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Engineering analysis of slope stability includes three separate but interrelated phases: (a) experimental strength measurements, (b) determination of a strength envelope that best fits the experimental results, and (c) formal limiting equilibrium analysis using the resulting strength envelopes. Studying the interrelations between these phases leads to an integrated approach to slope stability analysis. The present work uses a single experimental database that is fitted with both linear (Mohr–Coulomb) and nonlinear failure envelopes and investigates the effect of different forms of the failure criterion on slope stability computations for both 2D and 3D problems. It has been indicated that calculated minimum safety factors could be significantly overestimated by the linear approximation of a nonlinear strength envelope. The effect of neglecting strength envelope nonlinearity is more pronounced under 3D conditions than in a 2D simplification. As a result, the use of nonlinear failure criterions in slope stability analyses is recommended to account for the stress-dependent nature of the shear strength of soils.Key words: nonlinear strength envelope, Mohr–Coulomb failure criterion, limit equilibrium, critical slip surface, minimum factor of safety, three-dimensional stability analysis.

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A limit equilibrium method for progressive failure analysis of slopes is proposed based on a technique of non-vertical slices . The local factors of safety are calculated by defining variable factor of safety instead of conventional single value factor of safety along a shear surface. Reasonable simplifying assumptions about the inter-slice forces and the line of thrust are made to render the problem statically determinate. Strain softening, though in an approximate manner, is considered in the solution procedure. The analysis of a field case history indicates that the proposed method realistically represent the behaviour of progressive failure of an actual slope. Another case study revealed that in certain conditions a reasonable solution can be obtained using non -vertical slice approach, but the vertical slice analysis does not work well in obtaining reliable results . The proposed method, which can also handle vertical slice division, enables the user to choose more appropriate results either from vertical or from non-vertical slice analyses.