ABSTRACT:

The study on the mechanisms affecting the strength and the elastic modulus of a crystalline rock has been focused on a marble, variable in texture, despite an almost monomineralic calcitic composition. A wide experimentation was conducted on three levels: macro-scale (scale of the laboratory sample, order of centimeters), mesoscale (several grains) and grain-scale (order of mm). The experimental results have confirmed, on the various scales, the theoretical models which assume failure occurs from preexisting flaws, acting as stress concentrators in controlling the rock’s strength. On the grain- and meso-scales, the indentations cause the brittle failure of the calcite grain along its cleavage planes and the indents are influenced by microcracks (intra-granular and inter-granular), whatever the indenter’s shape (Berkovich, Vickers, Knoop). On the macro-scale, the texture markedly affects the strength, the stress-strain behavior and the failure mechanisms in Rock Impact Hardness Number (R.I.H.N.) test of the investigated rock. Furthermore, the more intense open microcracking inside the granoblastic marble causes lower values of elastic modulus, strength and P-waves velocity compared to xenoblastic marble.

1. INTRODUCTION

The mechanical behavior of a crystalline rock, at the scale of the laboratory sample, is significantly affected by the shape, size and orientation of its intra-, inter- and trans-granular micro-discontinuities. Thus, even the smallest rock samples must be considered as “discontinuous” media. Such micro-discontinuities consist of grain boundaries, twinning and cleavage planes. These flaws influence the mechanical response of a rock at each scale of observation, as the stability of rock masses is closely related to the characteristics of the joints and the fractures networks. This paper summarizes the results of a wide experimental research, aimed at clarifying the relationship between the intimate structure, i.e. the texture, of the crystalline rocks and their mechanical properties on the scale of the laboratory sample.

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