Civil engineers have long understood how to build tunnels in complicated places: just think of the Chunnel or London’s Crossrail subway, which burrowed through Roman graveyards deep beneath the British capital. From a technical point of view, Premier Doug Ford’s scheme to build a 40-odd kilometre long tunnel under Highway 401 is doable. But this megaproject to end all megaprojects will be brutally expensive and glacial.
“It’s expensive, it’s time consuming, it’s slow,” says , an associate professor of civil engineering at the University of 海角社区官网and a Canada Research Chair in sustainable infrastructure. “This project is technically buildable. It’s just bad idea.”
Yet for the next two years, engineering consultants retained by Queen’s Park will grind out a feasibility study; when complete, it will reveal the complexity of burying a highway under a bloated highway that traverses a dense city built on sand and clay.
Herewith, some of the engineering issues that will rise to the surface:
Depth:听Highway 401 is an enormously heavy object built over ravines, storm sewers and buried utilities. A tunnel, according to Saxe, who worked as a consultant on the Spadina subway extension, would have to be situated well beneath existing infrastructure, such as the footings of the major bridges and the storm sewers that shunt water away from the highway’s surface. The required depth means entrances and exits will have very long ramps, thus limiting the number of access points.
Water: The bulk of the tunnel will be situated well below the water table, which necessitates extensive pumping, both during construction and in perpetuity afterwards. The need to remove water will require loads of power and add strain to storm sewer systems along the corridor.
Ventilation: A highway tunnel requires constant ventilation to expel fumes and bring in fresh air. The locations of noisy exhaust vents will invariably become contentious, especially given the fact that much of the 401 is bounded by residential neighbourhoods. “You have to have really big fans to get the air going all the time,” says Saxe. “Even as we have more electric cars, just the tire wear makes for very polluted air.” The ventilation system, she adds, “has got to be redundant, because you don’t want the fans going down in a power outage.”
Construction Access: There’s a limit on the distance tunnel boring machines (TBMs) can dig before the blades wear out. That’s why contractors building such projects typically deploy multiple TBMs advancing in opposite directions, with the goal of meeting at a mid-point.
For that reason, the 401 tunnel will likely require several TBM access points adjacent to the highway. Anyone familiar with the Crosstown dig or the Scarborough subway extension project knows these staging areas are large, and include access for dump trucks hauling away the fill, as well as reservoirs for the grey slurry that’s pumped out of the hole.听听
Vulnerabilities: Highway tunnels tend to be paired, with lanes in one direction separated from those in the opposite, The parallel tunnels are connected at regular intervals by airtight fire doors, which also lead to escape exits for emergencies. Because the risk of car accidents rises with reduced sight-lines, the tunnel also requires shoulder capacity for emergency vehicles.
The most troubling risk arises from torrential storms, of the magnitude that have inundated the lower Don Valley Parkway and Union Station in recent years. Highway authorities typically close tunnels when such weather systems are forecast, says Saxe. “If we get a flash storm, this tunnel is very at risk of filling up with water.” The increased frequency of major storms due to climate change means weather-related closures will happen regularly
Time and Money: The lay public听likely doesn’t appreciate how much time a megaproject like this would take, according to Saxe. After the two-year feasibility study, the province has to develop a procurement strategy, conduct extensive geotechnical studies, carry out an environmental assessment, award the contract, and purchase a fleet of TBMs.
“We’re looking at five to ten years before construction could even start,” she says. “Then you’ve got to tunnel the whole thing, and it’s very long, and tunneling doesn’t move that fast. On the really optimistic end, we’re looking at 15 to 20 years before this could even open.” Saxe continues: “We’re talking about billions upon billions upon billions of dollars, which we could use to actually fight congestion.”
The colossal irony, of course, is that this tunnel, if it ever opens, will rapidly fill with cars — the paradox that has long bedevilled highway building. It’s why two decades of adding lanes to the 401 only made it busier. Now seems like a fortuitous moment for Queen’s Park to internalize that tough lesson and bury Ford’s tunnel scheme once and for all.
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