Scour in Cohesive Soils
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Scour in Cohesive Soils

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    This study of scour in cohesive soils had two objectives. The first was to introduce and demonstrate a new ex situ erosion testing device (ESTD) that can mimic the near-bed flow of open channels to erode cohesive soils within a specified range of shear stresses. The ESTD employs a moving belt and a pump to generate a log-law velocity profile in a small test channel to simulate open channel flow. Successful testing requires careful preparation of soil specimens to avoid slaking. Preparation of erosion test samples by compaction usually leads to soil slaking, which cannot be tolerated to generate meaningful erosion function data. Therefore, cohesive soil specimens with different percentages of clay, silt, and non-uniform sands were mixed and de-aired in a pugger mixer to prevent slaking. The testing confirmed that the ESTD is capable of determining erosion characteristics of cohesive soils for bed shear stresses within the range of 0.063 to 0.31 lbf/ft2 (3 to 15 Pa). Its capability of directly measuring bed shear stresses enhances the understanding of the erosion process in cohesive soils. The second objective was to develop a method for estimating the critical shear stress and erosion rates for a limited range of cohesive soils in the context of the Hydraulic Engineering Circular 18 scour framework. The method is based on more easily obtained soil parameters so that direct erosion testing is not needed in all cases. General relations are proposed for both best-fit and design applications. Estimates of critical shear stress are based on the water content, fraction of fines, plasticity index (PI), and unconfined compressive strength. In addition, an equation for estimating erosion rates when bed shear stress exceeds critical shear stress is proposed. For application, the designer must determine the critical shear stress of the soil (from the previous relation), the unconfined compressive strength, and the PI. The guidance may be used for engineering design within limits based on the range of values in the current data set and to a lesser extent the range from Illinois field data on which parts of the methodology were validated. A Texas data set on which additional validation was attempted represents a distinct data set. The recommendations apply to fine grained cohesive soils within a range of plasticity and liquid limit (LL) characteristics. The PI should be within the range of 4 to 25 percent and the LL between 15 and 50 percent. The fraction of fines should fall between 10 and 90 percent. These methods best apply to soils with at least 90 percent saturation but can be used with lower degrees of saturation.
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