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DIFFERENCES BETWEEN LE AND FE METHODS USED IN SLOPE STABILITY EVALUATIONS (2008)
A comparative study between Finite Element (FE) and Limit Equilibrium (LE) analyses is presented in this paper. The most advanced FE methods have today many advantages compared to the traditional LE methods that are applied in slope stability evaluations. Additionally, the study compares the results obtained from various LE based methods. For the purpose, a simple dry slope is analysed with both surcharged and earthquake
loads in LE based computer programs, SLOPE/W and SLIDE, and FE based software, PLAXIS. Morgenstern-Price (M-P) method is chosen, among LE methods, as the basis for comparison with PLAXIS. Moreover, a discussion is presented among LE methods that basically differ from their assumptions to define the inter-slice force function which relates to the localised normal and shear forces. Additionally, the position of line of thrust, defined in LE based Janbu’s Generalized Procedure of Slices (GPS method), is studied in PLAXIS and SLIDE. The comparative study shows that the inter-slice force function can neither be a constant nor a well-defined function as often assumed in most of the LE based methods . This function rather depends on the geometry of the slope and the applied loading conditions. For the same inputs and geometry, the assumed function in both SLOPE/W and SLIDE considerably differs from the computed inter-slice force function in PLAXIS. This difference may cause variation in localised normal and shear forces and thus have an effect in the factor of safety (FOS). Several other studies show that the LE based Morgenstern-Price method may compute higher FOS
compared to the results obtained from FE analysis, for example in PLAXIS (Aryal 2006). This variation may range from 5%, for dry slopes, to 15%, for fully saturated slopes without any external loads. Even larger
variations in FOS can be computed if a slope is loaded with external loads, such as surcharges and earthquakes, in fully saturated condition.
The 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India
Norwegian Public Road Administration, Region West, Norway