It wasn't like Montreal didn't have enough mega-projects on the go when the Louis-Hippolyte-La Fontaine bridge-tunnel was added to the mix in 1963.

The structure, which opened 50 years ago today, was built during the same period that saw the construction of Montreal's Metro system, Expo 67, the Turcot Interchange, along with the Décarie, Bonaventure and Ville-Marie expressways.

Fifty years later, the engineer who proposed the bridge-tunnel concept, Armand Couture, says the mega-structure remains one of the most solid and enduring road structures in Quebec.

An aerial view of the bridge-tunnel dry dock construction site in 1965. (Transports Québec)

That's good to hear considering that 120,000 cars pass through its immersed concrete casings at the bottom of the St. Lawrence River each day.

From the mind of a 32-year-old

Couture was only 32 when he proposed the concept and convinced the government of Quebec to discard its original plans for a suspension bridge.

"Engineers were respected in Quebec back then," Couture, now 86, said with a smile.

One of the prestressed concrete casings for the La Fontaine bridge-tunnel under construction. The lack of parallel walls helps to reduce noise from traffic. (Jean P. Lequy / Société d'histoire des Îles-Percées)

"It wasn't a question of age; it was a question of expertise."

Couture was inspired in part by the George Massey Tunnel in Vancouver and proposed a bridge-tunnel running along almost 1.5-kilometres of St. Lawrence seabed as the best bang-for-buck.

"The real criteria at the time, and one I really believed in, was to minimize the impact of the project on the economy of Quebec," he said.

Archival footage of the Louis-Hippolyte-La Fontaine bridge-tunnel under construction. 2:47

The $75 million project was not without its controversies, namely the expropriation and razing of the historic village of Longue-Pointe in Montreal's east end.

Practical purpose, symbolic value

The audacious project remains Canada's longest underwater tunnel, and stands as an engineering high-point in Quebec.

Historian William Gaudry said its purpose was practical, but the project also carried important symbolic weight.

"The innovation it represented was meant to show, in the context of Expo 67, that Quebec engineering was world class, was superior," Gaudry said.

The parish church in Longue-Pointe was demolished along with homes and businesses to make way for the Louis-Hippolyte-La Fontaine bridge-tunnel. (Bibliothèque et Archives nationales du Québec)

The engineering know-how that went into the La Fontaine bridge-tunnel was later imitated elsewhere, Couture said.

Lest it be compared to Montreal's crumbling infrastructure from the same period, Couture said his bridge-tunnel was a bird of a very different feather — or construction aggregate, for that matter.

The Louis-Hippolyte-La Fontaine bridge-tunnel was built from sealed, prestressed concrete casings that were fabricated onsite and submerged in the St. Lawrence River. ( Jean P. Lequy / Société d'histoire des Îles-Percées)

"In order for it to last, we didn't use the aggregate that was normally used for construction projects in Montreal," he said. "That aggregate reacts to the reinforcing steel and results in fissures that we couldn't afford."

We insisted on a special aggregate for the concrete. It was more expensive, but it resulted in much stronger concrete."