چكيده به لاتين
Even though the continuous welding rail method (CWR) is mainly used to connect the rails and eliminate the rail joints, in some parts of the track, the presence of the rail joints is essential for the track circuit purposes. The presence of the rail joint on the wheel-rail contact surface intensifies dynamic forces. It produces and distributes vibrations and noise, damaging the vehicles, the track structure and the surrounding environment. Under sleeper pads (USPs) have always been suggested as one of the available solutions to attenuate the dynamic effects and reduce the track stiffness in various parts of the railway track. Hence, by presenting the numerical model, this study has considered the presence of under sleeper pads in the rail joint to evaluate the effects of USPs on train-track dynamic interactions. Results manifest that utilizing the USP can reduce the track stiffness and increase the track resiliency, resulting in forces being distributed over the longer track length. Expanding the rail head displacements at the rail joint due to USP usage can amplify the impact forces. However, the softest USPs with stiffness equal to 50 MN/m can increase the impact force by up to 0.5%. Moreover, the results show that utilizing the USPs influences the first lumped masses of the sleeper and ballast layers, which are laid 0.3 m away from the rail joint. In both layers, the USP reduces transmitted forces to first lumped masses and distributes more along the railway track, given that the transmitted forces to third lumped masses (1.5 m away from the rail joint) are increased. Therefore, the USP use in the track structure could make different changes along with the track forces, and the distance of considered points from the rail joint should not be neglected. Generally, USPs, by increasing the dynamic behaviour of the sleeper layer, such as displacements, velocity and acceleration, can mainly improve the dynamic behaviour and attenuate the mentioned parameters of the ballast layer. For instance, Medium USP with stiffness equal to 500 MN/m can enlarge the displacement of the sleeper's first lumped mass by 7.36%, whereas it is diminished by 1.98% for the first lumped mass of the ballast layer. Thus, the most important consequence of USPs is focused on decreasing the ballast degradation and ground-borne vibrations.
By sensitivity analysis on different parameters, it is shown that the greatest impact force can be appeared equal to 195.96 kN when the wheel load is equal to 150 kN. Also, the greatest DIF of 1.63 is observed when the rail joint is supported. The most influences of the medium USP (500 MN/m) on some dynamic attributes of the sleeper and ballast layers first lumped masses are as follows: the most decrease in sleeper force is equal to 3.53% when the wheel load is 150 kN; the most reduction in ballast force is equal to 14.19% when the rail joint gap is 14 mm; the most increase of sleeper displacement is equal to 9.17% by considering the supported rail join (x/s = 0) and the most decrease in ballast displacement is equal to 2.01% when the rail joint gap is 6 mm.
Hence, the USP can improve the dynamic behavior of the ballast layer, especially for the first lumped mass, which results in growing the periods between ballast tamping and maintenance. It is notable to mention that for utilizing the USP in the track structure, the economic aspects of USP providing and installation should be considered in addition to technical ones to assess whether the USP use is efficient.
