Three dimensional dynamic interaction of adjacent foundations on a halfspace with local inhomogeneity applying a coupled ITM-FEM approach

The dynamic subsoil coupling for a finite number of adjacent foundations has a significant effect on the dynamic behaviour of the foundations involved and the structures based on them. As the mutual influence of the foundations on each other is decisively influenced by the properties and the composition of the subsoil, buried structures or inhomogeneities have to be included, in order to model the real soil conditions as exact as possible and to enable an accurate prediction of the interaction phenomena of adjacent foundations. In this contribution an efficient 2.5D coupled Integral Transform Method (ITM) - Finite Element Method (FEM) approach is used in conjunction with a conventional 3D FEM to compute the dynamic stiffness of foundations resting on the surface of a halfspace including a longitudinally invariant structure or inhomogeneity. Herein, the analytical ITM solutions of the dynamic wave equation allow to account for the infinite extension of the soil by satisfying the radiation condition. The 2.5D FEM formulation enables to model complex, spatially limited structures and a part of the surrounding soil within a cylindrical outer boundary, on which both methods are coupled. The finite foundations on the halfspace surface are modelled using 3D finite elements and are coupled to the soil substructure enforcing the compatibility conditions (full coupling or relaxed boundary conditions) at the common interface. The influence of the embedment depth, size and stiffness of the inclusion on the dynamic response of the foundations, taking into account the through soil coupling, is investigated for different configurations of the surface foundations (distance, width). Frequency dependent compliance functions at the soil-foundation interface are calculated for the different parameter settings and compared with existing solutions to verify the method as well as with the results for the homogeneous halfspace to illustrate the influence of an inclusion. Furthermore the displacement distribution over the total surface of a halfspace with a stiff inclusion due to a dynamic excitation of either one or both of the two considered adjacent rigid surface foundations obtained by a post-processing procedure is presented.