Strain compatibility and geogrid stiffness selection in the design of reinforced soil walls.

In designing reinforced soil walls, one of the most important, but often disregarded, aspect is that of strain compatibility. Strain compatibility has to be considered mainly 1) for a proper choice of soil shearing resistance; 2) for defining reinforcement characteristics, in particular axial stiffness; 3) for assessing acceptable movement values of the structure. Considering geogrid reinforced soil walls, the same mobilised reinforcement force (assumed as a design value by using safety factors or simply as the result of the interaction mechanisms) can be attained with tensile strains that are different by up to an order of magnitude, depending on the adopted geogrid type. This choice influences both the final aspect of the wall (i.e. its serviceability state) and the adopted design soil strength parameter. Therefore, to avoid incorrect (underestimated/overestimated) assumptions of soil strength level, that could lead to an oversize or unsafe structure, strain compatibility has to be taken into account. With the aim of exploring these aspects and supplying practical design indications, in the paper the results of a number of numerical analyses are reported. The performed FEM analyses have been carried out 1) with a commercial, robust and user-friendly code, especially developed for geotechnical analyses; 2) starting from the evidence provided by well documented and representative case histories, in order to provide confidence in the deduced results; 3) assuming different wall geometries and actual geogrid characteristics; and 4) calibrating soil parameters by considering triaxial test results, performed on dense and low confined sand specimens, in order to define strain levels and corresponding mobilised strength and stiffness. The evidence gathered from the analyses strengthens the importance of the strain compatibility concept and of the proper choice of geogrid axial stiffness.