Estimation of Critical Shear Stress in Sandy Lean Clay Soils: Case Study in Douglas County, IL

One of the main reasons for bridge failures is the development of scour around pier foundations. The scouring process occurs slowly in cohesive soils and depends on the soil properties. Accurate understanding and estimation of scour is necessary to ensure the safety and stability of the bridge foundations. The estimation of critical shear stress is a key factor for understanding the potential of scour around bridges. Relatively undisturbed cohesive sandy lean clay soil samples were collected using Shelby tubes from a bridge site in Douglas County, Illinois, U.S., and tested in the laboratory to obtain various physical properties of the soil. To determine critical shear stress, both erosion function apparatus (EFA) and a portable scour testing device, which simulate different erosion mechanisms, are used. This study provides insights into the effect of erosion mechanism in the determination of critical shear stress. The determined values are compared against existing empirical equations and methods reported in the literature to provide a comprehensive comparative analysis. An analysis was conducted to evaluate the effect of percentage of clay and undrained shear strength on the critical shear stress. The results indicate a linear relationship, where an increase in both the percentage of clay and undrained shear strength corresponds to a proportional increase in the critical shear stress estimated from EFA.