Abstarct: The stability of underground structures has always been one of the main concerns of engineers. Among different support methods, shotcrete, due to its ability to be applied immediately after excavation, proper bonding with the rock surface, and provision of initial strength, occupies a significant position in the design and construction of tunnels. However, the mechanical behavior of shotcrete is strongly time-dependent, and its strength characteristics gradually develop from the moment of application until the end of the service life. In this study, laboratory tests were first conducted to investigate the evolution of compressive strength and elastic modulus at different ages. The results indicate that the strength of shotcrete increases sharply during the early ages, rising from 6.51 MPa to an average value of 29.85 MPa. Subsequently, in order to better understand the influence of this time-dependent behavior on tunnel stability, three-dimensional numerical modeling was performed using PLAXIS 3D. The analyses revealed that temporal variations of the mechanical properties of shotcrete influence not only the magnitude of surface settlement but also the distribution of stresses and strains around the tunnel. Accordingly, the magnitude of settlement at the tunnel face in the time-dependent model is about 7% higher than that of the 28-day model. Moreover, to provide a more precise assessment of load-bearing conditions and to understand the behavior of the concrete lining under combined loading, force–moment interaction diagrams (P–M diagrams) were employed. These diagrams enable evaluation of the load-carrying capacity of shotcrete and comparison of different critical states (such as compressive or tensile failure). The P–M analysis results demonstrate that considering the time-dependent effects of shotcrete in design can determine the tunnel stability range more realistically and prevent early failures. Overall, the findings of this research indicate that the combination of laboratory testing, numerical modeling, and P–M analysis can provide a deeper understanding of the behavior of shotcrete under real conditions.