Ambient Vibration Testing and Modal Tuning of the Ridracoli Arch-Gravity Dam in Italy

Evaluating the performance of existing structures and infrastructures, particularly in accurately defining their numerical models, presents significant challenges. Unlike the design of new structures, where material properties, geometry, and boundary conditions can be precisely determined, these parameters in existing structures are often subject to uncertainties. To address these uncertainties, visual inspections, material testing, and structural monitoring become essential. Of specific interest in this paper are large concrete arch dams, where modeling complexities are extraordinary, and little experience exists on the use of dynamic testing as a tool for improving modeling fidelity. The paper specifically focuses on an ambient vibration test carried out on the Ridracoli dam; a major arch-gravity concrete dam located in central Italy. After a comprehensive description of the dam, the field-testing methodology is presented, comprising two different setups, each of them featuring 18 high-sensitivity uniaxial piezoelectric accelerometers. Subsequently, an advanced automated operational modal analysis (OMA) is conducted on the data collected on site to determine the dam’s dynamic properties, including natural frequencies, and mode shapes under operational conditions. The findings are then integrated into a numerical model to obtain accurate calibration of the dam’s material properties, with specific attention to the mechanical coupling between the body of the dam and the surrounding rocks on which the dam is founded.