The strength and deformability of anisotropic, folded metamorphic rocks strongly depend on rock mineralogy, texture and other microscopic features. Nevertheless, rock fabric controls on the failure mode of rocks which underwent severe ductile deformation are poorly understood and difficult to generalize. We studied the mechanical behaviour of gneiss samples of the Monte Canale unit (Central Alps, Italy). About 60 rock samples were tested under uniaxial and triaxial compression, and indirect tension tests. The samples revealed exceptionally low unconfined compressive strength (average 15MPa) andYoung's modulus (average<6GPa). Rock samples broke according to four failure modes, from shear failure along foliation to the development of centimetre-scale brittle shear zones, both at low and high confining pressure. X-ray Computed Tomography imaging and micro-structural analysis suggest that failure of strongly deformed metamorphic rocks is controlled by several anisotropies related to micro-fabric, not always readily detectable at meso-scale.
A sound definition of rock strength and deformability is a major issue in every rock engineering project, affecting the performance of rock slopes and underground structures. When dealing with anisotropic rocks, especially strongly deformed metamorphics, the mechanical behaviour of intact rock is controlled by mineralogy and fabric, including foliation and other micro-structural features. The influence of rock micro-fabric on the physical properties of rocks has also been investigated, especially for granitic rocks (Azzoni et al., 1996; Brosch et al., 2000; Nasseri & Mohanty, 2008). Nevertheless, fabric controls on the failure of anisotropic metamorphic rocks undergoing severe ductile deformation, including folding, is less understood. In this perspective we characterized the behaviour of gneiss samples from drill cores from Val Malenco (Central Alps, Italy). We show a preliminary evaluation of the influence of rock fabric on the failure modes of tested rocks, obtained by integrating petrographic, geomechanical, X-ray computed tomography, and micro-structural data.