<p>This study investigates the behavior of unsaturated subgrade soils beneath flexible pavement systems under varying environmental conditions, with a particular focus on the effects of moisture content, soil suction, and groundwater table fluctuations. Using numerical modeling with the CODE-BRIGHT finite element software, the analysis evaluates how changes in saturation levels and applied stresses influence vertical settlement, subgrade stiffness, and overall pavement performance. The results indicate that higher saturation levels significantly reduce soil stiffness and increase settlement, whereas matric suction in unsaturated soils enhances load-bearing capacity and minimizes deformation. When saturation drops from 100 to 70%, the subgrade’s elastic modulus goes up from 82 to 100.8 MPa, which noticeably reduces settlement. Additionally, a groundwater table positioned 6 m below the surface results in significantly fewer settlements compared to fully saturated conditions, highlighting the stabilizing effect of deeper water tables. Conversely, when the groundwater table rises closer to the surface, increased pore water pressure reduces effective stress and subgrade stiffness, thereby accelerating pavement deformation. In addition, the study indicates that seasonal changes in groundwater and long periods of wetting make pavement wear out faster. This makes it even more important to have effective strategies for managing moisture. These findings underscore the critical role of drainage systems, groundwater control, and moisture regulation in maximizing pavement design, enhancing subgrade stability, and ensuring the long-term durability of road infrastructure.</p>