In 2015, the Hydraulics Laboratory at Federal Highway Administration (FHWA) published a report with a critical shear stress (τc) equation for cohesive soils based on soil index properties, including water content, fraction of fines, plasticity index, and unconfined compressive strength. This equation was developed from tests on a simplified set of engineered soils, but practitioners inquired FHWA if the equation was applicable to a wider range of field cohesive soils. The Hydraulics Laboratory at Turner Fairbank Highway Research Center (TFHRC) reviewed erosion and soil indexing data of clay specimens collected from 11 states, and determined the equation gave reasonable estimates of critical shear stresses. Additional literature data was collected, resulting in a total of 124 data points. New regression equations between τc and index properties were developed. The measured and computed τc were compared, and more comprehensive analyses were conducted considering the variation of the soil index properties. A confidence interval of the prediction equation was identified using the residual error analyses, and the confidence interval identified erosion data outliers. Following the framework of the National Cooperative Highway Research Program (NCHRP) 24-34 report, the mean of the ratios between the measured and computed τc, i.e. the bias, was calculated. The ratios follow the lognormal distribution. A design equation to predict the lower bound critical shear stress was proposed based on and coefficient of variance (COV) of the lognormal distribution and a desired reliability. The design equation predicts a range of the lower bound τc from 0 to 32 Pa for clay with known index properties. This research proposes a framework to incorporate future erosion test data in the lower bound τc equation by examining whether the new data fit within the confidence interval. A simple procedure for calculating the new COV of the expanded dataset is explained to update the lower bound τc equation.

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