A metre-wide hole blew open in the pipe, which runs beneath Roland-Therrien Boulevard, just before 4:30 a.m. yesterday, spewing a geyser of water into the air.
About 20 homes surrounding the break were flooded and residents there remained without water this morning.
After several hours of water erupting from the damaged pipe, the flow was stopped and the damaged section removed Wednesday. A new section in now being installed.
"For the greater part of the day yesterday we were wondering what happened," said Jacques Tetrault, spokesman for the city of Longueuil.
When the pipe was installed 50 years ago, sand should have been put on top to cover the concrete and steel, he said.
When crews started digging yesterday, they realized that wasn't the case.
"Instead of sand, what we found was gravel," he said.
The engineers believe that the gravel may have ground against the concrete over time, allowing water to enter and corrosion to occur.
"Otherwise 50 years, it's young for a pipe that can last for 100 or 125 years – it's a young lady," he said, adding there was no freezing or dramatic temperature fluctuation, which often can be the cause when water mains burst.
"It's the only explanation we can see so far thing, but there will be some [testing] done on it to find out if our thesis is right."
The boil water advisory for 190,000 homes surrounding the main break is expected to remain in place until at least Friday afternoon.
"It's a preventative measure to make sure because when pressure is cut and there is no more water in the water pipe, bacteria could invade it," Tetrault said.
"So we want to make sure that nothing will happen. . . . We're taking no chances."