INTRODUCTION

Prior to the early 1970's the use of acoustic emission (AE) as a tool in geomechanics was relatively limited. More recently however the popularity of this technique has increased rapidly (Hardy and Leighton, 1980), and at present it is widely used in both laboratory and field investigations of the behaviour of geologic materials.

In such materials AE may originate at the micro-level as a result of dislocations, at the macro-level by twinning, grain boundary movement, or initiation and propagation of fractures through and between mineral grains, and at the mega-level by fracturing and failure of large areas of material or relative motion between structural units. It is assumed that the sudden release of stored elastic strain energy accompanying these processes generates an elastic stress wave which travels from the point of origin within the material to a boundary where it is observed as an AE signal or a discrete AE event.

In a recent study the nature of the observed AE behavior, and the possible petrographic interpretation, were investigated in five different Spanish granites ranging in their state of alteration from sound to Weathered rocks. These granites were loaded in the laboratory under uniaxial compressive stress, and the associated AE and longitudinal and transverse strains recorded. Later petrographic studies were conducted to study the internal rock evolution during loading. Particular attention Was paid to those petrographic factors of geomechanical significance: micro-fractograPhic network (especially the nature and location of cracks), grain interlocking, mineral alteration, etc.

PETROGRAPHY

The hercynic granitoids for this study were collected in NW Spain, (provinces of Pontevedra, Lugo and Orense).

A short description of each selected granite, mainly related to those petrographic factors of geomechanic significance is presented. In Table 1 their modal and granulometric analysis are summarized, the latter being performed by means of a Kontron MOP-1 image analysis system.

The specimens prepared for the petrographic studies allowed different optic and electronic microscope techniques to be applied to the same area of each granite: They were fluorescein-impregnated, goldmetallized, thin-polished sections, Montoto et al (1980), carefully prepared to avoid the appearance of surface "artifacts". "Ion-thinning" procedures were only applied to these granites when artifacted cracks, or pitting And plucking phenomena, masked fine pore observations that could be of interest.

The observations under light-reflected fluorescence microscopy (Fig. 2 and 3) combined with those under crossed polarizers (C.P.) in light-transmitted microscopy (Fig. 1) are undoubtedly the best way to clearly relate the open cracks with the mineralogy and texture of the rock. By means of this procedure, healed cracks which have no geomechanic relevance are so easily differentiated from the very significant open cracks.

The rock-forming minerals in these granites were studied to define their deterioration state, Table 2, according to the proposed petrographic index of Ordaz et al (1978). Table III in their paper is of valuable fractographic interest in this type of microscopic study.

(Table in full paper)

Gondomar granite

Dark grey heterogranular fine-grained granodiorite, with 99% of its grains in the 0.25–4×10 -3m. range.

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