What you need to know about high-resolution sonar scanning in underwater bridge inspection
Large multiple-span signature bridges that cross major rivers require detailed inspections to ensure the safety of the motoring public is maintained. The use of high-resolution sonar imaging is becoming increasingly relied upon as a method to augment underwater bridge inspections and provide information on bridge piers.
The purpose of underwater bridge inspection, as performed by qualified divers, is to find defects or deficiencies in the substructure unit and rate the condition of the substructure unit. The inspection report findings then quantify and locate the defects for tracking in future inspections. While this is manageable for most bridges over water, for owners of signature bridges, this can be a daunting task.
Signature bridges have numerous substructure units, often of massive size due to the bridge length and navigational clearance requirements, the depth of the waterway, the forces placed on the substructures from the superstructure (i.e. anchorage piers, bascule spans, etc.) and the forces from water and ice.
The findings and recommendations compiled in a typical biennial inspection report for a signature bridge can total several hundred pages. The effort and cost associated with performing underwater inspections on signature structures can be substantial. While typical topside inspections are logically affected by wind, weather, and vehicular and navigational traffic, in most cases, the defects are visible to inspectors, easily identified in daylight, and thus can be referenced to a known point in order to be located in future inspections.
Underwater inspections, on the other hand, are affected by current, weather, and navigational traffic, but have the added challenges of visibility and operational restrictions inherent in underwater operations, such as depth limitations, decompression time limits, and water temperature. These challenges make it more difficult to find defects and reference them to a known point for future inspections, but through new and innovative methods like scanning sonar, these challenges can be overcome.
In an effort to improve the quality of underwater inspections on a signature bridge located in the Chesapeake Bay, Pennoni Associates investigated the use of high-resolution scanning sonar. Among the challenges faced during the inspections, review of previous inspection reports determined that there was a lack of consistency in the previous findings. With at least three past cycles of reports at hand, past underwater inspections findings included cracks, spalls, honeycombing and cold-joint laitance bands that are typical for concrete substructures constructed under water. There were a number of concerns among these findings. The most recent underwater inspection report from 2009 indicated that more than 25 percent (15 piers) of the Potomac-type piers were classified NBIS Condition Rating 4 “Poor.”
A review of all accumulated underwater inspection reports indicates that the reports provide inconsistent and inaccurate information about the location, extent, and status of defects and repairs. The inconsistency from year-to-year included reported new findings and existing findings being reported as “not found.”
Pennoni’s underwater inspection team, led by Mark Sayger, P.E., and manned by six other engineer- divers and dive supervisors, in addition to Project Manager, Jennifer Laning, P.E., were tasked with investigating potential methods to increase the repeatability, and therefore the reliability, of these underwater inspections.
The investigation had a number of goals at the outset, including evaluation of the effectiveness of sonar imaging in providing information about the condition of underwater substructure units, and determining the role of sonar imaging in providing repeatable and reliable condition information.
The investigation focused on a particular substructure unit known as a Potomac-type pier, which is configured in two-column and four-column configurations. Reinforced concrete columns transition through a bell shaped section into a larger diameter column base. The larger diameter column base acts as a pile cap for the piles that have been driven into the bay bottom. Concrete web walls are constructed between the columns and extend from a depth of approximately 10 feet below the water level down to the bottom of the bell. The web walls connect all the columns in both the two-column and four-column piers. Steel forms were used during construction and remain in place on the piers. These piers, in dual- and quad-configurations, are massive, with a great deal of surface area and located in the very variable waters of the Chesapeake Bay.
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