Some limits of 2.5D models for sol-structure interaction are addressed. First, the problem of micro piles is studied in comparison with 3D FEM full models. In the 2.5D model, micro-piles are replaced by walls of a given thickness which was found to be directly correlated to the 3D pile density. Correspondence between full 3D and approximated 2.5D computations were found satisfactory. The second problem reported is the replacement of discrete (3D) damping pads with continuous (2.5D) ones of equivalent stiffness. Here again, this simplification lead to very satisfactory comparisons with full 3D computations. These two problems show that some limitations of 2.5D models can be overcome in a very simple manner.