Repairs following the sinkhole collapse in downtown Ottawa on Wednesday are expected to take at least a week, and determining the cause may take even longer, city officials say.

Concrete poured into the hole near the corner of Rideau Street and Sussex Drive to stabilize the area had reached the level of the water and sewage lines by Thursday afternoon, allowing crews to reinforce the walls so they can get in to fix the pipes.

It could be one or two weeks before the street is reopened to pedestrians and buses, said Ottawa Mayor Jim Watson at a news conference earlier on Thursday.

Watson said repairs should be made in time for Canada Day celebrations in the capital, but that the road would not reopen unless it was safe to do so.

The mayor also said it may take "a couple of months" before the cause of the collapse is known.

Bus drove over road minutes before collapse

The sinkhole formed about 10:30 a.m. ET Wednesday, at a light rail construction site next to the Rideau Centre just two blocks east of the Château Laurier Hotel.

Ottawa is building a light rail transit system across the city, with an underground tunnel connecting several stations in the downtown core, including one below where the collapse occurred. The light rail system is expected to open in 2018.

The mayor and city officials met with representatives from Rideau Transit Group, the consortium building the city's light rail transit system Thursday afternoon. The sinkhole area is considered an RTG worksite, meaning the group is responsible for the repairs.

City officials said it was fortunate no one was hurt.

OC Transpo general manager John Manconi confirmed that just minutes before the collapse, a city bus carrying about 50 people had driven over the section of road without incident.

Boil-water advisory for area

While hydro and gas have been restored to businesses in the area, a precautionary boil-water advisory has been issued for several buildings.

City officials from Ottawa held a news conference Thursday to update residents on repairs underway. From left to right: emergency services manager Anthony Di Monte, city manager Steve Kanellakos, mayor Jim Watson, drinking water manager Tammy Rose and rail implementation office manager Steve Cripps. (CBC) Some 2,700 cubic metres of concrete — or the equivalent of 300 cement truck loads — has been pumped into the hole to stabilize the area and adjacent buildings.

The hole is about 28 metres wide and 40 metres long, and though the bottom had varying depth, the average was about five metres deep, said Steve Cripps, the manager of the city's rail implementation office.

Construction workers had been excavating 25 metres below Rideau Street, as well as working in a trench above, when the street collapsed.

There was flooding in the five-metre high tunnel below, but crews have been unable to assess the damage while the sinkhole above is being filled with concrete, said Peter Lauch, the technical director of Rideau Transit Group.

A look at the <a href="https://twitter.com/hashtag/sinkhole?src=hash">#sinkhole</a> from earlier this afternoon.Glad 2 C the progress, concrete is at the level of the utilities <a href="https://t.co/0aBKnyJPaf">pic.twitter.com/0aBKnyJPaf</a> —@MathieuFleury

The sinkhole formed next to one of the city's largest shopping malls, the Rideau Centre.

The entire mall was evacuated Wednesday but reopened as usual Thursday, though The Bay and a glass walkway that crosses Rideau Street — providing an excellent view of the sinkhole — remain closed. The Rideau Centre also said its food court has shut down as a precaution, to ensure the drinking water quality is acceptable.

Some businesses nearby remain closed. The nearby Shaw Centre, also evacuated Wednesday, has reopened.

Cause of sinkhole unclear

Exactly why the sinkhole formed is unclear, but it formed over a vein of sand, silt and fractured rock so unstable that workers completing the light rail tunnel below had to inch along cautiously for fear of causing a cave-in.

"Everybody knew that that was going to be a bad area, that special precautions would have to be taken," a source familiar with the light rail project told CBC News Wednesday.

Signs on Milestone's door. <a href="https://t.co/JAX13Snd6G">pic.twitter.com/JAX13Snd6G</a> —@HallieCBC But city officials maintained during the Thursday briefing that it was too soon to speculate as to the cause of the collapse.

Watson said if the sinkhole is determined to have been caused by work on the light rail transit system, then RTG would be liable.

Watson would not say under what circumstances the city might be liable, but city manager Steve Kanellakos said it is unlikely the city would be liable for 100 per cent of the costs.

A major downtown Ottawa intersection was closed off after a sinkhole formed next to the Rideau Centre. 0:43

Parked van to remain in concrete

Soon after forming on part of the road, the sinkhole stretched from Rideau Street's south sidewalk to the north sidewalk.

A van parked on Rideau on the north side of the sidewalk, as well as a light standard, fell into the hole as it grew. A scissor lift that was in use in the above-ground worksite is also believed to have fallen in.

Guys I just saw a car get sucked into the sinkhole <a href="https://twitter.com/hashtag/ottawasinkhole?src=hash">#ottawasinkhole</a> <a href="https://twitter.com/hashtag/sinkhole?src=hash">#sinkhole</a> <a href="https://twitter.com/hashtag/ottawa?src=hash">#ottawa</a> <a href="https://t.co/7YyaHpzudr">pic.twitter.com/7YyaHpzudr</a> —@_Sophster Anthony Di Monte, the city's manager of emergency services, said in the minutes before the vehicle fell into the hole the owner went up to the city's command post and asked if he could move the vehicle and was told no.

Di Monte, speaking in French, said the van, light post and lift are all expected to remain in the sinkhole and be crushed down as the concrete continues to be poured in, for "future generations to discover."

Rideau should be 'stronger than most roads'

Rideau Street had been closed to all traffic except for city buses and taxi cabs while construction was happening in the area.

When asked about potential dangers to the section of road in the future, rail office manager Steve Cripps said the repairs underway should bolster the street.

"That mass of concrete will probably be stronger than most roads in North America," said Cripps.