A water main break that caused a major flood in downtown Montreal has been attributed to shifting ground caused by construction work, according to the results of an investigation by City of Montreal officials released on Thursday.

In a statement, city officials said close to 40,000 cubic metres of drinking water — enough to fill 17 Olympic-sized pools —gushed from an 88-year-old, 120-centimetre wide broken main onto city streets in about two hours on Monday afternoon.

Moments before the water main broke, crews were working at a construction site at the McTavish reservoir near McGill University's downtown campus.

The city's water service said it's possible the construction work near the water main caused the soil to shift, which led to the break.

Saeed Mirza is a professor emeritus at McGill University who specializes in structural engineering and the rehabilitation of infrastructure.

Mirza said the city should have known the pipe was vulnerable and taken the necessary precautions.

"It's nearly a 100-year-old pipe. If the pipe was weakened, then any soil movement could easily fracture it," he said. "Some engineer should have visualized this earlier."

Four aqueducts, including the one that leaked, were set to be replaced as a part of a major underground rehabilitation project near the McTavish reservoir. The construction work began mid-January.

McGill 'suffered tremendously'

"I'm a member of McGill, and we suffered tremendously on Monday. I think I have the right to know why it happened," Mirza said.

Michael Di Grappa, the vice-principal of administration and finance for McGill University, said the flood has had a major impact on thousands of staff and students.

He said it will take weeks, or possibly months, before things get back to normal on campus.  He estimated the damage could cost as much as $100,000.

The section of pipe that was breached on Monday will now be replaced in the coming weeks, city officials said.

About 500,000 Montreal residents depend on the reservoir for their drinking water.