Dynamic response of railway ballastless track-bed under various saturated conditions

Abstract Localised saturated zones formed within ballastless railway subgrades owing to rainfall infiltration and moisture accumulation may significantly alter the dynamic response of the track-bed under moving train loads. This study numerically investigates the influence of different saturated conditions on the dynamic response of a ballastless track-bed using a three-dimensional FLAC3D model. The numerical model considers one unsaturated reference condition and six cases containing localized fully saturated zones with different depths and thicknesses in the subgrade bottom layer. Under train-induced cyclic loading corresponding to a train speed of 100 km/h, the variation of dynamic soil stress, vibration velocity, and vibration acceleration along the depth and transverse direction of the track-bed were systematically analysed. The results show that the presence of saturation zones substantially increases the dynamic response of the track-bed compared with the unsaturated condition. Increasing the thickness of the saturation zone leads to greater increases in dynamic soil stress and vibration responses, while saturation zones located closer to the track surface produce more pronounced effects than those at greater depths. For example, at the top of the saturation zone, the vertical dynamic soil stress increases by up to 8.90%, whereas the vibration velocity increases by up to 18.88% compared with the unsaturated condition. The findings enhance the understanding that both the depth and thickness of saturated zones govern the dynamic response of ballastless track-beds. Under the investigated loading and material conditions, shallow and relatively thick saturated zones produced the greatest increases in the predicted dynamic response of the ballastless track system, underscoring the importance of effective drainage and moisture management in the design and maintenance of high-speed railway substructures.