UNDERWATER INSPECTION OF HIGHWAY BRIDGES – RECENT TRENDS AND TECHNOLOGIES BY DAVE SEVERNS, P.E. Underwater engineering evaluations of transportation assets have historically relied largely upon conventional, crewed commercial diving operations, using visual testing (VT) and tactile examination methods to detect surface discontinuities and evaluate site conditions. In practical application, this approach alone is often found to be suboptimal, due to multiple challenges inherent in conducting inspections in the underwater environment. Modern underwater inspections are increasingly reliant upon new technologies, and nondestructive testing methods beyond VT are used during conventional diving inspection to gain a broader picture of the asset and its condition, increasing efficiency while lowering risk in the process. Underwater engineering inspectors today employ traditional nondestructive technologies, including VT, ultrasonic testing (UT), and magnetic particle testing (MT) techniques, in concert with acoustic (sonar) imaging techniques and remotely operated vehicles (ROVs) to obtain more detailed information about the asset and adjacent waterway conditions. This approach enhances the inspection’s safety and efficiency and reduces risk to the bridge owner and end user. This article discusses today’s underwater bridge inspection approach, emphasizing the NDT technologies utilized and their benefits. Introduction As transportation assets within the United States continue to age, the need to manage their performance and ensure their safety becomes increas- ingly important. This is especially true for the nation’s bridges, which rely on periodic safety inspections to assess con- dition and determine needs for main- tenance, repair, and eventual replace- ment. Bridge safety inspections are mandated by federal regulation, falling under the jurisdiction of various federal agencies including Federal Highway Administration (FHWA), US Department of Defense, Bureau of Reclamation, Bureau of Indian Affairs, US Forest Service, and others. According to 2021 National Bridge Inventory (NBI) data, there are approximately 500 000 bridges in the United States that span waterways. Of those, nearly 30 000 bridges exhibit submerged substructure elements that require stand-alone underwater bridge inspection. This article will discuss the historical state of practice regarding con- ducting regularly scheduled National Bridge Inspection Standards (NBIS) underwater bridge inspections, includ- ing their use of NDT techniques, as well as a newer, more innovative approach marrying conventional, commercial diving techniques with more advanced nondestructive technologies, including sonar imaging and remotely operated vehicles (ROVs). Some example proce- dures employed in this enhanced work approach will be presented, along with their advantages. Historical Inspection Approach Underwater engineering evaluations of bridge assets have historically involved conventional diving operations, using visual testing (VT) and tactile examina- tion methods to detect surface discon- tinuities and evaluate site conditions. Underwater inspections are required on a maximum 60-month inspection interval for all highway bridges in the United States, as per 23CFR650 National Bridge Inspection Standards (NBIS), with oversight provided principally by the FHWA as well as individual bridge owner-agencies. Aided by high-intensity underwater lamps and small hand tools to facilitate cleaning and rudimentary measurement, the formally accepted inspection practice is for the inspec- tion team to conduct a “Level I” “swim by” cursory inspection of the asset, in conjunction with a “Level II” hands-on, tactile examination of the asset elements, involving the localized removal of bio- fouling to expose the element surface. Unless otherwise scoped, the use of specialized testing to evaluate material properties or identify subsurface condi- tions has typically been conducted on an as-needed basis, as a “Level III” detailed, or “in-depth” inspection. The water body spanned by the bridge is also typically evaluated during the underwater inspection, to evaluate both geomorphologic aspects (the shape and physical characteristics of the waterway) as well as to detect scour (erosion of the banks and underwater channel bed caused by flowing water). Waterway inspection procedures during a typical NBIS underwater bridge inspection rely on VT, in conjunction with a conventional echo-sounding fathometer, to chart channel bottom profiles both along the exterior edges of the bridge as well as adjacent to individual substructure units. Channel bottom depth data is post-processed and converted into elevations, which, in turn, can be compared to past data as FEATURE | 36 M AT E R I A L S E V A L U AT I O N • J A N U A R Y 2 0 2 3 2301 ME Jan New.indd 36 12/20/22 8:15 AM

