Current methodologies for predicting scour depths around bridge piers typically employ empirical equations derived from physical model studies using non-cohesive, uniformly graded sands. This approach represents a worst-case condition since non-cohesive sands are one of the most erodible soils found in nature. In practice, these equations are commonly applied to all soils that cannot be strictly classified as non-erodible. Since very little, easy-to-apply information is available to evaluate potential scour in erosion resistant soils, a great deal of engineering experience is necessary for one to feel confident about quantifying any reduction of the scour estimated from these equations. Consequently, because of the risks involved, predictions of scour in erosion resistant soils are frequently conservative, resulting in overly deep and expensive pier foundations. The unlimited range of soil types and combinations of soil types found in nature creates a full continuum of erodibility from the easily erodible, very fine silts to the non-erodible, competent rock. If even possible to fully describe this erodibility continuum, it will take significant time, effort, and money to develop reliable, practical methodologies and models for doing so. More immediate assistance is needed in this regard. An effective in-situ scour testing device could provide this assistance now on a project-by-project basis. Such a field device could more accurately define the scour potential for a given set of hydraulic design conditions and pier type, regardless of the foundation soil type or types present.

The objective of this research is to prove or disprove the viability of such an in-situ scour testing device for use as a foundation design aid by the highway engineering community. The field device for the proposed study could consist of a water jet directed vertically downward into the soils that are to support the bridge pier foundations. The jet would be calibrated through physical model testing and computer simulation to produce the predicted scour depth in a sand-bed channel for the design hydraulic conditions and proposed pier geometry. The water jet would be run until equilibrium conditions are reached in the resulting scour hole, or until some maximum period of time has elapsed (such as the expected cumulative time the foundation will be exposed to the design discharge over the life of the bridge). The in-situ soils would thereby be exposed to the energy necessary to develop the scour depth, as predicted by the currently used equation. Any equilibrium or maximum scour depth resulting from a field test that is less than the predicted depth for a sand-bed channel would be attributable to the erosion-resistant characteristics of the in-situ soils. It is envisioned that this scour-testing device would be used for foundation analysis and design in a manner similar to present-day soil borings in that several tests would be conducted across the channel and floodplain area to be occupied by a proposed new or replacement bridge. The scour hole information resulting from the field test(s) would be used, in conjunction with the subsurface soil boring information, to adjust the design scour depth predicted by the equations for sand-bed channels, as appropriate, for the actual soil conditions at the bridge site. The TFHRC Hydraulics Laboratory will collaborate on this proposed research and will provide Lab capabilities and technical assistance.

The scope of work consists of researching practicality of in-situ scour testing to predict scour and to improve scour prediction equations. The research will be based on a combination of data obtained from the historical scour research literature, laboratory experiments, field-testing, data collection, and data evaluation. Tasks The project will consist of the following tasks: Task 1: Assemble a technical advisory committee that will provide oversight and guidance on all aspects of the project. Task2: Identify a practical combination of prototype jet size (pipe/hose/nozzle) and variable speed pump (or throttle) that can be scaled down for laboratory use. Acquire and/or manufacture this scaled-down apparatus. Task 3: Replicate a set of design hydraulic conditions (discharges, flow depth) and sediment sizes in a laboratory flume that have been used in past physical models and augment tests using high performance computing. Task 4: Fix the scaled-down jet at the level of the sediment bed in the flume flow field. Orient the jet to direct the flow of water vertically downward. Record the resulting equilibrium scour depths over a range of jet discharges, sediment sizes produced by the variable speed pump (or throttle). Determine the jet discharge and total energy required to duplicate the depth of pier scour hole produced in past physical model tests. Repeat tests for 75, 50, 25, and 0 percent of design hydraulic conditions in the laboratory flume. Task 5: Conduct a search of an appropriate database to locate a stream site with suitable sediment characteristics to test a full-scale prototype of the in-situ scour testing device. Task 6: Conduct several full-scale field tests at various sites with known scourable material to calibrate the in-situ scour testing device. Task 7: Compare field test results to those obtained from the laboratory for all total jet energies tested. Evaluate the impact of scale effects and other factors on the results. Identify and define the types of adjustments and/or correction factors that may be necessary to achieve a good, consistent correlation between the lab and field test scour depths. Task 8: Evaluate the results of Task 7 and draw a conclusion on the viability of the in-situ scour testing device as a practical field tool for establishing more appropriate and economical bridge pier foundation design scour depths. Prepare a report outlining the study approach, results and resources needed to extend the laboratory and field testing to cover a full range of anticipated scour depths, sediment gradations, and stream flow conditions.

Suggested contribution: $15,000/year The Federal Highway Administration will serve as the coordinator for this pooled-fund project. State DOT's will be solicited for their interest and participation in this study. FHWA will issue a task order contract to the support services contractor to conduct the study. Periodic reviews will be arranged to keep participating states and agencies up-to-date on current developments. These reviews may include meetings in Washington D. C. during the annual TRB Session, e-mail submittals and conference calls.