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Approximately 584,000 bridges in the National Bridge Inventory (NBI) are built over streams. Many of these bridges span alluvial streams that are continually adjust- ing their beds and banks. Many of these bridges will experience problems with scour and stream instability during their useful life. In fact, the most common cause of bridge failure is scouring of bed material from around bridge foundations during flooding. Scour and stream instability problems have always threatened the safety of the U.S. highway system. The National Bridge Inspection Standards (NBIS) require bridge own- ers to maintain a bridge inspection program that includes procedures for underwater inspection. Furthermore, a national scour evaluation program as an integral part of the NBIS was established by the FHWA in 1988. As a result of the scour evaluation program, more than 26,000 bridges were classified as âscour criticalâ in 2002. FHWA policy specifies that a plan of action should be developed for each bridge iden- tified as scour critical. The two primary components of the plan of action are instructions regarding the type and frequency of inspections to be made at the bridges and a schedule for the timely design and construction of scour countermeasures. The purpose of the plan of action is to provide for the safety of the traveling public and to minimize the potential for bridge failure by prescribing site-specific actions that will be taken at the bridge to correct the scour problem. A well-defined monitoring program is an important aspect of the plan of action and could incorporate various fixed and portable scour instrumentation devices. During the last 10 years, significant research and progress has been made with fixed instrumentation. However, fixed instrumentation is not suitable, practical, or cost-effective for all bridges. In many cases, portable monitoring during a flood is a better solution; however, not much research has been completed to improve this type of technology. Conventional methods of portable monitoring are built on physical probing and sonar; however, both these techniques have limitations during flood events when the flow depth and/or velocity are high. As a result, critical decisions on bridge safety during floods have been hampered by the limitations of the existing equipment and its application. Without adequate scour data during flood events, some bridges have been closed unnecessarily, causing traffic delays and increased expenses, while other bridges that should have been closed were not, resulting in increased risk and liability. The objective of this research was to improve deployment, positioning, and data col- lection procedures for portable scour monitoring during flood conditions. The research SUMMARY PORTABLE SCOUR MONITORING EQUIPMENT
concentrated on developing a truck-mounted articulated crane to position various mea- surement devices quickly and safely. The use of a crane for scour monitoring provided a solid platform for deployment, even under flood flow conditions, that could be instru- mented to allow precise measurement of the movement of the crane. Given the avail- ability of knuckle booms or folding cranes in the construction industry, the research was designed around modifying and instrumenting this type of articulated crane for use in scour monitoring research. The result of the research was a fully instrumented articulated arm truck. The artic- ulated arm truck was designed using readily available components whenever possible. These components and pieces were also designed to be a bolt-on installation, so that the articulated arm truck could be readily used for other purposes outside of the flood season. In fact, many transportation agencies already have articulated arm trucks that could be retrofitted for scour monitoring work based on the design concepts developed through this research. A streamlined probe to position a wireless sonar was developed to allow measurement in the high-velocity conditions during a flood. Using this probe, the sonar could be positioned directly in the water nearly 30 ft (9.1 m) below the bridge deck. Once in the water, the crane could be rotated in an arc to collect data continuously upstream of the pier. The truck could also be driven across the bridge with the crane extended to collect a cross section profile quickly. A dual winch application of traditional cable suspended techniques was developed to facilitate working off higher bridges. Other deployment methods were developed to allow working off various bridge configurations, including a wireless sonar in a sounding weight and in a kneeboard deployment with a rigid frame. Using the articulated arm truck with different sensor deployment methods allows a wide variety of bridge geometries to be monitored during flood events, including high bridges, bridges with limited clearance, and bridges with large overhangs. A compre- hensive data collection software package was developed that facilitated the use of the articulated arm, providing the inspector immediate access to the data collected. Collec- tion of position and scour data is automated, and a data file is written that allows plotting of channel section or scour hole bathymetry. The articulated arm met most, but not all, of the criteria defined for this research. The research did not solve the measurement problems associated with debris and ice, which have been, and probably will continue to be, the nemesis of portable scour monitoring (as they are with fixed instrument monitoring). However, the articulated arm did substan- tially improve the ability to make measurements in high-velocity flow during flood con- ditions. These measurements can be completed from various bridge geometries using a truck that is affordable and maneuverable. The data collection process has been auto- mated, and the scour data are presented in the bridge coordinate system, allowing rapid evaluation of scour criticality. Overall, the ability to make portable scour measurements during flood flow conditions has been substantially improved. 2
