The Waterways Journal
5 January 2009
By David Murray
The Pittsburgh Engineer District announced December 30 that a contractor has successfully stabilized Allegheny River Lock and Dam 6.
Last fall, divers on a routine inspection discovered a large void nearly 200 feet discovered a large void nearly 200 feet long, up to 20 feet deep and extending about two-thirds of the way underneath the dam.
The Corps deemed the dam unstable and took measures to ensure it was operated safely while the damage was repaired. Officials were concerned that ice loads on the dam or large chunks of ice slamming into the dam could knock it off its exposed wood piling foundation and into the river.
The district initiated an emergency repair contract with Brayman Construction Corporation, which quickly mobilized to the site and began shoring up the dam. Crews first installed a steel wall along the toe of the dam to seal off the along the toe of the dam to seal off the void, and then filled it with 60 truckloads of concrete designed for underwater of concrete designed for underwater placement.
"We were able to place 600 cubic yards of concrete into the void to stabilize the dam while we had low flows in the river," said Brian Greene, project manager for the Corps. River flows have remained too high the past two weeks to complete the project, which entails filling a few smaller voids and placing rock along the downstream portion of the dam to prevent further erosion. Workers have also installed foot-high wood panels called "flash boards" on top of the dam, which "flash boards" on top of the dam, which divert water away from the work area.
Corps officials report that while more work is required to complete the project, critical repairs needed to shore up the dam have been completed.
"We're constantly monitoring the river conditions and looking for an opportunity to finish the project when flows are low enough to do the work safely," added Greene. "We have about a week's worth of work remaining."
Rain and melting snow have created high flows on the river that are three times the level needed for crews to safely complete repairs to the 80-year-old structure. The contractor has staged a high-speed drill rig on site and materials nearby so that the crews can react quickly when the water goes down.
An inspection of the remaining seven Allegheny River lock and dam facilities did not reveal severe erosion problems at other structures, though Lock and Dam 5 shows similar, less severe erosion that will require some repair work.
Pittsburgh District officials say that age, fatigue and the deteriorating condition of the region's navigation system coupled with hundreds of millions of dollars of unfunded critical maintenance work on the Allegheny, Monongahela, and Upper Ohio rivers will probably result in additional unplanned repairs.
"We're in fix-as-fails mode," said Colonel Michael Crall, Pittsburgh District engineer. "We're making critical repairs [of things] that threaten the reliability of the system and trying to slow the increasing number of unscheduled breakdowns," he added.
Crall applauded the engagement and coordination of local stakeholders during the repair effort, including Sithe Energies, which owns and operates the hydropower station on the dam adjacent to the work site. During the repairs, Sithe Energies drew as much water as possible through its hydroelectric plant to reduce flows over the dam and to facilitate lower flow conditions for construction crews. The Coast Guard's Marine Safety Unit Pittsburgh assisted by placing warning buoys around the work site to alert commercial vessels and recreational boaters.
The quick response may have spared the region from several serious impacts had the dam failed, the district said. A loss of the pool would have shut down commercial and recreational navigation on the river. Severe water quality degradation for the first week would have impacted water intakes and sanitary facilities. The hydropower facility located at the dam would have suffered severe damage and ceased operations. One of the largest wetlands in the region — Cogley's Island Complex downstream of Kittanning, Pa. — would have dried up and the the federally listed mussel species would have been negatively impacted, the Corps said.