This paper focuses on the contribution of the soil-foundation compliance on the measured dynamic response of the Chiaravalle viaduct subjected to ambient excitations. The viaduct, linking the SS76 with the airport of Ancona (in Central Italy), is 875 m long and is constituted by four kinematic chains separated by expansion joints. The r.c. deck is formed by three V-shaped beams supported by column bent piers characterized by circular cross-sections. The piers foundation is constituted by a group of six concrete piles. Ambient vibration measurements have been performed to evaluate the actual natural frequencies and mode shapes of the bridge and data have been used to calibrate a numerical model that was adopted to design the bridge seismic retrofit. Experimental modal properties are evaluated by means of the Operational Modal Analysis; due to the great length of the viaduct, different configurations of sensors placed on the deck are used to study the overall behavior of the viaduct and the Pre Global Estimation Re-scaling method is adopted to merge mode shapes obtained from each configuration. Furthermore, with reference to the transverse behavior, some tests are performed in correspondence of one pier, measuring accelerations of the foundation cap (both translational and rotational components) and the pier bent, in order to identify contribution to the transverse modal displacement due to the elastic deflection of the pier and the foundation rocking. Test results are compared with those obtained from a 3D Finite Element model of the viaduct, including or not the Soil-Structure Interaction problem. The numerical model on compliant restraints is able to capture the foundation rocking, experimentally detected, and provides modal parameters in good agreement with experimental ones.