Characterization of geogrid reinforced ballast behavior at different levels of degradation through triaxial shear strength test and discrete element modeling

Recent research efforts at the University of Illinois have aimed at studying geogrid applications in railroad track structures, specifically focusing on ballast and subballast reinforcement. Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure to facilitate load distribution and drainage. The primary mechanism of load transfer within the ballast layer involves inter-particle contact between ballast particles. Similarly, the effectiveness of ballast reinforcement with geogrids is primarily governed by the geogrid-aggregate interlock. Such interaction and the effectiveness thereof can change significantly as the level of grain size and shape degradation or fouling increases in the ballast layer with accumulation of train traffic. Although several studies in the past have investigated the effects of geogrid reinforcement on ballast shear strength and permanent deformation behavior, the effectiveness of geogrid reinforcement at different levels of ballast degradation needs to be further understood. In this study, monotonic triaxial shear strength tests were conducted on both new and degraded ballast materials with and without geogrid reinforcement. Two geogrid types, with square- and triangular-shaped apertures, were used in the laboratory to calibrate an aggregate imaging-based Discrete Element Method (DEM) modeling approach, which is capable of creating actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. The DEM model was observed to adequately capture the shear strength behavior of geogrid-reinforced triaxial ballast specimens prepared using both new and degraded ballast samples.