Soil shear strength: Its measurement and soil detachability

Abstract

Recent research on soil erosion mechanics has pointed out that the condition at which rill flow becomes erosive is controlled by soil surface shear strength ( TORRI et al. 1987a, RAUWS & GOVERS 1988). Also splash detachment processes are strictly linked to soil shear strength as shown by NEARING & BRADFORD (1985), TORRI et al. (1987b). A comparison of the proposed threshold conditions for incipient rilling immediately shows that they differ in the two quoted papers. The same situation characterizes the relations proposed between splash detachment and soil shear strength. Those differences may depend on the different instruments used for measuring soil shear strength: a Geonor model g-200 laboratory cone penetration apparatus, a laboratory vane tester and a pocket vane tester. Those instruments differ in shape (pocket and laboratory vane testers) or in the physics involved in their functioning (vane tester and drop-cone penetrometer). The experimental results show that the drop-cone penetrometer is characterized by a low repeatability. Moreover, it is sensitive to small variations in soil texture. The laboratory vane apparatus seems to determine a surface of shear close to that of a sphere circumscribed to the vane. The pocket vane apparatus, instead, is characterized by a cylindrical surface of shear. Those results seem to explain the main differences in soil detachment. In particular, incipient rilling conditions are clearly reduced to a single relation between critical shear velocity and soil shear strength. It can consequently be concluded that soil detachability is strictly linked to soil shear strength.