It announced the lawsuit Friday upon releasing a report that said the collapse of a huge concrete slab onto an expressway happened because of shoddy preparation before maintenance work.
Transport Minister Pierre Moreau said the study revealed that the pressurized blasting of the downtown tunnel's walls was poorly planned.
"The finding of the experts that have been mandated by the (Quebec Transport Department) is the fact that the conception of the work was improperly done," Moreau told a news conference.
"It is very clear that a mistake was made during the planning of the work and that it is at the root of the accident that occurred last July 31 — and those responsible will have to live with the consequences."
Moreau said the Quebec government will seek financial compensation of "several million dollars" in damages from Cima+-Dessau-SNC Lavalin, the consortium of firms which was mandated in 2009 to prepare the maintenance work.
Last summer's near-disaster fuelled local concern about the decaying state of Montreal's roads and also some national discussion about funding levels for municipal infrastructure.
Luckily, no one was injured when the 25-tonne concrete paralume collapsed in the Ville-Marie tunnel. The span is usually filled with bumper-to-bumper traffic during weekday rush hour — but the incident happened early Sunday morning.
When asked Friday whether criminal charges might follow, Moreau said it's possible but it's up to police to decide.
The 40-page report focuses on work being done on the surface of a support wall which held up the structure known as a paralume, which ultimately collapsed.
It faults improper specifications provided to the crews that performed pressurized blasting, a process that scrapes away layers of old concrete before they can be replaced with new ones.
Put simply, too much wall support wound up being blasted away. That caused the giant paralume to cave in on the expressway.
"The collapse of the structure was the result of two situations which led to an insufficient structural capacity of the remaining concrete brackets supporting the paralume system," Moreau said.
"That's the reason why we think we have the legal grounds to make a case with this."
Laco Construction Inc., the firm hired to perform the repairs, performed the water-blasting — or "hydro-demolition" — under the supervision of the consortium.
The paralume that collapsed measured about 15 metres in length and comprised several concrete blocks designed to prevent glare as drivers entered the tunnel.
The transport minister also says his department can't take any responsibility for work that was not done properly.
"The court will have to decide on that issue as well," Moreau said.
The report did not find any fault with the original 1970 construction plans for the tunnel, which were up to code.
At the time of the accident, police said it could have had disastrous consequences had it had occurred on a weekday. About 100,000 vehicles use the expressway during an average weekday, according to Transport Quebec.
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