True Triaxial Testing of Anisotropic Solids

ABSTRACT: Accurate determination of rock mechanical properties (particularly sedimentary shales, mica and schists with foliation and bedding planes) is critical to the safe design and excavation of underground mines and tunnels. Traditional techniques to calculate rock elastic properties often involve testing cylindrical or disc-shaped specimens under uniaxial compression or diametrical loading. But, these stress conditions may not represent the actual stress state under which rock is subjected at depth. A true triaxial testing technique on cubed specimens are, therefore, preferred as it better represents field stress conditions. This paper introduces and verifies a modified step-compression true-triaxial based technique to measure the elastic constants in fibre-reinforced epoxy samples, selected as a low-porosity anisotropic solid. The elastic constants obtained from the proposed method (even under higher stress levels) are found to be in good agreement with results from the benchmark tests with uniaxial compression but in the meanwhile offers other anisotropic parameters, which cannot be obtained from conventional measurements. 1. INTRODUCTION Accurate determination of rock directional elastic properties has always been a hot topic in rock mechanics with immediate applications in most geotechnical and mining engineering (Eberli et al., 2003). While rock is frequently treated as a CHILE (continuous, homogeneous, isotropic, and linearly elastic) medium, this assumption provides only limited insight into the true rock mass deformations (Chou & Chen, 2008; Serati, Alehossein, & Williams, 2016). A more practical rock behavior is therefore the consideration of rock anisotropy, since many rocks exposed near the Earth’s surface show various levels of directionally dependent properties due to bedding, stratification, foliation, fissuring, schistosity, jointing, and faulting (Amadei, 1996). In the stress-strain relationship study for a loaded rock sample, rock behavior can be generally classified into four categories: isotropic, transversely isotropic, orthotropic, and anisotropic. The number of elastic constants to represent the stress-strain relation of a complete anisotropic rock is 21. However, due to the elastic symmetry of three isotropic planes, the number of stiffness constants of an orthotropic material can be reduced to nine (9) constants only. It can be further reduced to five elastic parameters for a transversely isotropic material (E1, E2, ν1, ν2, and G2) and two (namely the Poisson’s ratio and Young’s modulus) for a perfectly isotropic material, where the subscripts "1" and "2" refer to in-plane and out-of-plane directions in transversely isotropic materials (Ding et al., 2006).