Effects of Soil-Wall Separation on Static Earth Pressures

In engineering design practice, seismic earth pressures on retaining walls are typically estimated based on limit analysis techniques such as the Mononobe-Okabe method. Nevertheless, such analyses are pseudo-static in nature; they do not account for the dynamic response of the backfill soil, nor do they provide details on the distribution of earth pressures over the height of the wall. In this context, a number of alternative elasto-dynamic solutions have recently been proposed (e.g. Veletsos & Younan, 1994, Kloukinas et al, 2012, Brandenberg et al, 2015) which properly incorporate the kinematics of the problem and soil-structure interaction. Although these solutions consider soil stiffness and the rigidity of the wall and its foundation, they are limited by the assumption of no separation between the wall and the backfill. To evaluate the effect of possible soil-wall separation, a parametric numerical investigation is conducted, with the aid of the Finite Difference Code FLAC. Focus is given on smooth rigid walls resting on a flexible base. The soil material is assumed to be non-homogenous elastic, with shear modulus varying with depth, hence ensuring that the numerical simulations remain aligned with existing analytical solutions. A first set of analyses involved the static application of a base rotation to the wall, with no separation being allowed. The numerical results were employed to calibrate simplified relationships for the rotation-induced reduction of earth pressures. Imposing a zero-tension requirement to those relationships leads to simple expressions for the distribution of soil pressures in the case of soilwall separation. In this way, a set of expressions were formulated for the total soil pressure force, its point of application, and the depth of soil-wall separation, as a function of the rotation at the wall base, for different types of shear modulus variation with depth. The accuracy of these expressions was verified against a second set of numerical analyses, where separation was simulated with the aid of interface elements. It was demonstrated that the proposed relations provide reasonably accurate predictions, hence they can be incorporated into existing elasto-dynamic solutions to extend their range of applicability.