ABSTRACT:

The dynamic characteristics of an anisotropic rock ('Antigorio gneiss' from the Central Alps) were determined by means of measurements of P- and S-wave velocities. The investigated gneiss was found to be characterized by an orthotropic symmetry, its lower elastic modulus being in the direction of the schistosity pole. On unstressed specimens the elastic moduli have been found to be lower and the anisotropy degree higher than the intrinsic values of the rock matrix, indicating that what prevails is the influence of sets of microcracks. At increasing stresses, the elastic moduli become higher, whereas the anisotropy degree decreases. In order to identify the distributions of the orientation and of the aspect ratio of the cracks, non-interacting crack models were utilized.

RÉSUMÉ:

Les caracteristiques dynamiques d'une roche anisotrope ('Antigorio gneiss' des Alpes centrales) ont ete determinees en mesurant les vitesses des ondes P et S. La roche examinee est caracterisee par une symetrie orthotrope, le module elastique le plus faible etant dans la direction de la normale à la schistosite, Sur les echantillons non soumis à des contraintes, les modules elastiques se sont averes plus faibles et le degre d'anisotropie plus eleve que les valeurs intrinseques de la matrice rocheuse, ce qui indique que l'influence des microfissures preveut, Si on applique des contraintes superieures, les modules elastiques augmentent, alors que le degre d'anisotropie diminue. Pour etablir les distributions de l'orientation et du facteur de forme des fissures, on a utilise des modeles à fissures non interactives.

ZUSAMMENFASSUNG:

Die dynamischen Eigenschaften eines anisotropen Felsen ('Antigorio Gneiss' aus Zentralalpen) wurden durch Messungen der Geschwindigkeiten der S- und P-Wellen ermittelt. Es wurde gezeigt, daß der untersuchte Fels eine orthotrope Symmetrie hat und daß der niedrigste Elastizitatsmodul in der Richtung der, Senkrechte zur Schieferung ist. Bei den unbelasteten Proben waren die Elastizitatsmoduln niedriger und die Anisotropie höher als die eigenen Werten des Felsbindemittels. Das bedeutet, daß der Einfluß von Haarissen ueberwiegend ist. Bei größen Lasten steigern die Elastizitatsmoduln und die Anisotropie verringert. Die Verteilung der Orientierung und des Gestaltverhaltnisses der Risse wurden durch nicht aufeinander einwirkende Rißmodell festegestellt.

INTRODUCTION

Elastic anisotropy in rocks derives from preferred orientations of crystals and from the presence of sets of microcracks or fractures, at the scale of laboratory specimen and of the rock mass.

At the scale of laboratory specimens, the measurement of the ultrasonic velocities allows to evaluate the elastic characteristics of rocks, whose comparison with the intrinsic characteristics of the rock matrix is useful in identifying some aspects of crack distribution (orientation, aspect ratio of cracks, etc.). Even the development of new cracks due to applied stresses (rock 'damage') can be evaluated.

In the following, the theoretical framework encompassing the elastic characteristics of anisotropic rocks affected by sets of cracks is briefly presented; moreover, an example of application of the identification of crack distribution, based on dynamic measurements carried out on a gneissic rock, is proposed.

The investigated rock, which outcrops in the river Toce valley (Central Alps), belongs to the Antigorio gneiss formation; it is extensively quarried 'as an ornamental stone (known as 'Serizzo'). Interest in the mechanical properties of this rock stemmed from the studies carried out during the investigations for the design of a power plant cavern (Piedilago plant).

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