Geotechnical behavior of gravity dams built on sedimentary rocks: pore pressures and deformations analysis of Dona Francisca HPP foundation

In Brazil, some dams have been built on sedimentary rock masses, which usually present greater deformability and permeability in comparison to metamorphic or igneous rock masses. This article 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 dam, whose foundation is essentially constituted by sedimentary rocks. Dona Francisca gravity dam is a hydroelectric power plant (HPP) and was built in 2000, on the Jacuí River, in the central region of the Rio Grande do Sul State, Brazil. The analysis of the foundation behavior was done in terms of pore pressures and deformations recorded during seventeen years of dam operation. 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 mass. 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 HPP dam. It was carried out an analysis of the 18 vibrating wire piezometers data, allowing a global assessment about the uplift water pressures behavior. Most piezometers indicated a reduction in the pore pressure values over time with a current trend of stabilization, and readings below the control values recommended in design.