Influence of pore structure on grain bulk modulus of underground rock masses under hydro-mechanical conditions

We present an evaluation of the grain bulk modulus of rocks through experimental and theoretical methods. Unjacketed tests were performed on three rock types—Berea sandstone, Idaho sandstone, and Hwangdeung granite—to measure grain-scale compressibility. Theoretical estimates were obtained using mineral composition data and the Voigt–Reuss–Hill averaging method. To examine the role of pore structure, X-ray computed tomography (CT) was employed to visualize and quantify pore geometry, including isolated pores. Experimental results indicated material-dependent variation in grain bulk modulus not directly correlated with conventional mechanical properties. The theoretical values, as represented by the Hill average, were consistently higher than the experimental measurements, with overestimations of 7.2% for Berea sandstone, 37.3% for Idaho sandstone, and 31.7% for Hwangdeung granite. X-ray CT analysis confirmed that isolated pores contribute to additional volumetric deformation, which is unaccounted for in volume fraction-based models. A correlation between theoretical estimates and experimental values was derived to improve the applicability of the model. Conclusively, we present a systematic methodology for accurately evaluating grain bulk modulus by integrating experimental and theoretical approaches, offering foundational data and insight critical for geomechanical modeling in deep subsurface applications such as radioactive waste disposal or underground liquid hydrogen storage.