The Ministry of Labour is investigating after three workers had to seek shelter Thursday afternoon following an industrial incident in a light rail tunnel near the University of Ottawa.

One of the workers suffered a hand injury, paramedics said, after building material came loose during work on the tunnel's side wall. The other two workers were unhurt.

All three sought shelter in a safe zone underground until firefighters could reach them, according to emergency officials at the scene.

Ottawa Fire Services say they were dispatched to the area for a possible rescue at around 4:20 p.m.

Ottawa Police and other emergency services responding to the scene of a tunnel collapse on Waller Street. More updates to come. —@OttawaPolice

BREAKING: emergency crews racing to LRT tunnel collapse at Laurier & Waller, police say <a href="https://twitter.com/hashtag/ottnews?src=hash">#ottnews</a> <a href="https://t.co/lAEmwoBngo">pic.twitter.com/lAEmwoBngo</a> —@AshleyBurkeCBC

Rideau-Vanier Coun. Mathieu Fleury, who represents the area, told CBC News that a portion of the tunnel that was being reinforced with a building material called shotcrete fell.

City councillor for area says that rescue crews are underground working to get 3 trapped workers out safetly after shotcrete fell <a href="https://twitter.com/hashtag/ottnews?src=hash">#ottnews</a> <a href="https://t.co/cv5wrW3Ytm">pic.twitter.com/cv5wrW3Ytm</a> —@AshleyBurkeCBC

As its name might suggest, shotcrete is concrete that's shot out of a hose-like piece of equipment.

It's intended to reinforce the sides of the light rail tunnel and was used during repairs following the Rideau Street sinkhole this summer.

In audio from the dispatch calls, provided by Broadcastify, an emergency responder warned "the side of the wall is caving in" where the three workers are trapped, and that "there could be additional injuries as this thing continues to evolve."

The three workers made it out safely.

'Always a risk'

Mayor Jim Watson told reporters that pieces of rebar came loose while workers in the tunnel were spraying concrete. Watson said he was told that approximately six kilograms worth of material fell.

Ontario's Ministry of Labour has been called in to investigate, Watson said, and no one would be allowed in the tunnel until they arrived. He reiterated that the tunnel itself did not collapse.

"Anyone who's been involved in any tunneling realises there's always a risk of challenges in the tunnel," Watson said.

"We have some of the best tunnel engineers working on this project. Safety is our first and foremost concern."

'Minor construction incident'

Crews were applying the concrete to the tunnel's side wall when "reinforcing steel" came loose and ended up resting on the three workers' machine, said John Manconi, the general manager of the city's transportation services department.

The "minor construction incident," as Manconi put it, happened near where an eight-metre wide, 12-metre deep sinkhole formed in 2014 while crews were digging an entrance to the city's light rail tunnel.

Laurier Avenue had been closed from Nicholas Street to King Edward Avenue. It was reopened at around 6 p.m.

<a href="https://twitter.com/hashtag/OttawaParamedics?src=hash">#OttawaParamedics</a> responding to 9-1-1 call at tunnel entrance near Ottawa U. Please keep access/egress routes clear for emergency crews. <a href="https://t.co/pxMBSICUFh">pic.twitter.com/pxMBSICUFh</a> —@Ottawa_9_1_1

Philbert Carter was working further down the light rail tunnel from where the incident happened. He was able to get out safely.

"It's just one of those things that happens sometimes," Carter said.

"We worry about our safety all the time, because it's a hectic job. And we always have to work safe. And that's the main thing — to work safe and make sure the guys around us are working safe."