The compressive strength and deformability are the most important mechanical properties of rocks in engineering applications. The critical factors influencing these parameters include constituent mineral composition, porosity, water content, temperature, anisotropy, and confining pressure (Li et al. 2012). Some rocks show well-defined fabric elements in the form of bedding, stratification, layering, foliation, fissuring or jointing. In general, these anisotropic rocks have physical, mechanical and hydraulic properties that are varied in different directions. Metamorphic rocks usually display the highest degree of anisotropy (Ramamurthy et al. 1993). The strength anisotropy of various rock types has been studied in compression tests by different researchers such as Donath (1964), McLamore and Gray (1967), Hoek (1968), Attewell and Sandford (1974), and Brown et al. (1977) on shales and slates, Ramamurthy et al. (1988) on phyllites, Akai et al. (1970), McCabe and Koerner (1975), Behrestaghi et al. (1996), Nasseri et al. (1997, 2003), Singh et al. (2001) and Zhang et al. (2011) on gneisses and schists.
