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USE OF TIME DOMAIN REFLECTOMETRY TO DETECT BRIDGE SCOUR AND MONITOR PIER MOVEMENT
Written by Charles H. Dowding, Professor and Charles E. Pierce, Research
Assistant June 1994
Table of Contents
Time domain reflectometry (TDR) technology can be deployed to detect bridge scour and monitor pier and abutment movement during flood events, when the maximum amount of scour is likely to occur. However, application of TDR to bridge foundation monitoring requires development of an innovative means to package and install the cable system underwater as well as a low power consumption, intelligent pulser. This paper discusses the background of bridge scour and required instrumentation, in addition to initial design considerations for TDR-based instrumentation. The envisioned TDR scour detection system measures the length of a buried cable, where flanges attached to the cable are expected to be torn off by high drag forces as the river bed is scoured away, thus shearing the cable at the flange connection. The shearing process shortens the cable and a signal reflection produced at the cable break marks the scour depth. Measurement of lateral pier or abutment movement is accomplished with a similar system without flanges. As the footing displaces relative to the sediment, the cable is deformed and a voltage pulse is reflected at the location of distortion. The reflection magnitude is directly proportional to the amount of cable deformation, which in turn corresponds to the footing displacement.
Scour has been linked to nearly 95% of all severely damaged and failed highway bridges constructed over waterways in the United States (10). There are two issues associated with such scour-induced damage to bridge pier footings. The first effect is the loss of foundation material, shown in COLOR PLATE 1, which exposes the footing and lowers its factor of safety with regard to sliding or lateral deformation. The greatest loss of sediment to scour occurs at high water velocities, such as during floods. Secondly, pier movement may occur as a result of material loss beside and beneath the base of the footing, which produces undesired stresses in the bridge structure and ultimately results in structural collapse, as shown by one of the many scour-induced failures during the 1993 Mississippi River floods in COLOR PLATE 2. For these reasons, the study of bridge scour has drawn much attention in recent years.
Measurements of scour at bridges founded on shallow footings indicate maximum scour occurs around the upstream side of the pier footing during floods, as shown in FIGURE 1. As scour progresses, the footing may be exposed and eventually undermined, leading to intolerable movement of the pier. However, as the waters recede, sediment may be loosely redeposited in and around the scour holes, leading to false postflood measurements of actual scour depth during floods, indicated by the higher bed elevation in FIGURE 1. Optimal placement and employment of scour monitors would therefore be in front of the footing on the upstream side and be operative at high water flows.
Four important stages of bridge scour research have been identified as 1) scour measurement during flooding conditions, 2) comparison of field measurements with scour equations, 3) identification of bridges sensitive to scour, and 4) development of monitoring and protection systems for these bridges. Current scour investigations are focused on the first two stages, where existing techniques to measure scour are being evaluated. In addition, Jones (7) lists new instrumentation as one of the most important research needs, particularly because field data collection during floods is severely hampered by currently available equipment and testing methods.
Two basic instrumentation needs have been identified. First, an inexpensive, temporary, and deployable instrument is needed to allow a large number of measurements to be made with limited funds for stage 1 and 2 research. Second, fixed systems are required for stage 4 to continuously monitor bridges that have been determined to be sensitive to scour in stage 3. Most importantly, there is a need for diverse instrumentation, as one device or method is not applicable to all field conditions.
This article discusses the possibility of employing TDR cables to measure bridge scour and pier or abutment movement. It also describes conceptual design of new schemes to deploy TDR cables to address problems of both scour and pier/abutment movement.
Total scour is comprised of long-term channel aggradation and degradation, contraction scour, and local scour. A thorough description of these effects on total scour and a detailed review of scour evaluation for bridges can be found in (11). Local scour involves the removal of material from around underwater structures such as bridge piers, abutments, spurs and embankments, thus it has the greatest impact on bridge integrity. Local scour results from acceleration of flow and subsequent development of vortices around obstructions, as shown in FIGURE 2. The action of both horseshoe and wake vortices is to displace material from the base of the obstruction, in this case a cylindrical pier, which can result in undermining the structure.
