St. Paul’s makeover of a busy downtown thoroughfare, which the city had planned to finish in November, won’t be completed until next spring due to unanticipated issues with the underlying sandstone.

But officials said that, weather pending, they still hope to end months of traffic detours by partly opening Kellogg Boulevard and the Wabasha Street Bridge’s southbound lanes by late December.

The construction delay will drive up the cost of the $12.2 million project, but how much won’t be clear until the city negotiates compensatory claims with the contractor, said Brent Christensen city project engineer.

Design adjustments and savings from other city projects will cover at least some of the cost overruns, he said.

The original construction schedule, from April 1 through November, “was aggressive, but we felt it was reasonable to complete construction in a single season,” Christensen said. “But the delays were more significant than we imagined.”

St. Paul is rebuilding two mostly unseen bridges that carried Kellogg along the river bluffline for more than 80 years — one across from the Central Library and the other from City Hall. The complicated project has closed Kellogg from Market to Wabasha streets for more than six months, shunting traffic off to 7th and Robert streets.

As the existing bridges were torn up, Christensen said, workers found portions of the sandstone bluff dangerously weathered. They also found four or five service tunnels that hadn’t shown up on their records. Debris was packed behind some buildings, and in one case, workers found a void below a slab where they had expected rock.

“We could work for a number of feet, but then have to redirect to another area of the project while we figured out what we were dealing with,” he said.

So crews were forced to dig much deeper than expected to pour concrete for foundation supports, he said. In other places, they drilled solid steel bars 60 to 80 feet into the bluff to spread the load to more stable rock.

Contractor Kraemer North America adjusted the construction schedule to minimize delays, but the schedule was pushed back to the extent that pouring of the final deck surface will have to await spring’s warmer temperatures.

In the meantime, Christensen said, crews will patch or fill in some areas on the deck to pass traffic through. “We can’t say yet what exact configuration will be ready by Christmas, but we should be able to open Kellogg and southbound Wabasha,” he said.

Ramsey County is in the process of tearing down a line of nearby buildings on Kellogg, a project that is about 20 percent complete and on schedule to be finished next year, spokesman John Siqveland said. It’s possible that the delay in the city’s work could affect the way the county goes about razing the former jail, he said.

“However, at this juncture we’re optimistic that any material impacts to the overall project budget and schedule be avoided by having our contractor modify their approach,” he said.