Laboratory tests on two field shale materials and one outcrop shale have revealed significant strength anisotropy. The tests have been analyzed in terms of the patchy weakness model; which describes the rock as isotropic apart from a set of parallel weak planes with heteroge-neous strength distribution. The strength heterogeneities are modelled as weak patches which may be mobilized prior to global failure. The presence of the weak patches yields a wider range of re-duced strength; as compared to the classical plane of weakness model in which the strength of the weak planes is assumed to be uniform. According to these analyses; heterogeneities within the weak planes play a significant role for the strength anisotropy for the outcrop shale and for one of the field shales. For the second field shale; heterogeneities within the weak planes appeared to be insignificant.

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