Revisiting bearing capacity analysis of MSE walls

Abstract

Common design of MSE walls determines the layout and strength of reinforcement by using a synergy of various analyses. One such analysis is bearing capacity in which the reinforced mass is considered a rigid body exerting uniform pressure over reduced area due to eccentricity. In some codes, Meyerhof's method for an eccentrically loaded footing is used to assess bearing capacity and to ensure a sufficient margin of safety. In these codes, the horizontal resultant of lateral earth pressure on the reinforced mass affects eccentricity but is ignored in assessing the bearing capacity coefficients in this analysis; i.e., the analysis does not consider the impact of load inclination. Using rigorous upper bound in limit analysis of plasticity, the critical failure mechanisms of the analyzed equivalent footing are identified. It is demonstrated that for a footing subjected to a vertical eccentric load only, Meyerhof's approximation is reasonable. However, ignoring the impact of horizontal force on bearing capacity in such an equivalent footing is significantly unconservative. Conversely, it is shown that some compounded conservatism in the equivalent problem stems from ignoring the interface friction between the reinforced and retained soils while implicitly considering an unfeasible failure mechanism for bearing of the footing. With conservative selection of soil shear strength properties and a typical value of factor of safety against bearing failure, the end result is likely conservative. The use of this flawed bearing capacity analysis in design is questioned. Also questioned is the applicability of bearing capacity calculations developed for a rigid footing when dealing with a flexible reinforced mass. It is suggested to replace it with a robust and more realistic, albeit simple analysis that considers failures through the foundation soil.