Performance of multi-layer geogrid stabilized roads over soft subgrades under cyclic plate loading: Effects of geogrid configuration and its interaction with subgrade strength

While most previous studies have focused on a single geogrid layer at the subgrade–granular interface, this study investigates the performance benefits of multi-layer geogrid stabilization. The key objectives were to improve rutting resistance, evaluate the potential for road thickness reduction, and investigate the influence of spacing between geogrid layers. A total of seven large-scale cyclic plate load tests were conducted on model road sections incorporating different geogrid configurations and two subgrade California Bearing Ratio (CBR) values of 1% and 2.5%. The laboratory results demonstrated substantial reductions in surface deformation with geogrid stabilization, with double-layer configurations consistently outperforming single-layer configurations. The section constructed over a subgrade with a CBR of 1% and stabilized with two geogrid layers exhibited performance comparable to that of the section constructed over a subgrade with a CBR of 2.5% and stabilized with a single geogrid layer. Reducing the geogrid spacing below 150 mm remained beneficial to long-term road performance; however, it did not necessarily provide additional improvement relative to a spacing of 150 mm. The findings further indicate that, where the subgrade is weaker than anticipated, incorporating an additional geogrid layer within the granular layer may compensate for the reduced subgrade strength.