The mechanical properties of a cohesionless granular medium are strongly dependent on the confining stress, which varies three-dimensionally under a surface foundation due to gravitational and surface loading effects. Success in predicting a foundation’s response to dynamic loading is therefore highly dependent on the appropriate choice of material properties for a given theoretical model. Presented in this thesis are critical results from an investigation of dynamically loaded square foundations on granular soil using a geotechnical centrifuge. From the results, it is shown that the characterization of the vertical mode of vibration of the square foundations via the homogeneous half-space theory is found to be directly feasible, while measurements from lateral excitation tests are found to require a significant reduction in the equivalent homogeneous shear modulus in order to match the measured behavior. These observations are confirmed by new vertical eccentric excitation tests. To provide a rational format for data synthesis, the new concept of Impedance Modification Factors is introduced. The resulting modified half-space theory is shown to allow an accurate prediction of the measured behavior of surface foundations on granular soils.

To gain an understanding of the physical phenomena requiring the use of Impedance Modification Factors, alternative continuum models are investigated. It is shown that the inhomogeneous shear modulus profile which exists in the absence of a surface foundation, along with the locally stiffened zone resulting from the static weight of the foundation, can together provide possible explanations for the required modification of elastic half-space theory.