Effects of retaining walls on the dynamic response of retained structures

During any dynamic loading of a structure there exists interaction between itself and the underlying soil. In current practice this soil-structure interaction (SSI) is taken into account via equivalent soil impedances that are imposed at its foundation to simulate the compliance of the soil. In this procedure the soil layers are usually assumed to extend horizontally to infinity. However, in the case of soil layers constrained by a retaining structure, two-dimensional soil conditions are present. In the present study, utilizing the finite element method, two simple SDOF oscillators (a slender and a squatty) founded on a soft soil layer, which is supported by a rigid bedrock, restrained by a retaining wall, and subjected to harmonic excitations, are studied in terms of their deformation response spectra. The results for the various components of the systems are compared with those obtained via the development of modified equivalent soil impedances which account for the soil compliance and the presence of the retaining wall. In addition, two parameters which quantify the kinematic and inertial dynamic interaction respectively among structures and soil (and retaining walls) are directly calculated, and useful results are derived from their variation with frequency. It is shown that SDOF systems lying near retaining walls can be approximately simulated as equivalent fixed-base SDOF systems with modified eigenfrequency and damping ratio (as is the case in simple SSI). A necessary prerequisite for this simulation is the utilization of the modified equivalent soil impedances developed in the present study (that take into account the wall presence) and not their usual values corresponding to one-dimensional soil conditions.