Monitoring Scour Critical Bridges (2009)

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51 This chapter discusses improvements suggested by the survey respondents and needs of the equipment. Scour monitoring solutions reported in the surveys and in the search for recent literature were discussed in chapter four. Suggestions that have been made regarding possible alternatives for improving scour monitoring technology are also included. Information on current guidelines, programs, and manuals for scour mon- itoring systems is documented in Appendix G. CURRENT STUDIES ON SCOUR MONITORING INSTRUMENTATION In addition to NCHRP Project 21-03 on fixed scour instru- mentation (Lagasse et al. 1997), several state DOTs have or are currently conducting research in this area. States that have conducted research in this area in the past include Iowa, New Jersey, New York, Oregon, Texas (University of Texas), and Vermont. The reports from these studies can be found in the references. The following sections discuss some of the more recent on-going projects. Arkansas The ASHTD reported that they are currently conducting a project called “Development of a Bridge Scour Monitoring System.” Arkansas has identified approximately 100 bridge sites that are scour critical and therefore require an FHWA Plan of Action. They report that their current method of monitoring is ineffective because it is dependent on personnel visiting the site and taking measurements, which can be before or after the maximum scour has occurred. Its objective is to recommend or develop scour monitoring systems that can be used to continuously monitor and record scour depths with corresponding water surface elevations at a bridge site. The agency designed and installed a system using sonar for streambed monitoring, an ultrasonic distance sensor for stage, and a Pan Tilt Zoom network camera for sonar and video still images. The system automatically transmits data to the Internet so that it can be accessed by the proper authorities. The project started in the year 2005 and is on-going. Michigan The Michigan DOT reports that in an effort to reduce uncer- tainty associated with scour prediction models it has a project to collect field-scale data related to pier scour. The research project is being done by Wayne State University and Lawrence Technological University. The goal is to reduce uncertainty to help reduce the cost of bridge construction without sac- rificing safety. The project includes the instrumentation of a bridge over the Flint River with one sonar device and one water stage. Bridge scour data were collected once per hour and transmitted by means of cellular telemetry back to the project computer. The results are stored on a network storage device shared between the universities and the Michigan DOT. The project started in the year 2008 and is on-going. Minnesota The Minnesota DOT is conducting a project called “Bridge Scour Monitoring Technologies: Development of Evaluation and Selection Protocols for Application on River Bridges in Minnesota,” which was awarded to University of Minnesota in 2008. The project is to take the first steps toward develop- ing robust scour monitoring for Minnesota river bridges. The project methodology includes the identification of the vari- ables of scour critical bridges that affect the application of scour monitoring technology and incorporation of the findings into a “Scour Monitoring Decision Framework” that will aid the Minnesota DOT in selecting the best technologies for specific sites. The final component of the project involves testing the Decision Framework on five bridges in a case study- type demonstration. Ohio The Ohio DOT is conducting a project called “Field Monitor- ing of Scour Critical Bridges: A Pilot Study of Time Domain Reflectometry for Real-Time Automatic Scour Monitoring Systems.” The project was awarded in 2008 to Case Western Reserve University and GRL Engineers, Inc. The Ohio DOT reports that the effectiveness of current scour countermeasures is still unclear and collecting scour data directly from the field is necessary to improve the current specifications. They also note that existing field scour measurement equipment is not completely satisfactory in that it is not sufficiently rugged, does not provide real-time monitoring during flood events, and it is not automated. The project will develop and deploy a rugged and inexpensive TDR real-time automatic scour monitoring system with several innovations. Tennessee The Tennessee DOT reported in its survey with the Univer- sity of Memphis that they were working on a research project CHAPTER SEVEN ON-GOING RESEARCH AND INNOVATIVE SOLUTIONS FOR SCOUR MONITORING

