Investigating influencing mechanisms of under-sleeper pads on lateral resistance of ballasted railway trackbed via hybrid DEM-FDM simulations

The under-sleeper pad (USP) is extensively used in railway track structures due to its desired damping performance. The impact of USP on the stiffness and dynamic responses of track structures was well documented in the majority of the existing studies; however, few studies focus on its influence on the lateral resistance of the ballast bed. To address this deficiency and further disclose its governing mechanisms, a refined numerical model coupling discrete element-finite difference methods (DEM-FDM) was established for the three-dimensional (3D) sleeper-USP-ballast bed-subgrade system. The numerical model was subsequently calibrated and verified by using field-measured lateral resistance results of a typical heavy-haul railroad. The influencing mechanisms of USP on the multiscale performance indicators including lateral resistance of different parts (i.e., the bottom, side, and end) of the loaded rail sleeper, ballast particle motion, and contact forces were revealed from a variety of hybrid numerical simulation scenarios. The results show that at the same level of lateral displacement, the mobilized ballast particles under the rail sleeper with USP had a deeper range of motion than those under the rail sleeper without USP, which led to greater lateral resistance. The unevenness of the USP surface is a major contributing factor to the increase in lateral resistance. Greater USP stiffness results in higher lateral resistance, i.e., USP increases total lateral resistance by 20% to 27.46% and contributing proportions of sleeper bottom by approximately 6% to 9%. The use of the USP could increase the lateral shear stress and the maximum normal contact forces under the sleeper, both of which increase with increasing USP stiffness.