Evaluation of scour potential of cohesive soils : final report, August 2009.
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Evaluation of scour potential of cohesive soils : final report, August 2009.

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      Final report; Aug. 2009
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      Prediction of scour at bridge river crossings is an evolving process. Hydraulic models to estimate water velocity and, therefore, the shear stresses that erode soil are reasonably well developed. The weak link remains methods for estimating soil erodability. Procedures in use today for highway bridges, such as HEC-18 and HEC-20 (FHWA engineering circulars), contain erodability models for non-cohesive soils. In these models a soils erodability is directly related to the soil grain size, i.e., larger soil particles require higher velocities for removal and transport. This basic erosion model is inappropriate for cohesive soils since their erosion resistance will generally increase with decreasing grain size and, therefore, increasing plasticity. The term generally is used because the influence of particle mineralogy (which would differentiate clay particles from less active silt particles) and density are also factors that affect soil erodability.

      Briaud et al. (1999, 2001a, 2001b, 2004) developed a model (erosion function) for characterizing the erodability of soil. This model, shown in Figure 1, is a plot of soil erosion rate (ż in units of mm per hour) versus shear stress at the soil/water interface (τ in units of N per m2). Key components of the erosion function are (1) the critical shear stress (τc) below which the soil will not erode; (2) the initial soil erosion rate (Si) defined as the slope of the erosion rate-shear stress curve at the critical shear stress; and (3) the post critical shear stress erosion rate relationship. This relationship is shown in Figure 1 as a curve where the erosion rate decreases with increasing shear stress, but a range of curve shapes is possible as will be shown later.

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