Indian River Inlet bridges survive earthquake, hurricane


Despite long-running concerns about its stability, the 46-year-old Indian River Inlet Bridge came away from two natural disasters in a single week with a clean bill of health, as did its replacement.

The Delaware Department of Transportation (DelDOT) announced mid-day on Wednesday, Aug. 31, that an inspection Tuesday, involving both a fathometer survey and visual pier inspection, had shown no significant changes in the existing bridge’s structure or the channel beneath its piers, even after it went through both the state’s first-ever hurricane and a nearly unprecedented earthquake in just a matter of days.

The dive inspection had already been scheduled prior to Hurricane Irene putting the Delmarva Peninsula in her sights, and DelDOT officials had done a sonar-based survey of the channel on Sunday, Aug. 28, after the storm had cleared the area, to speed the reopening of the bridge for travel.

State officials had closed the bridge before 4 p.m. on Saturday, as conditions deteriorated under Irene’s approach. The sonar survey had enabled the bridge to be reopened mid-day on Sunday, though officials had warned on Saturday that it might not be reopened for days, pending a positive report from the dive inspection.

The fathometer survey details the elevations of the channel bottom, including the area around the piers and the existing scour holes near the piers, which have been a source of concern since they were discovered and left questions about how stable the existing bridge might be and how long it might last. The scouring phenomenon also led to a different kind of design for the replacement bridge, keeping its supports out of the water.

Officials said the information from Tuesday’s survey was compared to that from previous surveys to determine if any significant change had occurred.

DelDOT conducts the same type of survey annually, as well as after any significant storm event.

“This recent inspection confirmed that there are no significant changes to the channel bottom near the existing piers since the last inspection,” Shockley said. “The bridge remains safe for use by the traveling public. It will continue to be monitored until such time that it is taken out of service.”

New bridge pushes toward completion

That is currently scheduled to happen sometime in December, when the new Indian River Inlet Bridge is completed. The project, as of last Thursday, Aug. 25, when members of the Construction Advisory Group (CAG) held their monthly meeting, was just a few small sections short of completion, with two sections on the southern side of the new bridge deck yet to be poured. That work is set to be done this month.

Once the final sections are poured, the “form traveler” will be lowered onto a barge and shipped off-site. A similar procedure was completed Aug. 15 for the north-side form traveler, requiring an eight-hour closure of the inlet to marine traffic while the barge was positioned precisely beneath the form traveler. The process has just a 12-inch tolerance, complicated by the fluctuations in the inlet waters. It must be completed at a time when the tide will cooperate with the delicate operation.

Both form travelers are to be recycled, as they were custom-designed for the project and cannot be reused.

Once the second traveler is removed, the new bridge deck will get its “closure pour” – a 10-foot joining span that will connect the two sides. That is currently set for sometime in November.

The bridge will then be readied for use, which is scheduled to begin in December. The old bridge will be removed once the new one is opened to traffic. Work on landscape and roadway restoration in the area of the bridge will continue into 2013.

Both bridges fare well during hurricane and earthquake

Though an unfinished bridge hanging over a fast-moving coastal waterway might seem like a prime target for damage in a hurricane, officials working on the project on Thursday were confident the bridge would be able to stand the test of Irene.

“We’re in full hurricane preparation mode,” they said during Thursday morning’s meeting.

They had tied down sections of the bridge on Thursday, as well as employing dampers and weights – including the project’s construction cranes – placed at strategic locations on the bridge, to help stabilize it in its cantilevered condition, with the unjoined ends of the north and south sections hanging out over the water.

Workers had also moved to clean up debris from the site, secure construction trailers and equipment and move everything out of the path of possible high water. Construction itself was shut down as of noon on Thursday, giving workers time to complete battening down the nearly complete bridge and then tend to their own families and other storm-related concerns on Friday.

Some of those workers had a bit of a scare on Tuesday, Aug. 23, when a rare East Coast earthquake – registering 4.8 on the Richter scale and based in central Virginia – shook the bridge and everyone on it, some of them suspended far above the fast-moving inlet.

Both bridges were quickly inspected and found to be in the same condition as they had been before the earthquake. The dive and other inspections early this week helped confirm that.

Bridge engineers said they weren’t surprised it had weathered the unexpected earthquake so well.

“It was all pre-planned before. It’s all in the design,” they said of the tolerances built into the bridge design. Even though an earthquake is a rare happening in the area, both earthquakes and storms had been figured into calculations as occurrences the bridge would have to survive.

“The worst-case scenario would be to have it hit when it was in this cantilevered condition,” they noted of the earthquake, pointing out that the bridge design had also been successfully wind-tunnel tested with Atlantic storms in mind.

“Of more concern would be a tornado,” they added, noting that a tornado could easily bring winds in excess of the 120 mph maximum winds Irene was creating as of Thursday. The bridge, however, did not have to endure what would have been a trifecta of natural disasters this week, as the one tornado reported in the area happened miles to the northwest.

Bridges statewide undergo inspection

Officials had said on Thursday that they expected they would likely have to close the existing bridge to traffic during Hurricane Irene, as flooding of the roadway leading to the bridge was anticipated and would require a closure and post-storm inspection.

Statewide, inspectors pinpointed 182 bridges that were either flooded or were considered vulnerable to scouring as a result of Hurricane Irene. Of those bridges, 119, or about 65 percent, had been inspected as of Tuesday evening.

Officials said none of the bridges had any damage significant enough to require closure. Most of the damage was described as minor and said not to pose an immediate safety concern. Dams statewide were also being inspected following the hurricane. Of the 37 dams operated by DelDOT, 25 had been inspected as of Tuesday evening.

The bridge inspection teams expected to have all of the bridges and dams inspected by Thursday, Sept. 1.

Information on the construction of the new Indian River Inlet Bridge can be found at www.irib.deldot.gov.