ABSTRACT:

This paper considers currently methods of investigations used in Brazil to characterize hydraulic behaviour of rock masses related to dam foundations studies and subterraneous work. Emphazis is given to field hydraulic testing to determine rock mass permea bility and techniques used to this investigation. Recent inprovements such as the use of Hydraulic Multitest Probe and some other techniques used in Brazil are discussed.

1 INTRODUCTION

Since last decade a great progress Conducted to the development of mathematical modelling employed to re present fractured medium is observed in opposition to a proporcio nally less effort directed to the objetive of gain a better knowledge of the medium under consideration. 'I'he beginning of 1980,',asseamed to show a trend in the sense of an in creasing valorization related to knowledge of rock mass properties and to improvement in the techniques Used to determine field parameters to be applyed as input for mathema tical models.

For permeability studies in rock masses field testing and data acqui sition are under fast development and information of better quality is accomplished with the employement of recent techniques of investiga tions wich is directed to attaine ment of two kind of data: data con cerned to hydraulic testing and da ta concerned to structural parame ters which takes into consideration the use of sampling methods for da a collection of fracture geometry and characterization of fracture properties.

2 THE FRACTURED MEDIUM

Hydraulic characterization of rock masses must take into consideration heterogeneity and anisotropy of the medium. Heterogeneity has its ori gins in the litological differences and varied geological structures which interferes in the configura tion of piezometric heads and seepa ge forces depending on the proper ties of the medium.

Rock mass permeability is likely to be highly anisotropic due to net work of interconnected fractures which have a definite orientation and constitutes the main channels to the fluid flow. In the most ele mentary case anisotropy corresponds to the existence of two directions of differential behaviour influen ced mainly by the attitude and others geometrical characteristics of the discontinuities as opening, spacing, lenght and persistence.

As a rule rock masses are essentially heterogeneous, anisotropics and dis continuos media. The complexity of this fractured medium has its origin on the variety of diiferen tial genesis and tectonics of region under investigation. However in any investigation of a fracture domi nium one has to consider the scale of the volume. of rock mass involved relative to that of the excavation or other engineering work before to decide if the medium is to be analysed within a continuous or disconti nuous approach. Figure 1 shows an example of a fractured medium with, Three sets of discontinuities. Block diagram showed represents a volume of rock mass at a mine site in 'Brazil (Institutede Pesquisas Tecnol6gicas,1986)

3 METHODS OF ANALYSIS

The conceptual model often used to any hydrogeotechnique' analysis of a fractured medium considers rock mass as being formed by intact blocks separated by discontinuities (Louis, 1969).

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