Channels Stability in Undisturbed Cohesive Soils

The erosion resistance of cohesive soils is defined by a comparison of shear strength tests with field observations under specific flow conditions. Samples are obtained by pushing 1.9 in.-by-4 in., thin-walled tubes into undisturbed soils along channel boundaries. Coefficients of permeability are obtained. While still saturated, unconfined compression strength tests are made. Field observations and tests showed that the degree of erosion resistance and shear strength are positively related. Dry density, plasticity index, and percentage of soil finer than 5 microns show a moderate to high correlation with shear strength, individually and in multiple regression analysis, but fail to explain a majority of the variations. Permeability is better related with the coefficient of correlation r = -0.78. This variable also reflects the condition that slowly permeable cohesive soils that lose strength by saturation are better able to resist erosion from long duration flows. Shear stresses are symbolized by the product of channel slope, hydraulic radius, specific weight of water, and average velocity, and termed tractive power. This product and the unconfined compressive strength are used to define the boundary between channels that are observed to be stable or eroding.