The most critical factors contributing to local scour are the velocity and depth of flow, both of which are significantly increased during heavy storms and floods. As the velocity and/or depth increase, the amount of scour increases. Therefore it is important to measure the maximum amount of scour at these critical flood stages. Of course the most difficult time to measure the elevation of river sediment occurs during flood conditions. Other factors affecting bridge scour include the dimensions and orientation of piers, bed configuration and material size/gradation, and accumulation of ice and debris along the piers. Current methods for measuring scour can be divided into two groups, postflood and flood stage measurement systems (5). One of the most commonly employed methods for measuring postflood scour is visual underwater bridge inspection. Although accurate measurements of scour can be obtained with highly trained divers, underwater inspections do not provide continual surveillance of the scour process and certainly cannot be performed in high flow conditions, when maximum scour is likely to occur. Unfortunately, flood stage scour erosion during peak flow is filled in during low flow immediately following flooding. Therefore postflood inspection does not provide the information necessary to determine the actual risk arising from scour.
Sonic devices, such as depth sounders and transducers, and sounding rods are frequently used to measure bridge scour. Current practices call for manual operation of sonar and sounding rods, thereby discouraging measurement during dangerous weather. However, these instruments can be integrated with real-time systems to monitor flood stage scour (9).
Lagasse (9) is currently engaged in a project funded by the National Cooperative Highway Research Program (NCHRP) to develop, test, and evaluate existing and new instrumentation to measure maximum scour depth at bridge piers and abutments during floods. This research is divided into three stages. Phase I involved identification and evaluation of available instruments. Phase II research includes modification and field testing, which is currently underway, of select instruments recognized in the first stage as potential field devices for Phase III study. This final stage extends the work to development and deployment of these devices for remote measurement.
To guide the enhancement of existing methods and development of new devices, specific criteria were assigned as either required or desirable, according to the relative impact on the project. These scour monitoring devices would be required to (1) be installable on or near a bridge pier or abutment, (2) measure maximum scour depth within an accuracy of +/- 0.6 m, (3) obtain scour depth readings from above the water or from a remote site, and (4) operate during storm and flood conditions. Desirable criteria for the devices are installability on most existing bridges or during construction of new bridges, operability in a range of flow conditions, including the impact of ice and debris, operability and maintainability at relatively low cost by highway maintenance personnel, which includes prevention of vandalism.
The Lagasse (9) study recognized four general categories of present scour monitoring devices that could be employed during floods. These instruments were divided among sonar, sounding rods, buried or driven rods, and other buried devices. After laboratory and prototype testing of instruments from each of these categories during Phase I, three were selected from the sonar and sounding rod groups for modification and further field studies in Phase II. These included a sonic fathometer, and two sounding rods, one called the Scour Tracker and the other called the Brisco Monitor. Lagasse and Nordin (8) describe these instruments in greater detail. All three of these instruments have been previously used in an unmodified form for bridge scour measurements.
Sonar and sounding rods are relatively inexpensive and simple devices, but each has its own limitations. The sonar approach involves measurement of the distance to the top of sediment from a transponder located in a protected pipe at the mean water level. It does not perform well under high sediment loads, and can give misleading information if debris and/or ice is present. Sounding rods measure the elevation of a plate bearing on the top of the sediment as it slides up and down a large pipe embedded in the sediment in front of a pier or abutment. These rods can induce additional scour around the baseplate and create vortex shedding along the unsupported rod length. Trapped sediment between the support and free sliding rod can also interfere with its mobility. These limitations substantiate the claim that any one scour monitoring device cannot be successfully employed in all environments.
A TDR method being developed to measure scour depth and lateral pier or abutment movement involves accessible electronics and pulsing of an inexpensive, robust cable. Most importantly, it would utilize an existing infrastructure for digital telecommunication. These attributes, especially the telecommunication possibilities, make TDR an ideal candidate for Phase III of the national effort to monitor bridges that have been determined to be sensitive to scour.
