Elastic anisotropic behaviour related to crack distributions in two gneisses

Stress-strain response of foliated rocks shows that mechanical behaviour and degree of anisotropy are influenced by the spatial arrangement of phyllosilicates. But the anisotropy of these rocks is essentially due to the characteristics and distribution of cracks aligned with mica beds. At the laboratory scale, the elastic symmetry can be represented by the transverse isotropy, with the lowest elastic modulus perpendicular to the plane of aligned microcracks. These issues are discussed with reference to experimental data obtained for two gneisses of the same geological formation. The gneisses show a quite similar strength behaviour, but very different deformabilities. The measures of dynamic and static deformabilities under loading prove the influence of the progressive closure of open cracks on the compliance tensors of both gneisses. The relationship between the elastic parameters and the characteristics of the crack distributions is discussed in the framework of non-interacting crack models. Assuming the presence of two different sets of cracks, crack densities have been estimated. The different deformabilities of the two gneisses can be ascribed to their different microcrack distributions. The degree of anisotropy due to cracks reduces as stresses increase, differently for the two gneisses.