Soft Clay Erosion and Its Undrained Shear Strength Measurements

Characterization of the in-situ critical shear stress of cohesive soils plays a vital role in bridge scour design. This stress is closely related to the undrained shear strength, which is conventionally obtained using a field shear vane (FSV) in situ or an erosion function apparatus (EFA) in laboratory. However, neither method is appropriate for assessing the erosional resistance of top surface layer of soft clays in streambeds, because: (1) FSV lacks sufficient sensitivity for the strength of very soft clays; (2) it is very difficult to obtain high-quality samples from such soft clays to be tested in laboratory; and (3) the properties of cohesive soils vary significantly with consolidation, organic matter content, microorganisms and biofilms, and seasonal changes. This paper describes the development of a new Louisiana Scour Vane (LSV) for in-situ measuring the undrained shear strength of surficial soft clay layer. Laboratory testing on a reconstituted clay was conducted to validate the LSV's capability of measuring shear resistance changes at small depth intervals. This new portable and easy-to-operate apparatus will act as a suitable supplement to the FSV for geotechnical engineers.