This paper describes a case study whose goal is to present and analyze the main data related to the monitoring of foundation behavior of the Dona Francisca HPP gravity dam whose foundation is essentially constituted by sedimentary rocks. The analysis of the foundation behavior was done in terms of deformations recorded during seventeen years of dam operation by multiple rod extensometers. Dona Francisca HPP dam was built in 2000, on the Jacuí River, in the central region of the Rio Grande do Sul State, Brazil. The geological and geotechnical conditions of the foundation are related to the Formação Caturrita rocks, made up mainly of sandstones and intercalated levels of siltstone and argillite. In the first five years of operation there was an intense stabilization process of the foundation rocky massif. After this period, it was verified the occurrence of stabilization at a lower rate. The deformation of the Dona Francisca HPP foundation is higher when compared with other larger dams, such as the Itaipu.
In Brazil, some dams have been built on sedimentary rocks, which usually present greater deformability in comparison to metamorphic and igneous rocks – other examples of materials that also constitute the foundation of many dams around the world. Foundations formed by soft rocks – such as sedimentary massifs – can even be suitable for gravity dam’s construction because a dam failure is the product of more factors than just the foundation geology type. In such a case, a large deformation of the foundation can take place during the dam construction and its operational period, because the loadings are usually higher than other structures.
It is very important to evaluate the dam massif safety and its foundation through the prediction of deformation levels during the design stage. In addition, it is also necessary to investigate the dam massif performance during its construction and operational period, in order to verify the field safety and confirm that the assumptions adopted in the design stage were appropriate. The investigation process related to dams performance is commonly done through the auscultation by in situ hydro-geotechnical instrumentation, which basically includes the measurement of pore pressure, flows, deformations and displacements. The comparison with the performance of other similar dams is very important for safety analysis based on dam’s behavior experience. However, in the case of dams built on sedimentary rocks, only a few instrumentation data from others structures can be found in the literature to help professionals working in dam monitoring.