Mechanical characterization of pink lac du Bonnet granite: Evidence of nonlinearity and anisotropy

Abstract

A comprehensive laboratory investigation of the mechanical properties of pink Lac du Bonnet granite was carried out. Specimens were tested in three orthogonal directions. In uniaxial and triaxial compression the strength and deformability were practically isotropic. Sonic velocity tests yielded minor directional differences on the order of 10%. Brazilian and uniaxial tensile strengths, however, were both found to vary up to 25% between orientations. The elastic parameters in both compression and tension varied continuously with the stress magnitude, indicating nonlinear stress-strain behavior. The tensile modulus of deformation was substantially lower than that in compression. An unexpected observation in our uniaxial tension tests was the recorded phenomenon of lateral strain decline beyond 50% of peak load, indicating diametrical expansion, and resulting in decreasing Poisson's ratio values. This apparent lateral strain paradox, exhibited by lateral enlargement even as the specimen is being pulled axially, can be explained as the result of the opening of oblique microcracks. Our work brings into question the use of isotropic models in excavation design, in which compressive material constants determined at one level of loading are employed regardless of the local state of stress.