Evaluation of scour potential of cohesive soils - phase 2.

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, ten cohesive soil formations were sampled with the assistance of the Alabama Department of Transportation. Specimens from 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 ten tested formations were scour resistant. Scour resistant formations has SPT N value 60 or more with moisture content less than 22% and mean grain size less than 0.008 mm. Velocity and shear stress based erosion functions were generated for the seven scourable formations with scour rates upwards of 15 mm per hour.