This TDR system has been employed in the mining industry for over 5 years and has thus been field tested. For instance, COLOR PLATE 3 shows a sequence of photographs of the installation of TDR cables upside down in a potash mine in New Mexico.
Two approaches are being taken to deploy TDR cable systems for Phase III bridge monitoring. First, cable systems are being designed to allow direct measurement of scour. Secondly, systems are being designed to measure small lateral movements of bridge piers/abutments relative to the foundation materials. This second approach involves monitoring the onset of lateral movement which would reduce the stability of a bridge during floods. Either of these two cable systems would be attached to a low cost, low power consuming, intelligent voltage pulser that is compatible with a wide range of telecommunication data loggers. This intelligent pulser would automatically detect changes in the cable length.
SCOUR DEPTH MEASUREMENT DURING FLOOD
The proposed design of a TDR system for measuring scour depth s based on recording voltage reflections arising from shear deformation at known weakness locations along a vertical, metallic coaxial cable embedded in front of a bridge pier as shown in FIGURE 3. Flanges are positioned at these weakened locations to precipitate shearing of the cable. When the bed sediment erodes to a flange elevation, it will be exposed to the high water velocity environment. The water drag forces will load the flange and deform the cable. As the cable deforms at a flange site, a TDR signal reflection will develop, indicating that the sediment bed has scoured to the flange elevation.
The most challenging aspect of this design is the installation of the cable and flanges in the sediment in front of a pier footing. Placement of the device is envisioned on the upstream side of a pier and in front of the spread footing. A borehole would be drilled at this location and cased to allow installation of the preconstructed cable system. As shown in FIGURE 3, the cable system will consist of a single metallic cable placed vertically inside a protected cableway to its open end, where the cable is bent 180 degrees to extend upward alongside the cableway. Metallic, vane-shaped "shear" flanges will be attached to the exposed cable at predetermined locations. To meet the accuracy requirements of the NCHRP project, the flanges should be separated by a maximum of 0.6 m.
The single cable with multiple flanges would then be lowered into the cased hole, after which the hole should be carefully backfilled with a clean sand. As the backfill is tremmied to the existing river bottom elevation, the casing is removed, leaving the exposed length of cable with the attached flanges buried in a sand column adjacent to the footing.
As scour occurs near the cable, the backfill will be removed along with the natural bed materials. Exposing the cable will subject a flange to high velocity water flow, thus causing flange rotation and subsequent shearing of the cable. This operation is depicted in the detailed drawing of the flange shown in FIGURE 3.
Two alternative schemes for measuring bridge scour are also currently being explored in addition to the primary design. The first scheme involves locating the river bed elevation by measuring voltage reflections due to changes in cable impedance at the interface of the saturated soil and flowing water. This method invokes a technique from previous research on water level detection (2), where an air-water interface was of interest. A second alternate for direct measurement of scour involves monitoring signal reflections produced by a magnet, which marks the elevation of the sliding collar of a Scour Tracker (9). This magnetic sliding collar follows the top of sediment by moving along a pipe driven into the sediment. As the bed material beneath the magnet is scoured away, the collar moves simultaneously downward to rest on the bottom of the scour hole. With TDR, continuous movement of a free-sliding magnet could be tracked by monitoring the waveforms along the entire cable length. Preliminary measurements indicate that the approach may be possible, however weakness of the voltage reflections may preclude its use, thus a great deal of work would be necessary.