well as analytical data identifying critical bridge foundation elevations raising structural concerns. Trusted NDT Technologies Since the 1960s, transportation assets have been evaluated by divers using a combination of NDT devices that migrated inland largely from the offshore oil and gas fields, combined with modified “topside” (above water) NDT testing equipment and a few improvised pieces of gear. In addition to VT using ubiquitous handheld or helmet-mounted high-definition photographic and video cameras, measuring devices (usually wooden folding rulers) and clear water boxes (used to aid still photography in murky waters), inland diver inspectors conduct underwater structural inspec- tions using a small variety of hand-held NDT devices. Ultrasonic Testing (UT) Other than VT, select UT methods are by far the most common NDT tech- nology employed during underwater bridge inspections to identify, locate, and size discontinuities in steel, timber, and concrete members. The equip- ment used underwater comes in two varieties: modified topside ultrasonic equipment, typically consisting of a transducer connected with a long cable to a conventional topside UT scope or a self-contained, water-tight unit handheld by the diver (see Figure 1). In the former instance, the diver manipulates a straight-beam or angle-beam transducer underwater while readings are taken from a topside technician who controls the UT instrument in a clear, benign environment. More common today is self-contained equipment built specifi- cally for use underwater (see Figure 2). Ultrasonic thickness testing (UTT) is the most common UT technique employed during underwater bridge inspections. It is commonly used to obtain thickness measurements in steel members. The member to be tested is cleaned to establish a clean, smooth surface, and measurements are obtained in predetermined locations. For sub- structure supports using steel piles or columns, section measurements are typ- ically obtained in the splash zone, at the mudline, and near the midpoint of the water column. The results are typically archived in a matrix format, for ease of comparison to future measurements taken in the same areas. From this, deterioration rates may be established. Section thickness data of concern is evaluated analytically and appropriate actions are taken, which might include corrosion mitigation measures, structural repairs, or, in extreme instances, mod- ifications to loading of the element(s) until repair or replacement actions are effected. UTT equipment used in the field is almost exclusively of the self-contained variety. Ultrasonic angle-beam testing using shear waves (such as shear wave and phased array ultrasonic testing) is con- ducted on a very limited scale (primarily during in-depth, Level III inspections) on submerged bridge members for the inspection of welds, crack detection, and for sizing of discontinuities. While both single-element and multi-element phased array systems have been adapted for use underwater, the latter equipment type is seldom used during underwater bridge inspections. Unlike UTT, angle- beam testing in the inland environment is usually conducted using a topside scope, and the diver is merely manipu- lating the transducer, due to limitations introduced by water turbidity. Magnetic Particle Testing (MT) MT plays a diminished role in underwa- ter bridge inspection as compared to its use in the offshore underwater inspec- tion industry. Diver-manipulated yoke systems are at times used, but princi- pally only during Level III inspections in clear, calm waters, where the powdered metallic filings can be applied and indi- cations can be seen by the diver (see Figure 3). Unfortunately, these condi- tions are not often experienced in the inland environment. Other NDT Methods Other NDT methods and techniques are available to the underwater bridge inspection team for testing of steel members. Techniques such as acoustic emission testing (AE), time of flight dif- fraction (TOFD), and underwater pulsed eddy current (PEC), while in use in the offshore inspection arena, have not readily advanced into the inland bridge inspection industry. Underwater PEC in particular holds promise, consider- ing that the technology is specifically designed to detect corrosion hidden under marine growth or coatings. Steel wall thickness can thus be measured Figure 1. Underwater-capable UTT unit with long transducer cable. Figure 2. Diver held UTT unit. Note display visible to diver. Figure 3. Underwater MT system, with diver- held yoke and lamp. J A N U A R Y 2 0 2 3 • M AT E R I A L S E V A L U AT I O N 37 2301 ME Jan New.indd 37 12/20/22 8:15 AM