A downtown tunnel planned for LRT trains, pictured, may have to be shortened, according to a geological survey. (City of Ottawa)

The city should consider shortening its plans for a downtown transit tunnel, an engineering report released Tuesday said.

The 3.2-kilometre tunnel for light rail trains was approved by council in June at an expected cost of $2.1 billion. The tunnel is set to run between LeBreton Flats and the University of Ottawa campus, with four underground stops.

The geotechnical study of the route was presented after a summer of drilling to determine what type of rock the tunnel would bore through. Rock samples came up as expected, except for one area north of Laurier Avenue near the Rideau Centre, said city planner Gary Craig, who works in Ottawa's Rail Implementation Office.

"The bedrock that we have found is anywhere from two to 10 metres deeper than we'd anticipated in the functional design," Craig said.

"That is one of the reasons why the preliminary engineering team have suggested to us that we seriously look at shortening the length of the tunnel."

The engineers' recommendation is that the tunnel be cut 600 to 900 metres shorter, leaving the campus stop above ground in its current location.

City staff said it’s too soon say whether that's the only option.

Staff also said shortening the tunnel likely won't affect the cost of the project.

The study also found several inactive fault lines along the proposed route, but staff said they're not a major concern.