BRIDGE PIER OR ABUTMENT MOVEMENT
TDR technology has been successfully deployed in the field to measure rock mass deformation (1,4) and is currently being developed to measure localized soil deformation (3). The cable- grout system described in (3) can also be applied to monitor footing or abutment movements resulting from scour and other processes. As shown in FIGURE 4, lateral footing movement produces local shearing of the cable-grout structure (i.e. cable deformation) at the interface of the footing and foundation soil. This deformation in turn produces a distinguishable voltage reflection. FIGURE 4 illustrates the anticipated orientation of the cable extending from the cable tester, down along the pier, and into a hole drilled through the footing and foundation material. The cable is encased in grout from the bottom of the hole to the top of the footing. A protective pipe, needed to screen the cable from debris, encloses the cable from the top of the footing to the bridge deck.
Horizontal movement of the pier footing can be detected from voltage reflections generated by local cable deformation. Lateral translation or rotation of the footing would progressively shear the cable as illustrated in FIGURE 4, thereby producing a unique signal reflection at that point along the cable. As shown in (4), the reflection amplitude can be directly related to the amount of shear deformation, which in turn quantifies displacement of the footing. Shearing of thin TDR cables would allow detection of pier movements on the order of millimeters, which is sufficiently small to initiate alarm.
The intelligent pulsers for either approach would be installed in vandal-proof enclosures beneath the bridge deck or other nonobtrusive locations. Data can be collected directly from the instrument on the bridge deck or remotely via a modem (or cellular telephone). Remote, telecommunication access as shown in FIGURE 5, and multiplexing capability have been demonstrated successfully in the field with TDR techniques to measure rock deformation (6). With the electronics enclosed on the bridge deck and a protected cable installed underwater, it is expected that a TDR device would be operable during storm conditions. In the case of electrical power outage, power would be maintained by a battery.
A miniaturized, low power, intelligent TDR pulser has been proposed for development by Northwestern University Infrastructure Technology Institute, HYPERLABS, and the U.S. Army Corps of Engineers for remotely monitoring groundwater levels and rock or soil shear deformations. This signal pulsing and recording instrument is designed to reduce the complexity and cost of current monitoring electronics. Most importantly for scour monitoring applications, it is expected to perform in harsh field environments, and operate with lower power consumption.
The miniature TDR pulser communication protocol will adhere to standard U.S.G.S. SDI-12 specifications for intelligent systems. Thus it will be compatible with a range of telecommunication hardware and software, and as a result no special equipment or personnel will be required for its adoption by industry, which will facilitate adoption of TDR cable monitoring approaches.
MULTIPLEXING AND REMOTE ACCESS APPLIED TO BRIDGE MONITORING
Huang and Dowding (6) describe the multiplexing capabilities and remote accessibility of a TDR cable testing system for rock mass deformation measurement. A single cable tester can be multiplexed to handle eight or more cables. The tester can also be controlled from a remote location via modem, as shown in FIGURE 5.
For the scour measuring and pier movement schemes presented in this paper, single or multiple cables can be installed on each bridge pier and continuously monitored from a central unit on the bridge deck. Thus a survey of all footings can be performed from one unit, saving time and reducing costs.
In the schemes described in this article, a single cable is utilized for pulse transmission and is connected to a cable tester inconspicuously located on the bridge deck. However, a multitude of cable configurations can be installed on one or multiple bridge piers and monitored from the same unit.
Bridge scour has been identified as the major cause of decline in structural integrity of bridges and other waterway structures. Diverse instrumentation is currently needed to measure scour and monitor structural response during floods. It has been suggested that time domain reflectometry technology can be used in the development of fixed systems to collect real-time data on scour depth and pier or abutment movement. Several advantages of a TDR- based device include easy access to electronics, pulsing of inexpensive cables, and most importantly telecommunication enhancements, such as multiplexing and remote access. The conceptual designs and proposed applications of these instruments are described in this paper.
1. Dowding, C.H. and F.C. Huang. Early Detection of Rock Movement with Time Domain Reflectometry. Accepted for publication, Journal of Geotechnical Engineering, ASCE, v. 120, No. 8, 1994, 28 p.
2. _____. Ground Water Pressure Measurement with Time Domain Reflectometry. Proceedings of the First Symposium and Workshop on Time Domain Reflectometry, Northwestern University, Evanston, IL, 1994, 12 p.
