Comparison between the Erosion Function Apparatus and the Ex Situ Scour Testing Device: Preliminary Findings

A variety of laboratory devices are available to measure the erosion resistance of soils, each with its own advantages, limitations, and applicability. Two of those devices—the erosion function apparatus (EFA) and the ex situ scour testing device (ESTD)—are very similar in design and concept; however, there are notable differences in geometry and test procedures that can affect the results. As the Federal Highway Administration continues to perform both EFA and ESTD tests as part of its NextScour research efforts, it was critical to evaluate the inherent differences between these devices and more directly compare the resulting erosion rates and hydraulic shear stresses. As part of the initial work, three different engineered soils were prepared and tested in the EFA and ESTD using their respective procedures. Hydraulic shear stresses were then computed based on surface roughness of the specimens and the Moody chart. The preliminary findings indicate that the EFA and ESTD generally yielded comparable shear stresses for all soils. After application of best-fit power functions through the data, the resulting shear stress factors between the EFA and the ESTD were near unity, ranging from 0.92 to 1.13; however, there were larger differences in critical shear stresses—particularly in soils with higher sand content. Additional work is needed on a wider range of soils, but in the interpretation of erosion resistance from laboratory testing, it may be important to understand the device, the flow conditions, and the test methods involved.