Seepage‐Induced Mass Wasting in Coarse Soil Slopes

An expression is derived for the critical shear stress of surface flow on a noncohesive soil slope subjected to seepage‐induced mass wasting, i.e., erosion caused by a successive mobilization of flake aggregates of surficial grains. The expression is supported with data from experiments with stone fill subjected to throughflow. In current theories for hill‐slope erosion or riprap protection on slopes, buoyancy is assumed to be a constant vector directed vertically, or the seepage force is omitted as a variable. However, the present analysis shows that, for many conditions, buoyancy should be described in accordance with the dynamic pressure distribution and the seepage force should be included as a variable in the theory. Furthermore, a parameter denoted “minimum stable volume” per unit width of stone fill is introduced as a measure of global stability of the stone fill, along with seepage height on the downstream face and grain size. Experimental results indicate that the minimum stable volume decreases with increase in grain size and, for the materials investigated, is about 20% lower for crushed rock than for gravel.