United States Department of Transportation
i
Underwater Inspection of Bridge Substructures Using Imaging Technology
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2018-06-14
[PDF-10.53 MB]
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Abstract:As sonar technologies improve, interest in applying these tools for underwater bridge inspection is increasing. While the capabilities, standards, and processes are well-established for bathymetric surveying, sonar use for identifying important characteristics relevant for underwater bridge inspection is evolving. FHWA, working with several states, developed and implemented a series of field tests to evaluate the capabilities of sonar in underwater bridge inspection. This report provides an overview of sonar basics, the types of 2D and 3D sonar technologies available, and factors affecting the reliability of sonar data. The report also describes the field testing and results. The field testing evaluation focused on: 1) identification of relevant underwater features, 2) performance in adverse environments, 3) data collection/reporting time, 4) equipment costs, and 5) personnel requirements. Findings demonstrated that sonar technology is capable of identifying larger scale characteristics of interest such as scour holes, debris, and moderate to large size voids and protrusions. It is more limited in identifying small scale cracks or features hidden by marine growth. The results showed that sonar is particularly effective in adverse conditions where limitations on diver bottom time exist, such as deep water, and conditions where diver safety or mobility is of particular concern including swift currents or turbid water. In all environments, including those with adverse conditions, sonar technology is useful for identifying macro features quickly. These technologies can be used to inspect underwater structural elements where divers cannot work effectively, to guide divers for a closer look, or to provide independent inspection insights. Sonar can also be effective in covering large areas quickly. The study led to two broad conclusions. First, sonar inspections have not demonstrated the ability to identify some smaller scale elements of substructure condition that may be important in assessing the bridge and recommending maintenance. Second, sonar technologies offer significant opportunities for improving underwater bridge inspections, especially in adverse environments or to inspect extensive areas. The study provided recommendations that support the expansion of sonar technologies for bridge inspections: 1) develop guidance or benchmarking information for the use of sonar technologies in underwater bridge inspection, 2) develop a program of training for bridge inspectors in the use of acoustic technologies, and 3) encourage the use of sonar technologies to improve underwater bridge inspection.
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