Current methodologies for predicting scour depths around bridge foundations typically employ empirical equations derived from physical model studies using non-cohesive, uniformly graded sands, with the median grain size (d50) as the only geotechnical parameter considered. 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 deep and costly bridge foundations.

The variability of soil mixes and layering system found in nature creates a full continuum of erodibility from the easily erodible, very fine silts to the non-erodible, competent rock. An effective soil erosion test can provide assistance on a project-by-project basis to determine the erodibility of subsurface soil types and layers. Such tests could more accurately define the scour potential for a given set of hydraulic design conditions and soil type. This approach aligns with the FHWA Next Scour Program (NextScour) recognizing that the phenomenon of scour consists of two major components, (a) consideration of water and hydraulic forces (loads) causing (b) erosion of soils and their associated geotechnical effects (resistance). NextScour institutes a new direction that refocuses and aligns those geotechnical and hydraulic components within a true multi-disciplinary framework that provide more certainty and reduced project costs.

NextScour will build off research conducted over the past 10 years which developed several erosion testing devices to better understand and address soil erosion resistance. 

These include:

• Ex-situ Scour Testing Device (ESTD): The ESTD is a laboratory device that measures the erodibility of a cylindrical soil specimen under well-controlled flow conditions. Its rectangular testing channel has a dimension of 40 inches long, 4.7 inches wide, and 0.75 inches high. The maximum flow rate in the ESTD is 0.5 cubic foot/s, which translates to a maximum average flow speed of 18 feet/s in the testing channel. The ESTD features an innovative electromagnetic shear stress sensor that can instantaneously measure shear stresses on soil samples during the erosion process and is located upstream of the piston-controlled soil sample. The ESTD uses a robotic underwater laser scanner that captures the erosion process of a 12-inch-long soil sample in a Shelby tube with an outer diameter of 3 inches. 
• In-situ Scour Testing Device (ISTD): The ISTD is a field testing device that features an innovative erosion head that, when inserted into an ordinary Shelby tube, circulates water to produce a high-speed horizontal radial flow to erode the soil surface. A drill crew uses a conventional drill rig to auger to the desired testing depth, then a Shelby tube is mounted to a series of casing segments which are lowered into the borehole. Once at the soil surface, the tube is then pushed 14 inches into the clay. Next the ISTD testing team assembles the remaining ISTD components, including the advanced linear drive, water tank, pump, hoses, control box and laptop. The erosion head is then lowered into the casing to the soil surface. Once water begins circulating through the system, sensors in the erosion head continuously measure the gap between the head and the soil surface. As the soil erodes, an algorithm lowers the erosion head to maintain a constant gap, which is later converted into an erosion rate for the soil. 
• Portable Scour Testing Device (PSTD): The PSTD is a simplified version of the ISTD. The PSTD follows the same testing procedure and data collection as the ISTD, but instead of conducting the test in situ at the desired soil depth, the soil sample is retrieved and the test is performed at ground level. The PSTD is positioned in close proximity to the drill rig and its frame is mounted directly to the water tank, which simplifies the required components for assembly. Additionally, by separating the erosion testing from the drilling operations, the efficiency of the entire testing procedure is greatly increased.

The objective of these pooled funds is to provide and/or support soil and erosion testing services for bridge projects over water crossings managed or coordinated by State DOTs, to provide technical assistance to design, fabricate, and install erosion testing devices to support and seek to broaden the use of erosion testing devices among State Department of Transportations, and to compile and analyze the collected soil and erosion testing data in a broader research effort to more accurately estimate reliable scour design depths given the soil conditions and hydraulic load during a given storm event.

Task 1. Soil Erosion Test in the TFHRC Hydraulics and/or Geotechnical Lab for various bridge projects: The Hydraulics and Geotechnical Lab staff will conduct soil and erosion tests utilizing the ESTD and/or EFA on soil samples shipped to the Laboratories for bridge projects managed or coordinated by State DOTs.

Task 2. Soil Erosion Test in the field for various bridge projects: The Hydraulics Lab staff will conduct soil erosion tests in the field using the ISTD or PSTD and collect samples for ESTD and/or EFA tests in the TFHRC Hydraulics Laboratory for projects managed or coordinated by State DOTs.

Task 3. Laboratory and In-situ Soil Testing: The TFHRC Geotechnical Lab staff will conduct index testing (e.g. particle-size distribution, unit weight, moisture content, Atterberg limits, etc.) and other, more specialized laboratory soil tests (e.g. undrained shear strength, consolidation, etc.) in the TFHRC Geotechnical Laboratory to determine key soil parameters that may impact erosional resistance. Geotechnical Lab staff will coordinate Cone Penetration Testing at the site with the State DOTs.

Task 4. Fabrication of an Erosion Testing Device: The TFHRC Hydraulics Lab staff will design and fabricate an Erosion Testing Device (e.g. ISTD or PSTD) to conduct soil erosion tests for projects managed or coordinated by State DOTs.

Task 5. Scour Analysis Support: The TFHRC Hydraulics Lab staff will provide technical support for scour analysis, including 2D and computational fluid dynamics (CFD) modeling to determine hydraulic shear stresses. This support will also encompass analyzing soil erosion tests conducted either in the field or in a laboratory setting for projects managed or coordinated by State DOTs.

Task 6. Laboratory and In-situ Soil Testing Support: TFHRC Geotechnical lab staff will provide technical assistance for conducting and analyzing ex- and in-situ soil testing for projects managed or coordinated by State DOTs.

Task 7. Scour along Longitudinal Structures: This task will use NextScour principles (hydraulic loading functions versus soil erosion resistance), CFD, Flume Experiments and Case Studies to research scour prediction for various flow conditions on longitudinal structure types and configurations in a riverine environment.

Task 8. Development of Shear Stress Predictive Equation: FHWA is conducting research to develop a lower-bound shear stress predictive equation from soil indexing properties of high or low plasticity clay soils. A State DOT must provide representative soil samples for testing by FHWA to verify that the equation is applicable to the State’s soils. This task will assist States in developing a localized predictive equation of critical shear stress from typical indexing properties.

It is estimated that the proposed research will be $50,000. The minimum funding contribution from each partner is $15,000 per 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 to receive soil and erosion testing services for bridge projects managed or coordinated by State DOTs.