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 strongly condition water seepage mechanisms which are fundamental issues for the dam stability. This paper discusses seepage aspects within basaltic rocks and the use of drainage and grouting for control, as well as some design aspects related to uplift pressures and its influence in dam construction.
Since the 60's many of the innumerous dams in Brazil have been built in basaltic formations which led to the development of important concepts related to seepage forces acting in the foundations and to the use of cement grouting and drainage systems to assure sliding stability safety. The typical basaltic sequence of flows has sub-horizontal contacts and large scale joints and breccias representing preferential flow paths. These sub-horizontal features became of fundamental importance in the stability analysis of concrete dams, whose technology is being used as a reference for new projects. In view of the experience accumulated in the Parana Sedimentary Basin, where most of dams built in basalt are located, the technological development of dam design and construction led the authors to concentrate on water seepage evaluation techniques as tools for assessing dam sliding stability.
The Parana Basin (Figure 1) has an area larger than 1 million km2 formed by a series of continental deposits with some few intercalations of marine deposits. In the upper part there is a Triassic sandstone layer covered by basaltic lava flows (Cretaceous). The thickness of each basalt layer varies from few to tens of meters and the total basalt package may be thicker than 1km (Figure 2). Each layer may include a sequence of basaltic breccia, vesicular basalt and dense basalt in the middle of the flow pack representing 2/3 of the flow thickness. For designing the main important design features are: - discontinuities along flow contacts having influence on the sliding stability of the dam structures: with both shear strength and stiffness of low magnitude and high permeability; This feature often exceeds the project scale. - discontinuities within the lava flow: very large and having sub-horizontal dip. - high residual stresses-mainly sub-horizontal stresses remaining from tectonic events or erosional processes: difficult excavation procedures and the construction of underground structures.
The design deals with the sliding stability of concrete structures founded on basaltic rocks. Although stress and strain field analyses have been performed for many of the projects, the final decision has been supported by the fulfillment of the limit equilibrium relation Eq. In this equation, Fv is the orthogonal force on the surface under analysis, U is the corresponding uplift pressure, C is the cohesion available in the area S, ö is the friction angle, ãö 621 and ãc are the corresponding partial safety reduction factors established in the design criteria and Fh is the driving force parallel to the surface.