EuroGeo4 Paper number 187 1 STRAIN COMPATIBILITY AND GEOGRID STIFFNESS SELECTION IN THE DESIGN OF REINFORCED SOIL WALLS Riccardo Berardi 1 & Giovanni Pietro Pinzani 2 1 Dept. of Civil, Environmental, Architectural Engng. - Univ. of Genova, Italy (e-mail: riccardo.berardi@unige.it) 2 SEIC Geotecnica, Division of HARPO s.p.a., Trieste, Italy (e-mail: g.pinzani@seic.it) Abstract: 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. Keywords: Strain compatibility, reinforcement stiffness, isochronous curve, finite element, field performance, reinforced soil walls. INTRODUCTION As design approaches for reinforced soil walls are often referred to ultimate conditions, limit equilibrium methods are adopted, not properly considering soil-reinforcement interaction mechanisms and, consequently, walls movements. A different approach, especially as far as serviceability conditions are concerned, relies on the concept of strain compatibility. When analyzing the reinforced wall behaviour in these perspective, particular care has to be paid in defining soil shearing resistance parameters and reinforcement characteristics, such as axial stiffness and creep behaviour. 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 axial stiffness of reinforcing elements. In order to design a reinforced wall that could be safe and in an acceptable working condition, an approach based on strain compatibility could be taken into account. The equilibrium in the reinforced soil may be investigated using a compatibility curve (Figure 1) constructed by assuming that there is equal tensile strain in the reinforcement and in the soil in the direction of the reinforcement (Jewell 1996). The mobilised soil resistance (i.e. the value of the friction angle) that has to be considered, depends on the expected equilibrium in the reinforced soil mass and, therefore, by the experienced strain levels which, in turn, are strictly related on reinforcement stiffness. The question if it is the peak friction angle (φ p ’) or the constant volume (φ’ cv ) friction angle that has to be used in designing reinforced soil walls has been arisen by many Authors (e.g. Leshchinsky 2001; Zornberg 2002). Several methods are based on the peak strength parameter; otherwise, especially when Ultimate Limit State is of concern, analyses based on φ’ cv are often suggested. For Serviceability Limit States, when the design in mainly governed by allowable displacements, the problem is still open. Considering geogrid reinforced soil walls, with the aim of exploring these aspects and supplying practical design indications, the paper reports the results obtained by numerical analyses performed in order to investigate the influence of the choice of soil strength and geogrid stiffness values on the wall movements. Different geometries and reinforcements have been considered. The models have been calibrated by the comparison with the evidences provided by of well documented case histories. Current strain levels in geogrid reinforced walls and in the soil have been taken into account by the analysis of published case records as well as by triaxial tests performed on dense and low confined sand specimens. The evidence gathered from the analyses strengthens the importance of the strain compatibility concept and of the proper choice of geogrid stiffness.