Soil Properties Affecting the Onset of Erosion in Cohesive Soils

Since its establishment in 1991, Hydraulic Engineering Circular (HEC-18) has been the primary tool for United States transportation agencies to evaluate bridge scour. The empirical relationships in HEC-18 are based on cohesionless soils and have been proven to be overly conservative cohesive soils. To incorporate the scour behavior of cohesive soils in the design manual, the Federal Highway Administration recently developed a scour prediction model for cohesive soil. However, the new model is valid for a very narrow range of cohesive soils, focusing mainly low plastic silts and clays. The objective of this study was to determine significant soil properties that affect the critical shear stress (exerted by the flowing water on soils) for the onset of erosion using a broader spectrum of cohesive soils. A total of 71 soil samples from 27 bridge sites were used for this study. Erosion testing was performed in the erosion function apparatus and soil properties were measured in the field and laboratory for obtaining different variables. Among the various soil properties, percent fines, liquid limit, and electrical resistivity were found to be statistically significant variables for predicting the critical shear stress. For the 71 samples used in this study, liquid limit varied between 26 to 80, plasticity index between 3 to 52, and percent fines between 11 to 100. Therefore, the variables that were found statistically significant in this study can be utilized to build a comprehensive model for critical shear stress, applicable for a broader range of cohesive soils.