Abstract
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.
Highlights
Many rocks and rock masses exhibit anisotropic behaviour of deformability and strength, which often has to be taken into account to realistically predict the performance of engineering works
If deformability in anisotropic rock masses is investigated through plate-loading tests, more exhaustive results are provided if the measures are carried out in oriented boreholes, parallel and perpendicular to the principal deformability axis
The ultimate symmetry is due to the superimposition of the various elements of symmetry, and generally, it results of a lower order
Summary
Many rocks and rock masses exhibit anisotropic behaviour of deformability and strength, which often has to be taken into account to realistically predict the performance of engineering works. If deformability in anisotropic rock masses is investigated through plate-loading tests, more exhaustive results are provided if the measures are carried out in oriented boreholes, parallel and perpendicular to the principal deformability axis. In this case the proper elastic model accounts for anisotropy [30]. Investigations for geotechnical characterization of anisotropic rock masses often provide complex outcomes and discrepancies between the results of in situ and laboratory tests. Static deformabilities measured under deviatoric stress on specimens with different angle of schistosity allowed the estimate of the static compliance tensor. From static and dynamic measurements, crack density distributions of open cracks were estimated for both the gneisses, evidencing their differences
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