A method for stability analysis of vegetated hillslopes: an energy approach

Stability analyses of vegetated hillslopes are usually carried out by the limit equilibrium (LE) method where shear displacement is not taken into account. Experiments show that soil with roots produces a shear stress - displacement curve with higher peak shear stress at larger shear displacements than fallow soil. If the safety factor is obtained by the LE method, the ability of soil with roots to resist large shear strains due to soil-root interaction may be underestimated. A new approach is proposed that incorporates, within the stability analysis, the ability of soil with roots to withstand strain. It is based on a consideration of the energy consumed during the shearing process of the soil-root system. This is developed using characteristics of the shear stress - displacement curve of a soil-root system obtained from in situ direct shear tests under simulated overburden pressure and pore-water pressure conditions. The method is limited to vegetated hillslopes where the stability analysis can be approximated by a simplified infinite slope model. Shear stress - displacement data for two tree species were obtained from hillslopes where shallow landslides commonly occur in rainstorms under near-saturated conditions. Using these results the energy approach (EA) and LE methods are compared. A procedure is also outlined to predict the safety factor for hillslopes with different plant densities. Extension of the EA method for general two-dimensional slope stability analysis involving nonlinear shear planes is also explained.