Energy-based failure criterion for macro-porous collapsible rocks

The mechanical behaviour of macro-porous collapsible rocks cannot be fully represented by classical stress-based criteria, such as the Hoek–Brown model, which neglects failure induced by structural collapse under isotropic confinement (zero deviatoric stress). In this study, an energy-based failure criterion and a corresponding generalised flow rule are introduced to characterise the strength and collapse behaviour of macro-porous volcanic rocks. The proposed model defines a closed failure surface that incorporates multiple stress paths, allowing the prediction of the shear strength of macro-porous rocks within a unified energy framework. The criterion was validated through a comprehensive laboratory experimental programme, including uniaxial, triaxial and isotropic compression tests performed on undisturbed pyroclastic samples from the Canary Islands, as well as a dataset comprising Italian tuffs. The results demonstrate that porosity and bonding control the isotropic collapse pressure and shear strength, and that the model provides excellent agreement with experimental data. Validation using Italian tuffs confirms its robustness and applicability. The proposed approach offers a physically consistent tool for assessing the stability of macro-porous rocks in volcanic regions.