Pile-driving adjacent to an Interstate 65 bridge near Lafayette, Ind., appears to have started a subterranean chain reaction that left one of the structure's riverbank piers skewed out of alignment, forcing a 37-mile closure of the northbound lanes that could extend into mid-September.
Built in 1969 and reconstructed in 1988, the 394-ft, five-span continuous steel multibeam-girder bridge across Wildcat Creek was being widened as part of a two-year, $82.8-million project begun in April to expand a seven-mile stretch of I-65 from four to six lanes.
According to the Indiana Dept. of Transportation, the first indications of the settlement of Pier 3-a 43-ft-tall, 10.5-ft-wide column structure-appeared on Aug. 4, one day after lead contractor Walsh Construction Co., Crown Point, Ind., began driving steel piles adjacent to the piers and installing sheet piling so that crews could work below the Wildcat Creek waterline. The bridge was immediately closed, and an inspection revealed that Pier 3's rocker bearings were missing.
A May 2015 INDOT inspection reported that the bearings were in place, albeit with an "extreme tilt."
An interim measure to reopen the bridge by augmenting Pier 3 with H-piles failed to arrest the settlement, forcing INDOT to close the structure for good on Aug. 7.
According to INDOT Director of Bridges Anne Rearick, the bridge site's geology features sandy soil and compressed gravel over loam. The agency-led structural and geotechnical investigation, conducted with the Federal Highway Administration, Walsh, and engineering consultant Cardno TBE, determined that the newly driven piles pierced the watertight soils and hit an underlying sandy layer that was part of an artesian water pocket.
"Our investigation is still underway, but it appears the resulting release of high-pressure water and sand beneath the pier caused it to settle," Rearick says, adding that the pier sank as much as 10 in. and rotated 7 in.
To permanently stabilize Pier 3, Walsh Construction will install high-strength micropiles through the existing spread footings to a depth that has yet to be determined. INDOT expects that measure to be completed by mid-September, at which time it hopes to reopen the bridge and resume work. In the interim, INDOT is diverting northbound traffic along detours of more than 60 miles to prevent local roads from being overwhelmed by approximately 25,000 vehicles a day, more than a third of which are heavy trucks.
An adjacent bridge supporting southbound I-65 remains open and continues to be expanded with an additional lane.
INDOT has long been aware of potential stability issues at Pier 3, though none were considered serious enough to warrant corrective action prior to the Aug. 7 closure.
A 2005 inspection mentions tilting of the pier's rocker bearings, but speculates that the condition may have been the result of the structure's original construction. The May 2015 inspection cites previous reports of settlement and tilted rocker bearings at Pier 3, as well as erosion along both banks of Wildcat Creek. The report also notes a scour depression at Pier 3, but the foundations are determined to be stable and at only "moderate risk" for scour vulnerability.
Overall, the bridge's substructure received a "poor" rating in May 2015, while its superstructure and deck were rated as "fair" and "satisfactory," respectively.
The one silver lining from the I-65 bridge incident, says Rearick, is a heightened awareness of potential structural settlement issues, particularly since the conditions at Wildcat Creek are found across the state.
"This is a good opportunity to remind our design teams and inspectors that they need to watch for these things," Rearick says.