Just before the start of summer, part of West Seventh Street in St. Paul, Minnesota, collapsed — opening a 35-foot sinkhole that damaged a sewer tunnel built more than a century ago.

"All the ground under the street had been eroding for years, most likely from water, and it created this void, and then the void just collapsed," said Sean Kershaw, the city's public works director.

Crews spent the next four months diverting sewage, reinforcing the tunnel and filling the ground above. The work was slow and deliberate because crews were repairing a tunnel only 6 feet tall and 2.5 feet wide.

"We put in down below about 100 cubic yards of shotcrete," Kershaw said, referring to the special concrete used to stabilize the walls and ceiling.

Businesses along West Seventh felt the impact. At P

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