ST. PAUL (AP) Three weeks before the Interstate 35W bridge collapsed, engineers and maintenance crews had their hands full with a “pretty good size” hole that developed near one of the concrete piers supporting the southern end of the bridge.
E-mails obtained by The Associated Press show Minnesota Department of Transportation officials puzzling over the source of the hole, with one engineer suggesting it was part of a larger erosion problem in that area. None of the e-mails reflect concern that it would affect the structural integrity of the bridge.
On Sept. 4, state transportation department spokesman Kevin Gutknecht downplayed the deformity, labeling it a washout instead of a sinkhole, as several of the July e-mails call it. Gutknecht said it was caused by water draining off the bridge.
“It did not appear to have any impact on that particular pier. That pier did not move,” Gutknecht said. “In fact, it did not move during the bridge collapse.”
Engineering experts said in interviews with AP that the hole was worth studying, but they considered it unlikely to have caused the collapse.
But the documents, which include several e-mails and four photos, have apparently been turned over to investigative teams, according to an e-mail that made the rounds a few days after the Aug. 1 collapse that killed 13 people. AP obtained the correspondence through a freedom of information request.
In a July 12 e-mail, Tom Osthoff, a maintenance supervisor at the department, said, “I went and checked the sink hole out, it is pretty good size.” He said he had his crew put a barrier around it so no one would fall in.
The photos show a hole above and to the left of the support pier. A paved downslope leading to the pier also shows signs of slumping. The hole was estimated to be 4 by 6-ft. (1.2 by 1.8 m) in diameter and a couple of feet deep, Gutknecht said.
“The pictures of the slope paving seem to suggest that the sink hole is connected with ongoing subsidence of the slope paving,” wrote engineer Bruce Irish in a July 11 e-mail.
Gutknecht said that the hole was first discovered in December and some initial repairs were made in January. Cold weather hampered the work so crews put off additional repairs until July. Forty-five wheelbarrows of moist concrete had been poured into the hole, with work done as recently as July 25. The fix wasn’t complete by the time of the collapse.
National Transportation Safety Board spokesman Terry Williams wouldn’t comment on the hole’s place in the investigation.
The area where the hole was found is where the bridge deck broke away on the southern side. Crews spray-painted the word “keep” on a steel beam from that section, presumably so investigators can give it a closer look later.
A month after the busy bridge plunged into the Mississippi River, federal investigators have offered a few hints on where their probe is headed, but no firm conclusions about what led to the failure. A firm retained by the state to do a parallel investigation has been silent about its findings.
The NTSB has issued an alert about the gusset plates that tie steel beams together, taken note of the heavy construction equipment on the bridge at the time and raised questions about its automatic de-icing system.
The e-mails reviewed by the AP are the first mention of a hole near any support piers. The piers are the concrete pedestals that hold up the steel components of the bridge’s underbelly.
The department finished its last bridge inspection report in June 2006, and did not mention erosion problems near the piers. Officials have said they began a new round of inspections in May. No reports have been made public from those latest inspections.
On July 17, a week after Osthoff’s initial message, he shared with colleagues an appraisal by Dale Dombrowske, a bridge maintenance supervisor.
“He feels that there was a problem with the bridge that caused this sink hole [wash out]. At this point I would have to agree with him, there does not appear to [be] any drain pipes in this vicinity,” Osthoff wrote.
Along the way, state engineers checked in with their peers in the city of Minneapolis to determine if the hole could be related to underground sewer lines. J. Rich Profaizer, an operations manager with the city, quickly replied that it was not. In a phone interview, Profaizer said there were no sewer or storm drains adjacent to the hole.
Bashar Qubain, president and chief engineer of Wayne, Pa.-based GeoStructures, has done geological analysis for transportation projects in Pennsylvania, New Jersey and Delaware. At AP’s request, he reviewed the rock makeup detailed in the original I-35W bridge designs.
He said the limestone and dolomite are prone to caverns and other underground erosion, and they tend to be part of a network rather than a solitary deformity.
“If there is a serious sinkhole happening on the surface there is usually a deeper problem below,” Qubain said. But, he noted, the designs called for the piers to be built on solid rock, diminishing the chances a sinkhole would jeopardize the bridge.
Kent Harries, an assistant professor of civil and environmental engineering in the University of Pittsburgh’s School of Engineering, also had doubts about the hole’s contribution to the collapse.
He said that washouts and sinkholes — even deep ones — are relatively minor compared to the size of the bridge’s footings.
“Assuming it’s a relatively surface sinkhole, it probably didn’t have anything to do with it,” he said, adding, “Like anything else in a case like this, you can’t discount anything.”
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