Before the operation of newly constructed railways, tamping and stabilizing machines should be used to improve the quality of ballast beds. With the expansion of the railway network and increase of speeds and axle loads, higher quality and efficiency for tamping and stabilizing operation are required. However, previous studies did not involve the effects and parameters of three-sleeper tamping and stabilizing operation under complex working conditions. In the paper, the effect of a three-sleeper tamping and stabilizing machine on the ballast bed state has been studied by performing field experiments. The effect of important factors, including tamping modes, stabilizing frequency, and track lifting amount, are discussed in detail. The results show that the tamping operation on newly constructed railways causes a reduction of the lateral resistance by 56.5 % and a reduction of lateral resistance work by 64.9 %. After the stabilizing operation, the lateral resistance and lateral resistance work are increased by 168.6 % and 209.8 %, respectively. The tamping and stabilizing operation can significantly increase the support stiffness of ballast beds, which meets the requirements of train operation. Meanwhile, 2X tamping mode is more beneficial to improve ballast resistance. Besides, it is reasonable for a stabilizing frequency of 25 Hz to be used for newly constructed railways. The track lifting amount also has a large effect on the ballast bed quality, and it is recommended to keep the lift amount in the range of 20 mm ∼ 30 mm to achieve a better tamping quality.

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Turnout is the key component of the railway tracks for trains to change direction, which is vital to operational safety and passenger comfort. Therefore, it is of great importance to perform a scientific and reasonable tamping operation for turnout areas. In this paper, based on the commercial software EDEM and RECURDYN, a coupled simulation model of the large machine tamping device-rail-sleeper-ballast bed in the turnout area is jointly established, and the correctness of the model is verified by the test results of the lateral resistance of the ballast bed. The influence of tamping operation on the macro-and micromechanical properties of ballast bed at the switching part of railway turnout areas is studied and recommendations for the optimization of tamping operation are proposed. The results show that in the squeezing stage, strong force chains are distributed concentrately under sleepers, where the distribution range is approximately elliptical with a depth of 150 mm. After tamping, only the 200-mm ballast under the sleepers is compacted, where the compactness is increased by 5.9%. On the contrary, the compactness of the ballast in the sleeper crib is reduced by 27.4%, which is the weakest part. To ensure favorable tamping quality, the tamping sequence at the switching part of railway turnout areas is suggested to be conducted in order of first through track and then diverging track.

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The sand intrusion in railway tracks in sandy regions can significantly change the mechanical behaviour of tracks and thus threaten the safety of train operation. This paper presents substantial field tests on both sandy and clean railway tracks to study the effect of sand intrusion on the longitudinal resistance of ballast bed and the vibration behaviour of track structures. After that, a 3D multi-scale the discrete element model is developed to study the micro-contact between ballast particles and the vibration behaviour of sandy tracks during train passing in detail. Also, the effect of train speeds and axle loads on the mechanical behaviour of sandy tracks is discussed. The results show that the sand intrusion increases the vibration acceleration amplitude of rail and sleeper by 11.3% and 50.3%, while ballast bed decreases by 44.9%. Besides, the sand intrusion significantly changes the energy distribution in the track, wherein the frequencies of the highest energy of rail and sleeper are increased while that of the ballast bed is decreased. The parametric study shows the high train speed can cause the increase in overall acceleration of the ballast bed and high axle load can cause an increase in the micro-contact forces between ballast particles, diffusion angle of the contact force chain, displacements of ballast particles, acceleration of ballast particles, and sleeper displacements.

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The lateral resistance of ballast bed is an important parameter to prevent track expansion and maintain track stability. The invasion of sand particles can cause the change of lateral resistance of ballast bed and affect the stability of track structure, but little attention has been paid to the change characteristics of nonlinear lateral resistance of sandy ballast bed. In this paper, the field tests on the lateral resistance of windblown sand ballast bed were carried out to establish a multiscale three-dimensional discrete element model of sleeper-ballast bed. A systematic analysis on the evolution of lateral resistance, resistance to lateral deformation, micro-contact characteristics and lateral stability of ballast bed is performed. The results show that sand intrusion can increase the lateral resistance of ballast bed, which is approximately 40 % higher than that of clean ballast bed. In nonlinear strengthening stage and yield stage, the enhancement effect of sand particles on the lateral resistance of ballast bed is relatively weaker in comparison with the linear growth stage. With the increase in sand intrusion depth, the lateral resistance and resistance work of ballast bed both gradually go up, and the contribution of ballast shoulder to lateral resistance tends to play a leading role. Sand intrusion can increase the lateral stiffness of ballast bed and reduce the elasticity of track structure. Therefore, the maintenance operation should be carried out in time for the section with severe sandstorm.

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Due to the limitation of the geological environment, many railways around the world are built in desert areas. The intrusion of sand particles can increase the stiffness of ballast beds and cause corrosion and fracture of fasteners, turnout blockage, etc., even affecting the service safety of railways. In the paper, based on the results of field and laboratory tests, a three-dimensional discrete element model of the sandy ballast bed is developed to study the effect of sand intrusion on the load sharing ratio of sleepers, contact forces between ballast particles, displacements of ballast particles, and energy dissipation in ballast beds. The results show that the axle load in sandy tracks is mainly shared by three sleepers under the single-axle load and six sleepers under the dual-axle load. The maximum contact force under the sleeper in the sandy ballast bed is about 80% larger than that in the clean ballast bed and its normal and tangential contact forces are also larger than that in the clean ballast bed. The ballast particles at the ballast shoulder of the clean ballast bed move downward in the direction of less than 45°, while those in the sandy ballast bed move outward at the direction of 90°. The sand intrusion inhibits the movement of ballast particles and may cause a rheological phenomenon of ballast particles at the ballast shoulder. The intrusion of sand particles mainly affects the elastic strain energy and damping energy in ballast beds but has less effect on friction energy and kinetic energy.@en

In order to study the interaction between various fouling particles and ballast, a multi-layer and multi-scale discrete element model (DEM) including the sleeper, ballast bed and the surface layer of subgrade was developed. Two typical fouling particles, the hard particles (sand) and soft ones (coal fines), are considered. A support stiffness test of the ballast bed under various fouling conditions was conducted to calibrate the microscopic parameters of the contact model. With the model, the influence of fouling particles on the mechanical behavior and deformation of the ballast bed was analyzed from macro and micro perspectives. The results show that the increase in the strength of the fouling particles enlarges the stiffness of the ballast bed. Hard particles increase the uniformity coefficient of the contact force bond γ of ballast by 50.4%. Fouling particles increase the average stress in the subgrade, soft particles by 2 kPa and hard particles by 1 kPa. Hard particles can reduce the elasticity, plastic deformation and energy dissipation in the track structure. As the fouling particle changes from hard to soft, the proportion of the settlement in ballast bed increases to 40.5% and surface layer of swbgrade settlement decreases to 59.5%. Thus, the influence of fouling particles should be considered carefully in railway design and maintenance.@en