Geophysical characterization of dams: a case study of Aosta Valley (NW Italy)

Dam monitoring is crucial for long-term maintenance and structure safety, particularly for smaller dams, as they are more likely to accidents. For this reason, the PNRR under Nodes project has decided to establish and validate a methodology to characterize and monitor geophysical attributes of earthen embarkments for snow production. According to Italian legislation, small capacity dams are under the Regional Administration. The monitoring plans implemented by these entities, currently include technical assessments such as standard topographic and geotechnical measures, where these latter are generally carried out with invasive surveys. Integrating geophysical characterization allows a transition from punctual data provided by these conventional monitoring programs to a more comprehensive view. As a matter of fact, geophysical techniques represent a non-invasive, fast, and effective tool for the characterization and the monitoring of the mechanical parameters of the embankments, which are useful to define the stability of these types of structures. In this thesis Fourcare dam was taken as test site. This embankment is located in Aosta Valley (NW Italy) at an altitude of 2332 m asl. The dam was built in the nineties with a capacity of 127.120 m3 for snow production by Monterosa S.p.A. The objective of this research was focused on the verification of the mechanical and hydrological properties of the body and its local and global stability, with specific attention to find possible seepage or weakness pathways which could compromise the structure. For the analysis of the hydrological attributes the piezometric levels were considered. Concerning the analysis of mechanical parameters, Electrical Resistivity Tomography (ERT) was implemented with the objective to discover saturated zones. Additionally, Active Seismic surveys were carried out to identify possible structural discontinuities, as well as define mechanical parameters such as Poisson ratio, Young modulus and Shear modulus. For this purpose, P-wave from and S-wave velocities from Seismic Refraction Tomography and Surface Waves were analysed. From the analysis of seismic velocities, it was possible to observe high values of the mechanical parameters. Whereas, from the ERT results it was possible to highlight the presence of saturated zones. However, combining these last observations, with the Shear wave velocities it was concluded that these areas could represent zones of material degradation within the dam structure, potentially leading to weaknesses over time. Therefore, additional analysis and ongoing monitoring programs could provide valuable insights to anticipate future issues.