Scour Potential of Cohesive Soils

Abstract

Determination of erosion parameters in order to predict scour depth is imperative to designing safe, economic, and efficient bridge foundations. Scour behavior of granular soils is generally understood, and design criteria have been established by the Federal Highway Administration. The same is not true for cohesive soils, and because of their complexity, a universal scour prediction method has not been established by the industry. The Erosion Function Apparatus (EFA) was created to determine the rate of scour of cohesive soils under known shear stresses, which can then be used to predict scour depths under similar conditions. During this study, nine cohesive soil formations were sampled with the assistance of the Alabama Department of Transportation. Six of these formations were scour tested in an updated EFA featuring an ultrasonic sensor for quantitative erosion measurements. EFA tests were performed to determine erosion functions and whether any formations demonstrated scour resistance. Geotechnical index tests were also performed on these formations to correlate scour to geotechnical properties. Results of testing verified the performance of the ultrasonic sensor and updated EFA. Three of the tested formations were scour resistant, while three formations showed evidence of scour. Velocity versus scour rate curves were generated for the scourable formations with scour rates upwards of 15 mm per hour. The scour behavior observed was unique among formations limiting the ability to establish correlations between tests.