Unsaturated Soil Slope Stability: Finite Element Analysis And Computations

Slope failure is a common geotechnical disaster. A large part of slope failures may be induced due to the transition between the unsaturated state and the saturated state, due to the heavy rainfall or sudden water drawdown. Slope failures induced due to the rapid transition between the unsaturated state and the saturated state can be adequately interpreted by the unsaturated soil mechanics. The water content of the soil in unsaturated soil slope increased, made the shear strength of soil decreased with bulk density increased and matric suction reduced, so the slope safety factor reduce and landslide damage is possible. It's the unsaturated soil slopes failure mechanism as we know. For practical engineering, we need a more reasonable way to analyze the unsaturated soil slope stability in a variety of complex conditions.Three distinct phases are systematically introduced to clearly illustrate the development of finite element methods for unsaturated soil slope stability in this paper, that is, time-independent qualitative method, time-dependent computational method with decoupled pore fluid flow and deformation, and the fully coupled unsaturated consolidation analysis.Qualitative method uses a simplified way to analyze the unsaturated soil slope stability and deformation, without considering rainfall intensity and rainfall duration. There are two models of rainwater flow in unsaturated soil slope, that is, accumulated infiltration model under heavy rainfall and free infiltration model under light rainfall. The examples are employed to evaluate the two infiltration models, so we can know the evolution of the safety factor and deformation of unsaturated soil slopes. Under the conditions of heavy rainfall, when using accumulated infiltration model, landslide of unsaturated soil slopes prone to occur early.Uncoupled analysis is the mainstream approach to study the stability of unsaturated soil slopes. Firstly, we may obtain the transient seepage field by analyzing the seepage. Next, from the transient seepage field, the pore water pressure or suction of the transient seepage is used as the initial conditions to analyze the stability of unsaturated soil slopes. Based on the basic differential equation of saturated-unsaturated seepage, the differential equations are rewritten into the finite element format with the seepage parameters and boundary conditions. Combined with the shear strength reduction finite element method, a FEM computer program is developed. To improve the nonlinear iterative method for the computational efficiency, a new a-section search algorithm is proposed. Finally, the FORTRAN program is used to analyze the stability and deformation of embankment under unsteady and steady seepage.Coupled analysis is based on consolidation theory of unsaturated soil by considering the interaction between unsaturated flow and soil skeleton. Therefore, analysis of unsaturated soil slope stability could be more reasonable. Considering that consolidation theory of unsaturated soil is still immature and coupled analysis of soil slope stability is limited, more intensive research work is required.