This paper is intended to illustrate the applicability of Shear Strength Reduction (SSR) as a general technique for obtaining Factor of Safety estimates for slopes in variable geology, progressive or locally brittle yield behaviour and with ground-structure interaction. Comparisons are made with Limit Equilibrium solutions (LEM). Implications of the assumptions required to ensure this correlation are discussed including uniform stiffness, rigid-plastic behaviour, instantaneous interaction between geological units, and instantaneous generation of support loads. The paper uses Finite Element Modeling (FEM) as a vehicle for demonstration although Finite Difference solutions are equally valid. General applicability of the method is demonstrated and the limitations explored using Discrete Element simulation of multi-block slope failure.


The Shear Strength Reduction (SSR) technique (Matsui & San 1992, Dawson et al 1999, Griffiths & Lane 1999, Cala & Flisiak 2003, Hammah et al. 2005a, 2006) enables finite element (or finite difference) techniques to be used to calculate factors of safety for slopes, providing an alternative to limit equilibrium calculations and a potentially more reliable analysis of slopes with heterogeneous stiffness, strain-softening and passive structure-ground interaction. The methodology is general and can be applied to other non-linear problems such as multiblock discrete element simulations. Geotechnical engineers primarily conduct slope design based on calculated factor of safety values. Limit Equilibrium (LEM) techniques that compare resisting forces to driving forces (or moments) are ideally suited to the generation of a nominal safety factor (Krahn 2003). Instantaneous slide surface mobilization and consideration of stresses and forces independent of pre-failure movement are inherent in these techniques, and may result in inadequate representation of the system’s actual stability state. Non-linear modeling using the Shear Strength Reduction (SSR) technique can also used to determine factors of safety.

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