Strength and failure modes of macroporous rock: Results from laboratory testing and 3D numerical models

This study quantifies the effects of macropore spacing on unconfined compressive strength and qualifies the failure mode of rock material. Numerical modeling, with FLAC3D (finite difference) and TNO DIANA (finite element) software, is used to assess changes in unconfined compressive strength and failure modes of specimens containing macropores. Modeling results are verified using data from forty-four unconfined compression tests on 10 cm (4") cubic specimens of Hydrocal, a high-quality plaster. The specimens, both laboratory and numerical, are 10 cm (4") cubes with circular "tunnels" extending through the front and back faces. Results show that unconfined compressive strength increases as macropore center-to-center spacing increases with specimens of the same macroporosity. With increasing macroporosity, unconfined compressive strength decreases with laboratory results having higher scatter than numerical results. The failure modes of the specimens change from tensile to shear failure with increasing macroporosity center-to-center spacing. This change in failure mode explains the low unconfined compressive strength with small center-to-center spacing. At small center-to-center spacing, the macropores act as a single large void and the specimen exhibits relatively low strength and tensile failure. At large center-to-center spacing, the macropores act as individual macropores, resulting in relatively high strengths and shear failure between macropores.