On the effect of subgrade strength on the performance of geogrid-reinforced railway ballast

Abstract

This paper presents the results of a series of ballast box tests aimed at investigating the effectiveness of geogrid reinforcement in reducing track settlement in a 300mm-thick layer of railroad ballast supported by three different artificial subgrades. In each experiment, the ballast layer supports a model tie subjected to cyclic compressive loading applied at a frequency of 0.8Hz with stress extrema at the tie-ballast interface of 57kPa and 400kPa for a total of 40,000 cycles. The three artificial subgrades considered in this study have CBR readings of 25, 13, and 5. For each subgrade, four tests are performed whereby one corresponds to an unreinforced condition (i.e., no geogrid) and three are reinforced with a single geogrid placed at either 150mm, 200mm, and 250mm below the bottom of the tie. The results indicate that geogrids exhibit a superior ability to minimize the tie’s settlement when the ballast layer is supported by a weak subgrade. The experiments further allude to the fact that the influence of the geogrid’s placement depth is exacerbated by the subgrade’s strength. In ballast layers supported by competent subgrades, the geogrid placement depth wields a marginal influence on the resulting tie settlement. However, the geogrid’s location becomes a key factor in ballast beds underlain by soft subgrades, with geogrids placed closer to the bottom of the tie being the most effective at minimizing the tie settlement.