A coupled octahedral formulation of intact rock behavior

An incremental octahedral formulation is proposed to model the response of intact rock to multiaxial loading assuming isotropic and non-linear behavior. The octahedral stress-strain relations involve three tangent moduli: a tangent bulk modulus, a tangent shear modulus and a coupling modulus. This formulation accounts for the experimentally observed coupling between hydrostatic and deviatoric responses and has certain features which can be extended into the thermodynamics concept of entropy.