RESEARCH PAYS OFF

INSTRUMENTATION FOR

MEASURING SCOUR AT BRIDGE

PIERS AND ABUTMENTSJAMES D. SCHALL AND PAUL DAVIES

cour is the erosion of waterway soils andsediment that provide support for bridgefoundations. More bridge failures aredue to scour than to all other factors

combined. The resulting disruptions of rivercrossings represent a safety hazard for travelersand can have devastating impacts on localeconomies. Thus it is important for bridgeowners to know the scour susceptibility of theirbridges.

An ongoing scour evaluation program beingconducted by the Federal Highway Admini-stration and all state highway agencies has ledto the identification of more than 17,000 scour-critical bridges and nearly 100,000 bridgeswith unknown foundations. An additional86,000 bridges screened as scour-susceptiblehave not been evaluated. Given the limitedtime and funding available, the scour-criticalbridges cannot be immediately repaired or re-placed, nor can the scour-susceptible bridgesbe immediately evaluated, and the bridges withunknown foundations will require monitoring.

Problem

Scour is the primary cause of bridge failure inthe United States. Because scour holes gener-ally fill in as stream flows diminish, postfloodinspections are not adequate for fully deter-mining the extent of scour damage. Methodsfor measuring the maximum scour depth are

needed in the management of scour-susceptiblebridges.

SolutionTechnically and economically feasible instru-ments for monitoring scour depth were devel-oped under National Cooperative HighwayResearch Program Project 21-3, Instrumenta-tion for Measuring Scour at Bridge Piers andAbutments. Two instrument systems—a low-cost sonic fathometer and a magnetic slidingcollar device using a driven-rod support—wereinstalled and tested in the field under a widerange of bridge substructure geometry, flow,and geomorphic conditions.

The low-cost sonar device (see Figure 1) con-sists of a simple fish-finder-type sonar connectedto a data logger that tells the sonar when to turnon, how much data to collect, and when to turnoff. The systems were installed successfully onriverine and tidal bridges by a state highwayagency using in-house equipment and personnel.

The magnetic sliding collar device (see Fig-ure 2) consists of a stainless steel pipe driveninto the channel bottom with a sliding collarthat drops down the pipe as the scour pro-gresses. The location of the collar is detectedby the magnetic field created by magnets onthe collar. Installations conducted in coopera-tion with state highway agencies demonstratedthat this simple, low-cost instrument is adapt-able to various field situations, and can be in-stalled with the equipment and technical skillsnormally available at the district level of a statehighway agency.

Another innovation, developed independ-ently of the NCHRP project, is a float-out de-vice. This device consists of a radio transmitterburied in the channel bed at a predetermineddepth. When the scour reaches that depth, thefloat-out device rises to the surface and beginstransmitting a radio signal that is detected by areceiver in an instrument shelter on the bridge.A conventional drill rig with a hollow stemauger is used to install the transmitter. Afterreaching the desired depth, the transmitter isdropped down the center of the auger.

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NCHRP Report on Scour-Measurement Devices

NCHRP Report 396, Instrumentation for Measuring Scour at BridgePiers and Abutments, documents all phases of the developmentwork for the devices described in this article. Findings from labora-tory and field testing are presented for each device, and a detaileddiscussion of the significance of these findings is presented. Thecompanion manuals (NCHRP Report 397A, Sonar Scour Monitor:Installation, Operation, and Fabrication Manual; and NCHRP Report397B, Magnetic Sliding Collar Scour Monitor: Installation, Operation,and Fabrication Manual), provide guidance for selecting the devicemost suitable for a bridge and its location. Detailed instructions, in-cluding fabrication drawings and parts lists, are included to permitfabrication of the monitors in most machine shops. Instructions foroperation and maintenance are also given.

Application

In preparation for storm events driven by ElNiño, a variety of instruments were installed atbridges in the southwestern United States inlate 1997 and early 1998. Five bridges wereinstrumented in California, five in Arizona, andfour in Nevada. The equipment included auto-mated sliding collar, low-cost sonar, mul-tichannel sonar, and float-out devices. Theseinstallations provided an opportunity to test anumber of new concepts, including two- andfour-channel sonar devices, application of earlywarning concepts (through definition ofthreshold scour levels and automated calls topagers when that threshold is exceeded), anddevelopment and refinement of the float-outinstrument concept.

During the testing, the SR 101 bridge overthe Salinas River near Soledad, California, expe-rienced several scour events that triggeredthreshold warn-ings. In one case the automatedsliding collar dropped 1.5 m (5 ft), causing apager call-out. Portable sonar measurementsconfirmed the scour recorded by the slidingcollar. Several days later, another pager call-outoccurred from a float-out d-evice buried about 4m (13 ft) below the streambed. In both cases, thecritical scour depth was about 6 m (20 ft) belowthe streambed, and no emergency action wasneeded to ensure public and bridge safety. Be-cause pager call-out was ineffective in alertingmaintenance personnel during nonoffice hours, aprogrammed voice synthesizer call-out to

human-operated 24-hour communicationscenters was implemented at other bridges.

BenefitsThe instruments developed under this researchand through additional commercial develop-ment have been tested extensively and are fullyfield-deployable. Use of these instruments forscour monitoring will provide state highwayagencies with an essential element of their ac-tion plans for bridges that are scour-critical orscour-susceptible, or have an unknown foun-dation. Use of these devices will allow moni-toring of scour-critical bridges so that solutionscan be developed before the problem becomessevere. In some circumstances, this monitoringwill be able to provide a long-term alternativecountermeasure for scour.

For further information contact Peter F. Lagasse,Ayres Associates, 3665 JFK Parkway, P.O. Box270460, Building 2, Suite 300, Fort Collins, Colo-rado 80522 (telephone 970-223-5556).

EDITOR'S NOTE: Appreciation is expressed to DavidBeal, Transportation Research Board, for his effortsin developing this article.

Suggestions for "Research Pays Off" topicsare welcome. Contact G. P. Jayaprakash,Transportation Research Board, 2101 Con-stitution Avenue, N.W., Washington, D.C.20418 (telephone 202-334-2952, e-mailgjayapra@nas.edu).

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