Basaltic rock masses are a consequence of solidification of crust surface lava flows trough intermittent events that result in superposition of different flows giving rise to relevant discontinuities in the contacts between the flows. These discontinuities assume strong interest for water seepage as a fundamental parameter concerning dam stability and its control by means of cement grouting. The paper discusses seepage aspects within basaltic rocks and its control by means of drainage and grouting, as well as some design aspects related to dam construction.
The main development of Rock Mechanics in Brazil started in the early 50s, with the Federal Government Program for the construction of large hydropower plants that would fulfill future needs for energy a great number of dams were built in the country, most situated in the Central-West region on the Parana Hydrographic Basin. Important concepts related to seepage and cement grouting were mainly developed upon the need of studying the phenomena associated to the hydraulic behaviour of the basaltic rocks, conspicuously occurring in most of the project sites, concerning concrete structures sliding stability along large discontinuities and breccias. Those concepts should be applied in the analysis of the uplift pressures acting in the upward direction in some specific planes mainly in the contacts rock-concrete or large discontinuities. Key problems had to be antecipated related to investigations: the effect of subhorizontal planes and zones of weakness (e.g. breccias); the permeability of rock foundations and abutments leading to design and execution of grouting and drainage systems aiming the stability of the whole structure.
The Parana sedimentary basin (Figure 1) is located in the South of Brazil and have an area larger than 1 million km2 composed by a series of continental deposits with some few intercalations of marine deposits. In the upper part of the geological series there is a sandstone layer (Botucatu formation - Triassic) that were covered by the basaltic lava flows (Cretaceous), and later covered by sandstones again (Bauru formation - upper Cretaceous. The lava flows originated from crustal tensile cracks, in intermittent events, being sometimes split by sediments of various origins named intertrappean sediments. Each basalt layer is tens of meters thick.
Brazilian experience on designing dams, has dealt with sliding stability of structures laid on basaltic rocks, considering various safety concepts (external and internal safety factors, partial safety factors).
In concrete gravity dams (the most used in Brazil) the main effects caused by the structure are reflected in the changes in the state of stress and hydrogeological conditions affecting direction of flow and gradients in the whole region of influence of the dam.
Near the compact rock, the relict structures moderately weathered (the zones of the contacts) are the main responsible for water percolation. Results of hydraulic traditional water pressure tests performed in Agua Vermelha, Capivara and Prornissao reported by Guidicini (1972) showed this zone to be the most pervious.
