Recently discovered soil erosion under a footing of a bridge over Saltwater State Park has resulted in its emergency closure by the city of Des Moines.

Spotted by consultants on March 26 during a seismic evaluation, a large cavity was created by the sloughing of soil under the spread footing of the west column on the south end and Woodmont side of the bridge.

The affected structure is the north twin bridge on 16th Avenue South at South 250th Street.

City crews have marked a detour route.

Grant Fredricks, director of the Des Moines Planning, Building and Public Works Department, said repair work was expected to begin April 7 "unless something changes."

Engineers still were working on the details last Friday.

"We hope to have the repairs completed" by the end of this week, Fredricks said.

His target date for completing the work and re-opening the bridge is two to four weeks earlier than an initial time estimate that was given Des Moines City Council members March 27.

Fredricks noted the affected footing "sits on a fairly steep slope."

The seismic evaluation that led to discovery of the erosion was being conducted so the city can apply for seismic repair funds from the state later this month, he said.

When the city was notified of the problem, "Our engineering staff immediately evaluated the situation, determined that a potentially unsafe situation existed and that it needed further evaluation."

Crews closed the bridge to all traffic the next morning and the city retained a geotechnical firm to help department engineers "evaluate the situation, and we also brought in inspectors to help us look at the underside of the bridge for cracking."

Fredricks said any cracking would have indicated "the first sign of structural failure from settlement on that pier. Fortunately they found nothing."

"The full bearing capacity of the pier is on the underlying soils," he explained. "We knew we had to put something back in that cavity so the 25 foot wide spread of the footing will bear (weight) across the entire bridge width.

"We believe we have the design and repair solution that will solve the problem," Fredericks said.

The bridge will remain closed to traffic until all repairs are completed and evaluated to determine if a full traffic flow can resume, or if traffic limits should be imposed "that could keep off heavy vehicles like fire trucks and garbage trucks."