ABSTRACT
In comparison to sand and gravel, cohesive soils, and clays in particular, present significant challenges when critical entrainment thresholds and erosion rates are to be modeled in the laboratory. When modeling sand beds for example, buoyant weights of individual particles can be quantified. The water below the bed surface can be reasonably considered an extension of the water flowing above the surface in the sense that a hydrostatic pressure distribution can be assumed throughout. Therefore, aside from facilitating particle buoyancy, and possibly bed sediment dilation, it can be assumed that pore water has relatively little influence on erosion and transport mechanisms in non-cohesive sediments. The objective of this paper is to highlight important principles to consider when modeling actual field conditions involving the erosion of cohesive soils such as clayey sands. A specialized device presented herein was designed specifically to examine the influence of positive or negative pore water pressure on critical entrainment thresholds. Initial calibration of the device showed a very close correlation with critical shear values measured in the laboratory flume. Subsequent testing showed an increase in critical shear following a rise in pressure head applied to the surface of the sample.
