Evaluation of Slope Stability of Small Valleys Based on the Soil Strength Probe: An Example in Southern Gifu Prefecture, Central Japan

Road disasters are often caused by slope failure in small valleys following heavy rainfall. Field investigations and soil penetration tests were performed along Route 21 near the town of Mitake, central Japan, where five slope failures occurred in 2010 and 2011. The field investigations identified the following geological and topographical features: (1) knick lines around the hillside are active erosion fronts; (2) small valleys above the erosion fronts accumulate rainwater and ground water; and (3) thick colluvial deposits accumulate beneath most slope failures. The newly developed Soil Strength Probe Test is a useful method for estimating topsoil depth. The test data from selected traverse lines and slope failures were plotted on a slope angle-topsoil depth diagram, and vertical and horizontal critical lines determined. Slope stability was evaluated as relatively low in the field measurements, for which both the slope angle and topsoil depth are greater than the two critical lines in the diagram. Thus, by referring field-derived values of slope angle and topsoil depth to the diagram, it is possible to estimate the stability of a slope.