Critical Shear Stress of Cohesive Soils for Scour Analysis Using PSTD and EFA Testing for Two Case Studies

Reliable estimation of critical shear stress (CSS or τc) is essential for accurate prediction of erosion and scour around bridge piers with cohesive soils. The CSS is the shear stress required to initiate erosion, typically assumed at a rate of 0.1 mm/h. This assumption, although convenient, oversimplifies the erosion process and introduces significant variability across different testing techniques and flow regimes. This study compares critical shear stress values obtained from two erosion testing methods: the erosion function apparatus (EFA) and a portable scour testing device (PSTD), using undisturbed CL soil samples from two different bridge sites in Illinois. Two analytical methods are proposed: Method A, which determines τc as the range between the onset of erosion and the 0.1 mm/h erosion rate threshold for individual tests; and Method B, which aggregates data across multiple sites to account for soil and site-specific variability. Results show that EFA generally reports higher τc values than PSTD, likely because of differences in flow regimes. Turbulent conditions in PSTD consistently produced lower τc values, emphasizing the importance of flow regime-specific assessments in scour depth design. Comparison of τc determined in this study with τc determined from an empirical correlation presented in previous literature revealed substantial scatter, with prior studies often underestimating τc. The proposed methods offer a more field-representative and standardized approach to determining τc, which is imperative for the development of realistic and site-appropriate scour prediction models.