Modeling Strength Anisotropy

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.