Effect of geogrid reinforcement layers on soft subgrades in unpaved roads under repeated loading

Abstract

Geogrids are widely used in unpaved roads to reduce rutting and improve bearing capacity. Laboratory and field studies have demonstrated the benefits of geogrid reinforcement; however, the reinforcement mechanisms remain complex and difficult to quantify when the reinforcement configuration changes. This study examined the influence of the number of geogrid layers on the performance of unpaved road sections constructed on soft clay subgrades. The subbase layer consisted of single-sized artificial triangular-prism aggregates, and biaxial PET geogrids served as reinforcement. Repeated loading tests were performed using a rolling wheel. The test program varied the number of geogrid layers and the number of loading cycles. The results indicated that geogrid reinforcement reduced subbase layer settlement and decreased the vertical stress transmitted to the subgrade. Specifically, the four-layer configuration achieved a high Reduction in Permanent Settlement (RPS) of over 70% almost immediately at the onset of loading. In contrast, the three-layer system required approximately 50 cycles to stabilize, demonstrating that increasing reinforcement layers significantly accelerates structural stabilization and strengthens deformation resistance. These findings emphasize the need to optimize geogrid layer configuration to achieve effective reinforcement in unpaved road applications over soft subgrades.