[en] A finite element formulation for the solution of a three-dimensional soil-structure interaction problem is developed, based upon the use of the consistent lateral boundary. The method is applied to the determination of the dynamic stiffness of a square foundation and the results are compared to an equivalent circular footing to check the validity. The underlying soil interaction between two rigid masses is then investigated for the case of a harmonic force applied at the base of one mass, and for the case of a base motion representing seismic excitation, the values of the masses, natural frequencies, and distance between foundations are varied in the investigation