The three-dimensional elasto-plastic shear strength reduction FEM is used to evaluate the stability of slopes reinforced with piles or anchors in a general elasto-plastic FEM frame, where the soil-reinforcement interaction is simulated with zero-thickness elasto-plastic interface elements. The numerical results are compared with those obtained by Bishop's simplified method. It is shown that the slope stability is significantly influenced by the soil-pile interaction. The safety factor of slopes reinforced with piles obtained by the shear strength reduction FEM is significantly larger than that predicted by Bishop's simplified method. The piles should be installed in the middle of the slopes for the maximum factor of safety. For slopes reinforced with anchors, the prestressed load has almost no influence on the safety factor. The safety factor of slopes reinforced with anchors predicted with the shear strength reduction FEM compares well with that obtained with Bishop's simplified method. Thus the reinforced effect of the anchors can be considered only as an axial resisting force.
The use of piles or anchors to stabilize active landslides, and as a preventive measure in stable slopes, has been applied successfully in the past and proved to be an efficient solution, since they can be easily installed without disturbing the equilibrium of slopes. The current design practice for slopes reinforced with piles or anchors often use the limit equilibrium methods, where the reinforcements are assumed to only supply an additional sliding resistance. The soil-reinforcement interaction is not considered, and is very difficult to be considered in the limit equilibrium methods. The key to the limit equilibrium methods is an accurate estimation of the resisting force supplied by the reinforcements, and some researchers have made such attempts for piles (Ito and Matsui, 1975; Poulos, 1995; Chow, 1996).