Another look at the stability of unsaturated soil slopes considering nonuniformity and nonlinearity

Natural soils in site are mainly unsaturated under evaporation or infiltration effects, which cause nonuniformity along depth or present nonlinearity in strength properties even for uniform soil strata. Conventional upper-bound analysis is unable to handle the above issues, requiring alternative approach such as the discretization-based kinematic analysis to be adopted. Nonlinear matric suction profile along depth is theoretically formulated under one-dimensional steady flow. Within the context of extended Mohr Coulomb (MC) failure criterion, total cohesion is explicitly expressed for unsaturated soils. Meanwhile, an extended Power-law failure criterion is proposed to consider nonlinear characteristics of uniform soils in unsaturated zones. Based on these two failure criteria, upper-bound analyses of unsaturated clay slopes' stability are evaluated by slope safety factor (FoS). Optimal upper-bound FoS solutions are sought by a strength reduction technique and iterative algorithm. Effects of clayey soils' non-uniformity, nonlinearity and some other influential factors on unsaturated slope stability are investigated, which is of engineering significance for sound design and reliable assessment of unsaturated slopes. One of key findings is that stability of unsaturated soil slopes enhanced by their unsaturated soil strength is substantial and should be utilized to derive a more economical design or construction of temporary slopes in practice.