City facing challenges to fix corroding canal bridge on Lake Mitchell two decades after being built

·3 min read

Jul. 15—MITCHELL — City officials are racing to repair a busy bridge along the west side of Lake Mitchell to avoid potential structural failures down the road, and it's causing frustration among some city council members.

Just two decades after being built, the canal bridge finds itself in tough shape with rotting timber abutments and deteriorating beams that are key parts supporting the bridge.

During the July 5 Mitchell City Council meeting, Eric Prunty, a local engineer, provided details of an inspection report on the condition of the structure and warned officials key parts of the bridge are in "really tough shape."

"The north abutment timber pile cap is in really tough shape with decay and deterioration," Prunty said of the canal bridge. "That timber pile cap will need to be replaced with probably a steel I-beam. You'll want to have something in motion in the next five years to get it taken care of."

Public Works Director Joe Schroeder, a professional engineer, said the structural issues that the canal bridge is experiencing 22 years after being constructed makes it a "definite anomaly." Schroeder said rough costs of replacing the bridge hover around $2 million.

"A new bridge design would put 75 years of life expectancy on it," Schroeder said.

The canal bridge was constructed in 2000 as a swath of homes began going up in a lakeside housing development known as The Island. It sits above a creek channel that drains into the west edge of Lake Mitchell.

In an attempt to reduce the stress of the bridge, city officials implemented a vehicle weight reduction of 50% on the bridge roughly a year ago. The reduction that dropped the maximum weight limit on the bridge from 80,000 pounds to 40,000 pounds remains in place. If the city doesn't make major repairs to the bridge in the next few years, Prunty noted another vehicle weight reduction could be needed.

Considering the bridge isn't old in terms of bridge life expectancy, the corroding conditions of the bridge has some city council members frustrated. Councilman Jeff Smith said taking on a rehabilitation project just two decades into the bridge's existence is "a little disheartening."

"That's one of our newer bridges we have in town, and we're talking replacing it. It's a little disheartening," Smith said during the bridge inspection update at the council meeting.

Engineer Don Hammond said during the 2021 inspection update that the design of the bridge is the root cause of the ongoing problems. Previous inspection reports on the bridge do not name the entity that constructed the bridge.

The bridge was constructed with pre-stressed concrete beams and treated timber abutment caps, which support the structure. The 2022 engineer report conducted by Brosz Engineering stated some of the timber on the abutment caps supporting the integrity of the bridge are rotting.

The city's goal of repairing the canal bridge has faced some setbacks over the past year after the South Dakota Department of Transportation (SDDOT) denied a grant application to rehabilitate the bridge.

The city's first attempt at securing a grant to help fund major repairs on the bridge was denied by the SDDOT due to it being deemed "not a worthy project," according to Prunty.

"They said do it with your own forces or go down the replacement route," Prunty said.

Prunty indicated there are several grant opportunities the city can pursue to help fund replacing the Canal Bridge, including a Bridge Improvement Grant through the DOT's program. Schroeder said the steadily deteriorating condition could potentially enhance the chances of securing a Bridge Improvement Grant (BIG) through the SDDOT.

Schroeder will be seeking the council's approval to apply for a preliminary engineering grant at the upcoming meeting. He said the preliminary engineering grant will help determine the type of work that needs to be done on the bridge.

With $33 million available in the SDDOT's Bridge Improvement Grant program this year — a drastic increase from the previous year — Prunty indicated the time is right for the city to apply for another grant.