An anisotropic Hoek-Brown criterion for layered rock masses incorporating multi-factor weakening effects

In layered rock masses, the presence of weak planes (e.g. bedding, foliation, joints) and their geometry induce pronounced strength anisotropy that classical failure criteria fail to represent adequately. This work develops a generalized anisotropic Hoek-Brown criterion by integrating multi-factor weakening effects of weak plane dip angle (β), dip direction (α), and volumetric proportion (η) of weak layers, as well as the effect of the intermediate principal stress (σ2). Weakening functions are derived from a mechanistic basis: orientation functions for α and β are constructed to reflect stress redistribution and shear-normal partitioning; volumetric weakening is modelled via a statistical failure probability approach. The proposed model retains boundary self-consistency and preserves the nonlinear structure of the original Hoek-Brown law. Validation using both triaxial and true triaxial test datasets demonstrates superior predictive accuracy compared to classical and previously modified Hoek-Brown models. The model holds promise for incorporation into fracture and damage mechanics frameworks and for improved directional strength prediction in rock engineering applications.