The price tag for Ottawa's proposed downtown light rail tunnel could balloon because of engineering complications, says a local transit advocate.

City councillors voted last month to approve a $2.1 billion, 12.5-kilometre stretch of light rail track, to run from the Tunney's Pasture transit station in the west to the Blair station in the east.

It will include a 3.2-kilometre long tunnel between LeBreton Flats and the University of Ottawa, with four underground stations. Portions of the tunnel will go under the Rideau Canal, and at points will reach depths of over 100 feet.

David Jeanes, head of the transportation advocacy group Transport 2000, told CBC News digging that far underground could present unforeseen complications.

"Ottawa is proposing a tunnel [that] is very deep. It's passing underneath buildings and it's passing through rock that we really don't know much about," Jeanes said.

"Very few other building owners or land owners have ever gone as deep as we need to go for this system."

Ottawa officials have said they have budgeted extra money for problems like hitting fault lines, tricky geological conditions and even old sewers.

"We have limited information at the depth that we've done the design. As a result, our design is relatively conservative," said David Hopper, a project consultant hired by the city from Delcan Corporation.

Niagara tunnel comparison

But the project could run into problems similar to those encountered by a much-maligned hydro tunnel project in Niagara Falls, Jeanes said. The Niagara Tunnel project is intended to deliver a new water source to the Sir Adam Beck generating station near Queenston, Ont., in an effort to boost energy production.

The cost of that project has ballooned to $1.6 billion from $985 million, and its expected completion date has been pushed to December 2013, from the original target of June 2010.

The cost overruns and construction delays have been blamed in part on the geological complexities of the area.

Dennis Gratton, a senior project manager with the city, played down fears the Ottawa tunnel could face similar problems. Deep transit tunnels under bodies of water are becoming more commonplace around the world, he said.

"With that in mind, and going into preliminary engineering later on next year, I don't think we're going to have those types of serious cost overruns or mistakes that were made in Niagara or [with] anybody else," Gratton said.

Tunnel experts from England are advising Ottawa on the dig. The city says its project is modeled on ones in Pittsburgh and New Jersey.

The city has so far secured $600 million from the province to put toward the $2.1 billion project. However, even if the federal and municipal governments make equal commitments, the project will still have a $300 million shortfall.