Influence of Pore Structure Characteristics on Grain Bulk Modulus in Underground Rock Masses: A Hydro-Mechanical Experimental and Theoretical Evaluation

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. Theoretical values were consistently higher than those of experimental measurements, with deviations attributed to pore-scale characteristics unaccounted for in the theoretical model. 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.