A Permeability Model for Fractured Granite and its Numerical Implementation

Underground disposal repositories are considered the most effective solution for the safe disposal of high-level radioactive waste (HLW), yet excavation-induced stress variations will influence the permeability properties and form nuclide migration pathways in the host rock, posing significant ecological resource pollution risks. This study focuses on the impact of stress on granite permeability of the Beishan HLW disposal repository. Through triaxial compression-seepage experiments, we measured permeability and acoustic emission (AE) characteristics. Analyzing the experimental process revealed that variations in rock volume and permeability are predominantly influenced by cracks. To account for the contribution of multi-scale cracks and pores to these variations, we treated fractured rock as a dual medium, consisting of the matrix (with inner micro-cracks/pores) and fractures, following distinct Hooke's laws under identical stress. Based on the relationship between axial stress and rock volume, we propose a dual-medium model describing the stress-induced permeability evolution of fractured granite. Model calibration demonstrates its ability to accurately depict stress-induced permeability variation and reflect the mutual transformation between micro-cracks/pores in matrices and fractures. We further implement the proposed constitutive equations in FLAC3D, successfully reproducing the permeability evolution of the tested sample during deformation. Additionally, we simulate the permeability distribution around the HLW disposal repository.