What you need to know about high-resolution sonar scanning in underwater bridge inspection
The tripod mount was attempted and found to be high risk, given the swift currents and depths. In consideration of this, an alternative deployment method was developed that consisted of mounting the sonar head to the pier by way of an aluminum frame. This frame was designed specifically for the Potomac-type piers and included permanently mounted attachment brackets. The frames were designed to also be mounted by a strap and tightened with a come-along device.
After successfully obtaining sonar images and confirming the identified targets for the shallow pier, additional piers at varying other depths (21 feet, 45 to 60 feet, 57 to 61 feet, and 76 to 78 feet) were scanned using the pier frame to support the sonar head.
While weather conditions during the scanning time frame did not allow diving operations to verify the scans, in each case, the scans revealed previously undocumented structural details. Further, numerous areas of interest were visible on the pier surfaces. It was concluded that these images could certainly be used to prioritize a diver’s efforts and optimize the results and repeatability of the underwater inspection findings.
Further efforts were concentrated on two piers, with the intent of clarifying both positive and negative targets. Once they were identified, divers were deployed to verify that the positive targets on the sonar image indicated the presence of actual defects or areas of deterioration and that the negative targets indicated the lack of defects or deterioration. For one of these piers, the sonar image targets and the dive results were compared with the findings in the most recent underwater inspection report, demonstrating inconsistency in the report findings.
Ultimately, Pennoni’s study concluded that the sonar imaging was able to successfully direct the divers to areas of interest on the piers. This supports improved documentation and more repeatable inspection results, as the images can be repeated from inspection to inspection. The divers can use the sonar image prior to diving to evaluate and identify areas of interest on the pier face. Use of the sonar image can eliminate the need for the diver to have to locate the target to a known point of reference, which can be a difficult task in little or no visibility, strong current and on massive piers.
Use of sonar imaging can also improve the knowledge of a pier in general, as Pennoni found that the images identified the presence of underwater pier elements either not located or incorrectly located on existing plans and drawings. Ultimately, the use of sonar imaging cannot replace NBIS-required underwater inspections, but when managing the inspection of massive piers for signature structures in challenging environments, this can serve as an additional tool in the bridge manager’s toolbox, increasing reliability and repeatability of inspection findings.
About the author: Jennifer Laning, P.E., serves as the Bridge Inspection Practice leader at Pennoni Associates. Laning has 20 years of experience specializing in bridge and underwater inspection and management of complex bridge inspection projects. She is responsible for project and quality management for Pennoni’s bridge and underwater inspection agreements with local, state, and federal agencies across the Mid-Atlantic region and the United States. She can be reached at 443-449-2503 or at firstname.lastname@example.org.
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