A laboratory investigation to assess the functioning of railway ballast with and without geogrids

Understanding the complex load transfer mechanism and the subsequent accumulation of deformation in ballast and subballast layers under repeated wheel loading is essential to design resilient rail tracks. Large-scale cyclic tests have been conducted on railroad ballast instrumented with optical based fibre Bragg grating (FBG) sensors, LVDTs, pressure plates and the settlement pegs to explore the role of geogrid and its interaction with ballast in improving the track performance. Latite basalt and geogrids with different aperture sizes were used for the investigations. The laboratory experimental results indicate that the geogrid inclusions enhance track performance by arresting the lateral spreading of ballast and thereby significantly reducing the extent of its vertical settlement. In contrast, the reinforcement of ballast with geogrid has only a marginal effect on reducing the settlement in the subballast layer. The results also show that geogrid minimises the amount of particle breakage, the effectiveness of which is governed by its placement position, with lowest breakage occurring when the geogrid is placed at a location 130 mm above the subballast. In addition, geogrids also reduce the extent of vertical stress in the subgrade soil. The laboratory test results establish beyond doubt the effectiveness of FBG sensing system in capturing the ballast movement under cyclic loading

Improved performance of railroad ballast using geogrids

Geogrids are commonly used to stabilise and reinforce ballast, and over the various laboratory tests have been carried out to determine how geogrids affect the interface between geogrid and ballast aggregates. This paper presents a critical review and interpretation of the results of large-scale direct shear tests and cyclic tests on key parameters such as the interlocking effects of aperture size and the location of geogrids. Field investigations from sites at Bulli and Singleton as well as findings from Discrete Element Modelling, including the influence zone of geogrid and the linear relationship between geometric anisotropy and stress ratio are examined and discussed. It also includes a presentation and discussion of analytical modelling for quantifying the geogrid reinforcing effect (pull-out tests)