52 using thermocouples spaced along a piling to determine the elevation of the streambed. This instrument is similar to the piezoelectric film monitors, except the thermocouple measures the ground temperature instead of the water temperature. They have built a few prototypes that have been placed in a stream near a bridge. They reported that they encountered several problems including vandalism, lightening, and debris wash- ing the whole piling downstream. They have recently been reassessing the software and communication options for remote data storage. They note that their experience with fixed scour monitors is “maintenance intense to insure the instrument does what you want it to do.” Texas In 2007, Texas DOT awarded a contract to Texas Transporta- tion Institute/Texas A&M University for the development of fixed scour monitors for bridges. The 3-year project is called “Realtime Monitoring of Scour Events Using Remote Monitoring Technology.” They note that scour monitors are still in development and there is a need to make them less expensive, easier to install, more robust, and to optimize the remote and wireless data collection and warning systems. These are the goals of this project and it can include improve- ments to existing devices or development of new devices. The project will include the evaluation of existing technology, the development of new technology, laboratory testing, field installations and demonstrations, and the use of data to eval- uate scour depth predictions. Japan Inclinometers have been used to measure scour on railroad bridges in Japan (Suzuki et al. 2007). The early installations are discussed in chapter three. Suzuki points out that the current technology does not give enough advance warning of scour problems. He notes that even if the inclination angle of a bridge pier is minute, reconstruction of the pier tends to take a long time, is expensive, and includes suspension of train service. They are currently working on a project to develop a new device to alert of scour damage prior to the inclination of a pier. The system is a vibration-based health monitoring method that employs accelerometers set on the top of the bridge piers. They are using train-induced vibration analysis measured by two piezoelectric accelerometers to evaluate the stability of the bridge pier. The gradient of the linear regression line between vertical and transverse acceleration responses is being investigated as an indication of the stability of the pier. The study includes a 3D analytical model, physical model experiments using stable and damaged piers, and field mon- itoring of bridges with various parameters. Preliminary results indicate that this proposed indicator is related to sediment loss at the foundation, and the method has potential for health monitoring of railway bridge piers. They recommend that these accelerometer devices be installed in conjunction with the inclinometer scour monitoring device. Debris Scour Many of the owners who use or do not use fixed scour mon- itoring instrumentation stated in their surveys that debris is a problem with fixed monitors. NCHRP Project 24-26 on the “Effects of Debris on Bridge-Pier Scour” began in 2004 and is expected to be completed in 2009. The objective of this study of debris at bridge piers is to develop guidelines for predicting the size and geometry of the debris, and for quantifying the potential scour. The data from this study will provide infor- mation on debris that can be useful for analyzing a site, and for the design of the scour monitoring devices that can better withstand potential debris forces. INNOVATIVE SOLUTIONS FOR SCOUR MONITORING SYSTEMS It should be noted that solutions to some of the concerns expressed by bridge owners are already being implemented in the new monitoring installations or are currently under devel- opment. Remote access for downloading scour data is currently being used successfully on numerous sites throughout the United States. In Alaska, one of the monitoring system ven- dors has designed and fabricated a movable sonar scour mon- itoring system for a bridge with debris problems. This bridge had two fixed sonar scour monitors torn from it as a result of debris flows. This new system consists of a winch mounted below the bridge deck, which lowers the sonar scour measuring device into the water at set intervals. When the sonar assembly reaches the water, it stops the winch and the sonar can take a series of readings. The winch then raises the sonar back up, where it is stored and protected under the bridge deck. An adjustable mounting bracket has been developed for use in underwater sonar monitoring installations where the geometry of the pier or abutment is uncertain. This enables those installing the monitor to adjust the bracket so that the sonar device clears the footing to take readings of the streambed below. (Refer to Appendix G for a plan depicting this bracket.) This bracket design is in stainless steel for installation in tidal waterways. The downloading of data was at the bridge site for the earlier installations of fixed scour monitors. The design of a remote downloading technology using landline or cell tele- phones was first used in 1997 in the design of the scour monitoring system for the Woodrow Wilson Memorial Bridge in Washington D.C. (Hunt et al. 1998). Alaska has used satellite communications to retrieve the data in remote areas that do not have reliable telephone service (Conaway 2006). The data for all three communications can be retrieved to a base computer(s) equipped with the software to download the data. More recent installations are using the Internet to host and retrieve the data. This allows more flexibility for accessing the data and makes it easier for numerous persons to retrieve and analyze the data from a variety of locations.