“…As for the ballast degradation mitigation and performance improvement, the under sleeper pads [10], the geogrid [26,29,[56][57][58][59][60][61][62][63], the geocell [64,65] and the polyurethane [7,66] are the widely-used geomaterials. Plenty of studies with DEM models have been performed to demonstrate their effectiveness and propose application advices.…”