I-35W bridge runoff created 4-foot-by-6-foot hole

MnDOT

This photo released by the Minnesota Department of Transportation, taken on July 11, 2007, shows a hole near a concrete support of the Interstate 35W bridge in Minneapolis.

MnDOT says the hole, which was filled with concrete less than a month before the bridge collapse, did not affect the bridge's integrity.

By Tony Kennedy, Star Tribune

Last update: September 5, 2007 - 12:03 AM

Water runoff from the I-35W bridge created a 4-foot-by-6-foot hole in the ground near one of the bridge's concrete support piers weeks before the bridge collapsed, a Minnesota Department of Transportation spokesman said Tuesday.

Crews filled the hole with cement and the washout didn't affect the bridge's structural integrity, MnDOT spokesman Kevin Gutknecht said.

"The pier didn't sink. The pier is mounted on bedrock," Gutknecht said. "Nothing eroded under the pier."

He described the site of the erosion as being 50 to 60 feet up the hill and away from the base of Pier 5, which stood on the west bank of the Mississippi River. When the I-35W bridge collapsed on Aug. 1, the deck first gave way above Pier 5 at the southern end of the bridge's steel truss spans.

The bridge failure, which killed 13 people and sent more than 85 vehicles crashing, is under investigation by the National Transportation Safety Board. NTSB Chairman Mark Rosenker has never mentioned the soil washout as an area of interest by investigators, but he has said repeatedly that the NTSB's fact-finding and analysis of the collapse will be comprehensive.

Known areas of interest to the NTSB include the activities of a construction company that was resurfacing part of the bridge when it collapsed, the weight and placement of construction equipment and raw materials for the resurfacing project, a de-icing system that was installed on the bridge in 1997 and the chemical it sprayed, the original design of the 40-year-old bridge, the status of fatigue cracking in the bridge's steel beams, MnDOT's inspection methods for cracks in the beams and the bridge's gusset plates -- steel plates where truss members were joined.

The Minneapolis bureau of the Associated Press was the first to report the washout near Pier 5 on Tuesday. The AP looked at MnDOT e-mails concerning the washout, but reported that "none of the e-mails reflect concern that it would affect the structural integrity of the bridge."

The AP reported that a MnDOT maintenance supervisor described the hole on July 12 as "pretty good size," noting that a crew put a barrier around it so no one would fall in.

Gutknecht said the washout formed near a parking area under the bridge from water that ran off the bridge's deck through a drainage system. He said the water was "falling the way it should," landing on angled concrete on a slope under the bridge.

"Erosion happens," he said.

Tony Kennedy • 651-298-1543

Tony Kennedy • tonyk@startribune.com