SUMMARY:

This paper presents a summary of the geomechanical parameters of basaltic masses, obtained by a large number of "in situ" tests in Southern-Central Brazil. The meaningful development of the finite elements method and the possibility of analysis of stresses and deformations within the body of a dam and its foundations have forced a revision of the techniques of determination and definition of the geomechanical parameters necessary to the characterization of the foundations mass. A brief definition of these parameters, as well as a critical analysis of their present meanings and of the techniques used for their determination are also presented.

RÉSUMÉ:

Ce texte presente un sommaire des paramètres geomecaniques des massifs basaltiques qui ont ete obtenus à partir de nonbreux essais "in situ" effectues dans la Region Centre Sud du Bresil. Le grand developpement de la methode des "Elements Finis" et la possibilite d'analyse des tensions et des deformations dans la superstructure et les fondations des barrages nous obligent à reviser les techniques de determination et de definition des parametres geomecaniques dont on a besoin pour caracteriser le massif de fondation. Une definition rapide de ces paramètres et une analyse critique de leur signification et leurs methodes d'obtention sont aussi presentees.

ZUSAMMENFASSUNG:

Diese Arbeit zeigt eine Zusammenfassung, der Geomechanichen Parameter vom basaltischen Massiv. Diese worden aus einer groβe Anzahl von "in situ-Proben" erhalten, die in Zentrum des Suedlichen Brasiliens ausgefuehrt worden. Die rasche Entwicklung der "Finite Element Methode" und die Möglichkeit eine gsnaue Untersuchung der Druck-und Formveranderungen in einem Dammkörper und seiner Gruendung vorzunehmen, hat zu einer technischen Revision der Bestimmung und Entscheidung ueber Geomechanichen Parameter Gezwungen. Dies ist notwendig, zur Charakterisierung seine Gruendungsmasse. Es wird in diese Arbeit auch eine zusammenfassende Deffinition dieser Parameter und ihre Kritische Analyse, sowie ihre Bedeutung und der heutigetechnische Bestimmung gezeigt.

1. INTRODUCTION

The large number of in-situ tests performed on basaltic masses, both for the determination of their characteristics of deformability (by means of flat jacks) and for the determination of the shear strength of large blocks of rock has allowed the accumulation of large amounts of information, this situation has encouraged the Authors to try a global analysis of these data, aiming at a possible future establishment of brackets of variation for the main geomechanical parameters for lithologic types and for the specific characteristics of basaltic masses. An overall analysis of these data is not an easy task, mostly because of three reasons: - the constant evolution of the techniques for in-situ testing; - lack of standardized test procedure which is a direct consequence of their quick evolution; - lack of a more detailed characterization of the geological feature of the test site, and of a uniform description thereof. The summary of data presently existing and the analysis of the main geomechanical parameters presented in this paper are somewhat inexact, due to these factor but nevertheless they have shed some light on the intervals of variation of the geomechanical parameters of basaltic masses. The development of the analysis of stresses and deformation by the finite element method, both for continuous and for discontinuous media in elastic or elasto-plastic phase, has allowed detailed studies of the combined behaviour structure-foundation under the various possible conditions to which the dam will be subjected. The evolution of the means of analysis should be followed by a corresponding evolution of the techniques used in geological survey and in the definition of the geomechanical parameters, lest we come to analyse details of a geomechanical model which might have very little in common with the actual site under study. Unfortunately, these techniques, though they present a continuous evolution have been showing a development which is less meaningful than that of the means of analysis.

2. STABILITY ANALYSIS AND GEOMECHANICAL PARAMETERS A BRIEF SUMMARY

As a result of the continuous development of the means for the determination of the parameters, as well as of the methods of calculation, new approaches have been developed for the evaluation of the resistant forces involved in the determination of the stability of dams on top of rock masses. Both in the classical method and in the modern ones, the stability of the dam is calculated by a comparison between stresses (caused on the foundations by the presence of the structural and resistant forces (contributed by the rock mass and tending to guarantee the equilibrium of the structure-foundation assembly). In the classical method, the latter are evaluated by comparison to parameters obtained in test aimed at failure conditions and in these tests the deformations occuring up to that point may not be taken into account. Well, that ultimate state of equilibrium of forces may not be acceptable for the good behavior of the structure, either by excessive deformations or by the possible lack of simultaneous mobilization of the maximal resistances.

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