3. Dowding, C.H. and C.E. Pierce. Measurement of Localized Failure Planes in Soil with Time Domain Reflectometry. Proceedings of the First Symposium and Workshop on Time Domain Reflectometry, Northwestern University, Evanston, IL, 1994, 11 p.
4. Dowding, C.H., M.B. Su, and K. O'Connor. Measurement of Rock Mass Deformation with Grouted Coaxial Antenna Cables. Rock Mechanics and Rock Engineering, v. 22, 1989, pp. 1-23.
5. Fenner, T.J. Scoping Out Scour. Civil Engineering, ASCE, v. 63, No. 3, 1993, pp. 75-77.
6. Huang, F.C. and C.H. Dowding. Telemetric and Multiplexing Enhancement of Time Domain Reflectometry Measurements. Proceedings of the First Symposium and Workshop on Time Domain Reflectometry, Northwestern University, Evanston, IL, 1994, 12 p.
7. Jones, J.S., R.E. Trent, and D.L. Potter. Bridge Scour Research Needs. Proceedings of the 1991 National Conference on Hydraulic Engineering, ASCE, 1991, pp. 323-328.
8. Lagasse, P.F. and C.F. Nordin. Scour Measuring and Monitoring Equipment for Bridges. Proceedings of the 1991 National Conference on Hydraulic Engineering, ASCE, 1991, pp. 311-316.
9. Lagasse, P.F., C.F. Nordin, J.D. Schall, and G.V. Sabol. Scour Monitoring Devices for Bridges. Transportation Research Record, Third Bridge Engineering Conference, Transportation Research Board, v. 2, No. 1290, 1991, pp. 281-294.
10. Lefter, J. Instrumentation for Measuring Scour at Bridge Piers and Abutments. NCHRP Research Results Digest, Transportation Research Board, No. 189, 1993, 8 p.
11. Richardson, E.V., L.J. Harrison, J.R. Richardson, and S.R. Davis. Evaluation of Scour at Bridges. 2nd ed. Federal Highway Administration, Publication No. FHWA-IP-90-017, HEC No. 18, 1993, 132 p.
Time Domain Reflectometry (TDR) is an electrical pulse testing technique originally developed to locate discontinuities in power transmission cables that has been modified to monitor deformation of a coaxial cable grouted into rock mass as shown in FIGURE A1(4). As shown in FIGURE A2, rock mass movements deform the grouted cable, which locally changes cable capacitance and the reflected waveform of the voltage pulse. Ultimately large shearing deformations sever the cable and shorten the reflected voltage signature as shown by the decreasing signatures in FIGURE A2. By measuring changes in these reflection signatures, it is possible to monitor both local extension and local shearing. Furthermore, the sign, length, and amplitude of the individual reflection pulses define the location, type, and severity respectively of every cable deformity.
FIGURE A3 presents sample TDR records arranged by date and time (increasing to the right) and illustrates signature patterns that occur prior to severance as the face approaches and moves past the cable (negative values indicate distance past the cable). These records show that strata movement sheared the cable producing the leftward signal deflections, called reflection spikes marked as (a). These reflections increase in amplitude as the longwall face advances before strata movement severs the cable (which produces the open end marked as (b)). Additional shearing is also detected and is marked as (c). Precrimped distance indicators, marked as (d), improve location accuracy of measurements. Waveform 1 (May 28, 10:46) is the first signature recorded after cable installation. This record serves as the benchmark against which other signals are compared. Waveforms 2 (June 12, 16:35), 3 (June 13, 7:21), and 4 (June 13, 16:28) were recorded before, at, and after complete severance at depth of 66 m (217 ft). In a similar fashion, waveforms 5 (June 14, 11:17), 6 (June 14, 16:27), and 7 (June 14, 20:36) were recorded before, at, and after complete severance at a depth of 48 m (158 ft).
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