A study of the alterability of basaltic lithotypes from the Itaipu Dam site (southern Brazil)

The alteration of geological materials is a natural process and the measurement of these effects is of great importance in ensuring the safety of engineering works. This study presents results from a set of simple and low-cost tests using specimens lacking a specific geometry. The principal objective of this study was the geological-geotechnical characterization of various basaltic lithotypes, focusing on evaluating the alterability of the rock. Accordingly, this study included tests of point load strength, physical properties (apparent density, apparent porosity, and water absorption), natural alteration, pH, and electrical conductivity (dissolution of ions). Laboratory test results showed good correlation between these parameters, but the results should be interpreted with caution because they were strongly associated with the presence of secondary minerals, the size and distribution of vesicles and amygdules, and the state of alteration of the rock. The specimens subjected to natural alteration in the laboratory failed to develop a significant degree of alteration and, therefore, did not present large changes in point load strength parameters and physical rock properties throughout the alteration cycling tests. Nevertheless, the influence of plagioclase alteration on various laboratory parameters was clearly observed when specimens were analysed microscopically (200×).