Part of St. Paul's W. 7th Street is closed from Kellogg Boulevard to Grand Avenue after a sinkhole opened there Thursday evening, and officials said repairs could take up to two months.

Crews from St. Paul Public Works and Regional Water Services were assessing the damage caused by the sinkhole, which opened between Chestnut and Walnut streets not far from Xcel Energy Center and United Hospital.

One lane of the street will remain open in each direction to local businesses only. Drivers are asked to use alternate routes and follow posted detours. Sidewalks are open.

Surrounding businesses and homes still have water service, and sewer service has been diverted during the repairs.

Public Works Director Sean Kershaw said small voids that cause sinkholes are easy to repair and common. Water often erodes limestone under roads to create the voids, which can be around 12 inches deep.

But this sinkhole is unusual because it's much deeper, Kershaw said. The Public Works Department confirmed Saturday that the sinkhole is 35-feet deep.

Officials were investigating what caused the sinkhole. Kershaw said the city's oldest sanitary sewer line may lie beneath this stretch of W. 7th Street in layers of sandstone and limestone.

Part of the sewer line collapsed, Kershaw said.

"A sinkhole of this depth is very uncommon. I'm not aware of it happening in St. Paul anytime recently," Kershaw said. "You've got old cobblestones, old brick streets with tracks in it, with asphalt on top of it. ...

"So you've got a lot of things underground here, and all of this is surrounded by sandstone and limestone."

Kershaw said work crews will bring an excavator to the site on Monday to dig out rubble. They plan to rebuild the sewer tunnel and manhole shaft leading to the street.

"It's fairly straightforward, it's just that you're doing it in an area where you want to preserve pedestrian and car access," Kershaw said. "You want to make sure you don't disturb the water main, and that you are careful going into it because we don't know yet how big the void is."

Public Works spokeswoman Lisa Hiebert said further updates will be announced as crews gather information.