The scope of the present study is the characterization of the inherent anisotropy of metamorphic rocks, based on the most widely used laboratory tests for rocks. The proposed classification system is based on data from literature and results from tests carried out in metamorphic rocks. It takes into account the degree of anisotropy as it is determined from:

  • the uniaxial compressive strength, σ ci,

  • the point load strength, Is50 and

  • the longitudinal wave velocity, Vp.

A new anisotropy index for point load strength, determined from diametral tests on oriented core specimens, is also presented.


A number of indices have been proposed for the classification of inherent anisotropy of the intact rock, mainly that of the uniaxial compressive strength and the point load strength. The degree of strength anisotropy is determined as the ratio of the strength perpendicular to the planes of anisotropy (maximum strength) to that in the weakest direction (minimum strength), as described by ISRM (1981). A classification of the anisotropy of foliated rockswas proposed by Tsidzi (1997) based on the ultrasonic velocity. Tsidzi (1986, 1987) also proposed a descriptive petrographic index (foliation index) for the characterization of the development of anisotropic texture of intact rock and a modification of the classification of point load strength anisotropy (Tsidzi, 1990). Other indices that have been used for the characterization of anisotropy are that based on the deformation modulus and the Poisson ratio (Ramamurthy, 1993 and Kwasniewski, 1993), as well as that of the tensile strength of rock. The indices that have been used for the characterization of anisotropy, possess a different classification scale, as they refer to different mechanical properties of intact rock. These properties are influenced to a varying extend by the anisotropic texture of intact rock. The aim of the present study is the proposal of a concise classification scheme of anisotropic rocks, which will take into account the fundamental anisotropy indices of mechanical properties and will characterize the degree of anisotropy of intact rock, by considering also the uniaxial compressive strength, σ ci, perpendicular to the planes of anisotropy. Thus, the strength and velocity anisotropy indices are correlated to the uniaxial compressive strength.

2.1 Uniaxial compressive strength anisotropy

The strength anisotropy index has been widely used for the classification of anisotropy. The maximum uniaxial compressive strength occurs when loading is perpendicular to the planes of anisotropy (β = 90°). The strength anisotropy index is given as (Ramamurthy, 1993): The application of uniaxial load relevant to the planes of anisotropy in the direction ofmaximumand minimum strength is shown in Figure 1. Based on the strength data of anisotropic sedimentary and metamorphic rocks, Ramamurthy (1993) proposed the classification given in Table 1.

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