ABSTRACT
ABSTRACT: Rising confining pressure p at conventional triaxial compression (σ1 > σ2 = σ3 = p) of intact rocks causes a transition from localized brittle fracture to nonlocalized ductile flow making rocks monotonically less brittle. The following sequence of failure regimes is accepted today: brittle-transitional-semibrittle-ductile. Unlike the traditional approach the paper demonstrates that hard rocks can exhibit dramatic embrittlement within a certain range of p. A special shear rupture mechanism is proposed to explain this phenomenon. In accordance with this mechanism the embrittlement is resulted from reduction of friction within the rupture zone. This mechanism can create transient negative shear resistancereferred to as ‘negative friction’—which makes rocks superbrittle with vanishingly small rupture energy. The paper proposes the following sequence of failure regimes for hard rocks with rising p: brittle-superbrittle-brittle-transitional-semibrittle-ductile.
