Experimental Multi-Modal Foundation Vibrations and Comparison with Benchmark Half-Space Solutions

In this study, results from several field-scale experiments of a surface foundation on a natural soil deposit are presented for vertical, coupled lateral-rocking, and multi-mode vibration tests. An experimental setup was developed which employs an electromagnetic exciter as an inertial shaker to investigate the small-strain wave propagation characteristics of the soil-foundation system over a broad frequency range using random and swept-sine excitation types. Experimental frequency response and coherence functions are obtained to evaluate the relative effectiveness of the two forms of excitation. The various vibration modes are stimulated via vertical-centric (VC), horizontal-centric (HC), and vertical-eccentric (VE) dynamic loading, with the latter producing multi-modal (simultaneous vertical and coupled lateral-rocking) vibrations. As demonstrated in numerous scaled-model geotechnical centrifuge studies, a single multi-mode VE test is shown to be equivalent to separate tests of the vertical and lateral-rocking modes. The hybrid-mode VE test therefore offers improved efficiency while decreasing uncertainties regarding load history effects and differences in soil properties and contact conditions that may arise when the vertical and lateral-rocking modes are tested separately. The performance of the homogeneous half-space theory in modeling the multi-modal response of the soil-foundation system is evaluated, and a preliminary set of Impedance Modification Factors for improved predictive capability are calibrated for the in-situ soil conditions of the study.