Practical Determination of Critical Shear Stress in Cohesive Soils

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 determined for a particular basin or modeled in the laboratory. A wide variation in cohesive soil characteristics and behavior is usually encountered, even within relatively short reaches of river and creek beds. It is, therefore, evident that assessing the erodibility of such beds requires analysis of a large number of locations within reach, resulting in a costly and time-consuming undertaking. The objective of this paper is to introduce a practical procedure and test device designed to allow a fast and accurate determination of the critical shear stress of cohesive (and some noncohesive) soils, both in laboratory samples and in the field. The device, herein referred to as the erosionometer, is based on physical shearing of the soil surface. Initial calibration of the device showed a very good correlation with critical shear stress values for particle entrainment measured in a laboratory flume.