Intact rock strength, usually determined by Unconfined Compressive Strength tests, represents one of the key parameters for performance and tool wear assessment in any hardrock excavation. An accurate assessment of this parameter during preliminary site investigation and undergoing operation is therefore of crucial importance in order to allow appropriate predictions as well as objective contractual discussions. Unfortunately, anisotropic rock behaviour may significantly influence UCS testing and interpretation of intact rock strength. The presented paper resumes the current knowledge regarding this topic, including empirical experience on specific anisotropic behaviour of different rock types, definition and experiences for the "Anisotropy Index" (AI) as well as experiences on sampling strategies and actual project application.

1 The Effect of Anisotropy on Rock Strength Assessment
1.1 Defining "anisotropic behaviour"

In the context of engineering geology and geotechnical engineering "anisotropy" may be defined as directionally dependent geomechanical behaviour. The term is usually used in contrast to "isotropic" behaviour, which characterizes a material with uniform, directionally independent properties. In rock mechanicals literature, the term "anisotropy" is often traced back to the definition of Jaeger 1969, who used the term for the effect of a single plane of weakness on the strength of a cylindrical specimen. The models referred to nowadays do actually differ from this and usually refer to a specimen, that features a set of parallel discontinuities with theoretically infinitely dense spacing.

Mechanically anisotropic behaviour of rock is a result of the rock´s mineral composition and the geological processes that have formed it. If minerals with significant anisotropic properties are included in a rock, they might cause anisotropic rock behaviour when they are structurally distributed and orientated. Common minerals, which cause rock anisotropy, are layered silicates, like clay minerals or mica minerals, which may primarily be orientated by sedimentation (→ lamination, bedding planes) or secondarily be orientated under a specific stress regime during metamorphosis (→ schistosity / foliation, see Figure 1, left).

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