The sand/bentonite (S/B) mixture is considered as a candidate sealing and backfilling material for Switzerland’s deep geological repository (DGR). Using S/B mixtures as sealing material offers key advantages, such as higher intrinsic gas permeability and lower swelling pressure, which enhance gas transport efficiency. These properties make the S/B mixture a strong candidate for use as a gas-permeable seal within the Engineered Gas Transport System (EGTS) concept. This study investigates hydro-mechanical and gas transport (HM-G) response of the S/B mixture, incorporating the effects of dynamic compaction-induced variability in dry density. Key properties of the S/B mixture, such as porosity-dependent and strain-dependent water retention curves (WRC), effective and intrinsic water and gas permeability, and mechanical parameters, were calibrated while accounting for the multiscale heterogeneity in dry density and effective gas permeability of the system. Laboratory-scale simulations of oedometer and swelling pressure tests were conducted to support the parametrization of the mechanical model. The simulations were performed using CODE_BRIGHT, a Finite Element Method (FEM) program. Advanced geo-mechanical models, including the Barcelona Basic Model (BBM), the Barcelona Expansive Model (BExM), and strain-dependent permeability, were utilized to characterize the material behavior.