Every day, Pittsburgh District is faced with the task of using limited resources to remain good stewards of our region’s infrastructure.
Tuesday, Aug. 4, was no different.
On that day a multidisciplinary team converged on Allegheny Lock and Dam 4 (AR4) to investigate potential concerns with erosion of the river bed, also referred to as scour, downstream of the dam. It was the culmination of innovative brainstorming and careful planning to get at the root of the question – was there a serious problem with the dam’s foundation?
The planning and investigation team included personnel from a broad range of district organizations:
- A dive inspection team led by Jay Kochuga
- A hydrographic (sonar) scanning team (George Brkovich, Brett Kelly and Patrick Barger)
- Operations staff led by Jeff Dudley, Brian McFarland and Joe Ostrosky
- Engineers from the Dam and Levee Safety Section (Margaret Pelcher) and the Geotechnical Design Section (Paul Zinnecker)
- Project Management (Dave Heidish)
- Construction branch (Denise Polizzano) and dredging contractor, River Salvage
Scour and undercut concrete had been detected during a dive inspection downstream of the dam in 2019. Information from that dive inspection and a subsequent attempt to conduct a sonar scan near the dam were hindered by the turbulent flow over the dam. The preliminary findings from the inspection pointed to the need for further investigation to confirm and better define conditions in the dam’s foundation.
The dam has no provisions for flashboards, which are panels sometimes placed across the top of a dam, and there is no companion hydropower plant (as opposed to some of the other Allegheny projects) to reduce flow over the dam. The follow-up inspection and scan needed to be done under more controlled conditions to allow collection of more reliable information.
“Since AR4 is a fixed-crest dam, we had to break the current of the normal river flow, so that we could send divers into the water and inspect the concrete over-pour edge and the undercut wall of the dam,” said Jay Kochuga, the district’s dive program coordinator. “It took the efforts of four different offices, River Salvage who were the contractors, and many professionals with a broad array of backgrounds to develop an inventive approach to complete the inspection.
According to Kochuga, more than a foot of water flowing over the dam would make it too difficult for the divers to maneuver and gather clear information.
However, in preparation, the construction branch and operations teams coordinated with one of the district’s contracted partners, River Salvage, to dredge and set a barge along the upstream face of the dam.
The purpose of the barge was to limit flow and turbulence in the area downstream of the dam where scour had possibly occurred.
The project operations staff worked with the contractor to set, tie off and monitor the barge for movement throughout the investigation.
With the barge in place, the dive team tied its boat off to the lower river wall and sent two divers upstream to the dam. They determined that while concrete associated with a former scour repair downstream of the dam was undercut, undercutting at the dam itself was limited to only a small section (about 10 feet long).
The observations and measurements made by the divers were recorded by the engineers in real-time, which allowed interaction and feedback between engineering and the inspectors to make sure all the necessary information was acquired.
After the dive team completed their inspection, the hydrographic survey was conducted. The survey team made numerous passes over the area to get a clearer picture of the riverbed.
The survey team used several methods, including side-scanning to obtain good resolution from several angles, to pick up as much information as possible on potential undercutting features.
Careful planning and coordination for this investigation was essential and was conducted over the course of many months, but enabled the investigation to be executed within a single day. Although the district could have outsourced this work, the related contracting and coordination would have resulted in delay and a substantially higher cost to the taxpayer.
“A separate contract to complete this investigation would be over $200,000 and have taken years to program and award,” said Mark Jones, the Engineering and Construction Division Chief for the district.
The teamwork, employed by the broad range of professionals and partnership with its dredging contractor, enabled the district to exercise responsible stewardship of the public’s infrastructure, saving taxpayer dollars through innovation and carefully choreographed execution.
The planning involved in this inspection ensured the investigators and the public remained safe throughout the process – safety is the district’s number one priority. Jones communicated that this was “a team that executed a well-thought plan that put safety first.”
The information obtained from the investigation was used to inform the district’s fiscal year 2021 work plan submission, which had previously included a sizeable placeholder for scour repairs at Allegheny Lock and Dam 4.
David Heidish, a district project manager, said funds that had been set aside for the potentially extensive scour repair will be used to keep other infrastructure in safe working order.
“By doing this dive work and obtaining the information from the inspection, the district can now remove this project from the 2021 work plan submission and move another item that needs repairing or replacing onto it,” said Heidish.