Piezometric variations in the basaltic massif beneath the Itaipu hydroelectric plant (Brazil/Paraguay border): Right Buttress Dam

Abstract

This article describes a case study of the changes in the hydrogeotechnical behavior of a portion of the foundation block of the Itaipu Dam, which is located on the Parana River on the border between Brazil and Paraguay. The study focuses on the foundation buttress blocks of the Right Buttress Dam, where the most permeable regions of the massif are monitored by standpipe piezometers. The behaviors of two features within the basalt layers, a basaltic breccia and a fracture, were evaluated. In general, the uplift pressure distribution models of the foundations of concrete dams only consider the hydraulic loads caused by the presence of the reservoir and the tailwater elevation. However, this study demonstrates that the hydrogeotechnical behavior of the massif in a given period may also depend on the local variation in rainfall. The primary conclusion is that although the rocky massif has received several foundation treatments (waterproofing and drainage work), the natural hydraulic conditions of the massif govern the pressure variations within the monitored features. Studying the behavior of these features is of fundamental importance to all large concrete dams, as a statistical analysis of ruptures in several types of dams conducted by the International Commission on Large Dams has demonstrated that accidents involving this type of dam are primarily caused by foundation collapse, particularly by collapses related to internal erosion problems and low shear resistance.