.<\/p>\n Metrolinx opened submissions in October 2021 for its Scarborough machine and for the two borers on the Eglinton Crosstown West extension. Hundreds of names came in, a shortlist went up, and thousands of votes came back.<\/p>\n Scarborough got Diggy Scardust. Eglinton got Rexy and Renny, which are tidy nods to Rexdale and Renforth Station and which nobody has ever repeated at a party.<\/p>\n The winning entry came from Jean-Pierre Veilleux, who told Metrolinx he had been thinking about Britain’s Boaty McBoatface poll and that once he had “Diggy” the jump to Ziggy Stardust “was pretty immediate.” The assonance between Star and Scar did the rest. He also pointed out that 2022 happened to be the 50th anniversary of the Bowie album.<\/p>\n The entry that lost, and deserved better, was Diggy McDigface.<\/p>\n Metrolinx describes Diggy as the largest tunnel boring machine ever used on a project in Canada. The narrower claim is the safer one, because Canada has run a bigger machine before.<\/p>\n Big Becky, a Robbins hard-rock TBM, cut a 14.4-metre bore under Niagara Falls between 2006 and 2011, roughly 47 feet across and 10.2 kilometres long, to feed water to Ontario Power Generation’s Sir Adam Beck stations. That was a hydro tunnel, not a subway, and the design-build contract for it also went to Strabag. Twenty years and one province later, the same contractor is doing the same job with a smaller face and much worse ground.<\/p>\n Against transit machines, though, Diggy really is the biggest Canada has run. Montreal launched its Blue Line borer, Lisette, on May 19 this year, and that machine measures 9.7 metres<\/a>, just under 32 feet. Toronto’s other recent big borer, the 1,000-tonne water-tunnel machine that crossed a buried prehistoric valley under the city<\/a>, was smaller again.<\/p>\n By world standards none of this is exceptional. Herrenknecht just finished a 16.4-metre machine, nearly 54 feet, for what would be America’s widest subway tunnel in San Jose<\/a>. The finished Scarborough tube will still be the largest-diameter metro tunnel in Canada.<\/p>\n Before anyone ordered a machine, Metrolinx and Golder Associates put 366 boreholes and 575 monitoring wells into the alignment, ran 11 pump tests, 411 well response tests and 67 pressuremeter tests, and turned all of it into a Geotechnical Baseline Report the bidders had to price against.<\/p>\n The headline finding was an absence. No bedrock was anticipated in the shafts or anywhere along the tunnel alignment.<\/p>\n What is down there instead is fill, then granular sand and gravel deposits that swing between one foot and 85 feet thick, then plastic clayey silt, then broadly graded cohesive soils packed with cobbles and boulders sitting right in and below the tunnel invert. One band of silty sand runs up to 151 feet deep. Crews pulled a boulder roughly four feet wide out of the launch shaft alone.<\/p>\n That mix is why the machine is an Earth Pressure Balance shield with a mixed-face cutterhead rather than a rock borer. An EPB works by keeping the excavated muck in the chamber at enough pressure to hold the ground up in front of it. Get that balance wrong in soft, variable soil and the surface above starts to settle.<\/p>\n Plenty of coverage has described Diggy as having been stuck under Highway 401. The documents tell a more specific story, and it is worth getting right.<\/p>\n Redacted Metrolinx reports obtained by the Ontario NDP and reviewed by Global News<\/a> show the machine hit trouble almost immediately after launch, with subsidence issues recorded through 2023 and 2024. It reached the 401 in the summer of 2024 and got to the far side. Ontario’s transportation ministry closed lanes on the highway for about three weeks in June and July 2024 while it did.<\/p>\n Metrolinx has said the combination of ground conditions and wear on the machine raised the risk of soil shifting, and that the closures were a precaution posted through the province’s 511 system.<\/p>\n The repairs came after the crossing, not during it. Metrolinx documents recorded concern over “elevated cutterhead torque and thrust forces” across the machine’s final few advances, and the contractor concluded the fixes had to be done in atmospheric conditions rather than under compressed air. So crews built a safe haven, a stabilised section of ground where people could get to the cutterhead at normal pressure. That work put the drive on hold for eight months.<\/p>\n At the south end of the route, the exit is already waiting. Metrolinx finished the extraction shaft<\/a> at Eglinton and Midland in June 2025, which meant shifting Eglinton Avenue and diverting five lanes of traffic to make room.<\/p>\n The shaft goes down 24 metres, about 79 feet, with an opening 82 feet by 49. Crews drilled 240 tiebacks into the walls at angles between 15 and 45 degrees to hold the surrounding soil, dewatered the bottom and poured close to 1,570 cubic yards of concrete for the floor slab.<\/p>\n Diggy has since passed under the 401, through the Scarborough Centre station site and past the future station at Lawrence and McCowan. Everything left is the westward curve along Eglinton toward that shaft.<\/p>\n This is where a subway story turns into a highway story. Premier Doug Ford’s government is studying a tunnelled expressway running roughly 50 kilometres beneath Highway 401, and awarded a $9.1 million contract to look at whether it can be done.<\/p>\n Internally, Ontario’s transportation ministry had already flagged the risk of extensive settlement along the 401 based on what it watched Strabag deal with while crossing a single width of the same highway. NDP leader Marit Stiles has pointed at the same episode to argue the larger tunnel is not realistic. The ministry says the feasibility study is ongoing and the project remains a priority.<\/p>\n One machine needed a three-week lane closure and eight months of repairs to get across the 401 once, in ground with no rock in it. The proposed highway tunnel would run parallel to it for 30-odd miles.<\/p>\nDiggy Scardust won a public vote, and Boaty McBoatface is the reason<\/h2>\n
Canada has bored a wider hole, just not for trains<\/h2>\n
There is no bedrock down there, and that is most of the problem<\/h2>\n
The 401 crossing cost eight months and a safe haven<\/h2>\n
The hole it climbs out of has been ready since last spring<\/h2>\n
Ontario wants to try the same trick for 50 kilometres<\/h2>\n
Money and dates<\/h2>\n