An investigation into the cause of the Rideau Street sinkhole will look into why it took city crews two hours to stop water from a failed watermain from pouring into the LRT tunnel, Mayor Jim Watson told CBC News.

The sinkhole occurred shortly after 10:30 a.m. on June 8, but a gas leak forced workers to wait 20 minutes before they were able to begin shutting the water valves.

That work began at 11 a.m. and wasn't finished until 12.35 p.m., according to an emailed statement sent on behalf of Dixon Weir, the city's general manager of environmental services.

During that time, water from the burst pipes flooded the tunnel.

Part of the investigation, says mayor

According to Weir, seven valves had to be shut completely to cut off water to the affected area. Each valve takes between five and 15 minutes to close.

"The valves were located at different points on either side of the sinkhole and crews were required to travel from valve to valve in order to close them," said Weir's statement. "The process involves using a specialised tool and can require dozens of rotations, against the force of the pressurized water in the main, to completely close the valve. Care is required as well to ensure that the isolation process does not damage the surrounding water infrastructure further."

It's not clear if the valves were closed one at a time and if so, why. Another public works official, who is not involved in the sinkhole file and would only speak on background, said valves sometimes need to be shut in sequence. To close more than one valve a time, multiple crews with their own sets of equipment would have to be dispatched.

"I think that's a reasonable question," Watson told CBC News when asked why it took as long as it did to shut off the water. "And obviously we want to have answers to questions like that."

Ottawa Mayor Jim Watson shakes hands with workers at the Rideau Street sinkhole Friday, June 17, 2016. On Saturday, the mayor said Rideau could remain closed to vehicles until after Canada Day. (CBC)

Watson said he was told by city staff that there were "legitimate reasons" why shutting off the water takes so long.

Still, he said the issue "will be part of the review that we undertake. What's the cause? And secondly, are there things we could do better on a go-forward basis?"

It's still unknown exactly what caused the June 8 watermain break or whether the tunnel drilling was a factor. The city's and RTG's investigations into the cause of the sinkhole are expected to take two months.

Damage to tunnel, roadheader, unknown

When engineers went underground two days after the sinkhole occurred, they found standing water along 300 metres of the tunnel, from about Elgin Street to the Rideau Centre. The water was as high as three metres at one point, city officials estimated.

RTG has been pumping water out of the tunnel ever since.

Pumping out the water is slow going. For one thing, the tunnel opening is more than one kilometre from the sinkhole. Secondly, the water must be treated by RTG's own water-treatment plant built specifically for the LRT project, before being poured into the city's sewers.

The pumping is expected to continue for several days, Watson said.

One of three Rideau Transit Group roadheaders, like this one shown here, is in the tunnel under Rideau Street, at the deepest part of the flooding. It's not known if, or how much, the 135-tonne piece of equipment is damaged. (CBC)

One of RTG's three 135-tonne roadheaders is in the tunnel, where the flooding was the deepest. It isn't known how much, if any, damage has been done to the boring equipment or the tunnel.