Ballast Beds Research Articles

The influence of sand content on dynamic behaviors of ballast bed from a multiscale perspective

In desert regions, sand intrusion into the ballast bed is unavoidable, leading to a reduction in the elasticity of the ballast bed and posing potential risks to the service safety. Previous studies have focused on the behaviors of sandy ballast beds in 2D planes using discrete element method (DEM), where sand content is typically calculated utilizing sand areas within the ballast voids, different from that using sand volumes or mass in 3D space. Therefore, the impacts of sand content on the multiscale responses of the ballast bed are not fully addressed. In this paper, 3D full-scale half-sleeper models with various sand contents are established via DEM. Multiscale responses of sandy ballast bed obtained by laboratory tests are utilized to verify the reliability of established models. Simulation results show that macroscopically, sand intrusion increases the stiffness and consequently reduces the elasticity of the ballast bed. Microscopically, sand contaminant restricts the translational acceleration of ballast particles, while simultaneously intensifying the angular acceleration. In terms of particle contact, the anisotropy of the directional distribution of contact force among ballast particles is weakened by sand contaminant. Therefore, the inter-particle contact force among ballast particles becomes more uniform, particularly beneath the rail. A normalized parameter is proposed to quantify the filling effect of sand contaminant, based on which the relationships between multiscale dynamic responses and sand content are linearized. The linearization results indicate that the sand intrusion has more significant impacts on the multiscale responses of the ballast bed under higher loading magnitude.

Semi-Conv-DETR: A railway ballast bed defect detection model integrating convolutional augmentation and semi-supervised DETR

Railway ballast bed defects, including subsidence, mud pumping, and abnormal water, pose significant safety risks by destabilizing the railway ballast beds. Timely detection and repair of railway ballast bed defects are vital for safeguarding the security of both the trains and their passengers. Ground-Penetrating Radar (GPR) is widely used for railway ballast beds inspection and evaluation owing to its high speed and non-destructive characteristics. However, GPR image data contain considerable noise, and the distinct shapes and sizes of each ballast bed defect renders it challenging to apply a unified data annotation standard, which hampers the development of railway ballast bed defect detection models. Considering the distinct wave-like characteristics of GPR data and the vaguely contours of the defects to be identified, we propose a convolutional augmentation operation tailored for GPR images. Furthermore, we also investigate Semi-Supervised Learning by employing limited annotated railway ballast bed inspection data along with a vast amount of unlabeled data to joint train the DETR detection model. To sum up, we proposed a semi-supervised DETR model supplemented with convolutional augmentation for railway ballast bed defect detection, termed as Semi-Conv-DETR model. Experimental outcomes indicate that Semi-Conv-DETR shows an improvement of 58.6 % in accuracy when compared to the classical Faster-RCNN model.

Advances in research on the ballast bed stability on railway bridges

The rapid development of high-speed railway bridges in China has brought enormous challenges to the ballast bed stability on the bridge. With the increase in the bridge span, the bridge deformation under the temperature load increases gradually, as well as the stability difference of ballast bed between different regions. Moreover, the track vibration response under high-speed driving conditions is larger, which can easily cause differential settlement, flying ballast, and deterioration of the ballast bed, thereby affecting the bearing capacity and stability of the ballast bed. In view of this, this article summarizes relevant research work on a ballast bed on the bridge from the perspectives of the resistance, deformation, and acceleration response of the ballast bed. In addition, this study discusses the existing measures for improving the ballast bed stability on the bridge, including the optimization of the sleeper and ballast bed structures. Finally, the main issues that need to be addressed to improve the ballast bed stability on the bridge are pointed out, and ideas for future research on ballast beds on bridges with different spans and structures are proposed.

Influence of wind-blown sand content on the mechanical quality state of ballast bed in sandy railways

During the operation of sandy railways, the challenge posed by wind-blown sand is a persistent issue. An in-depth study on the influence of wind-blown sand content on the macroscopic and microscopic mechanical properties of the ballast bed is of great significance for understanding the potential problems of sandy railways and proposing reasonable and adequate maintenance and repair strategies. Building upon existing research, this study proposes a new assessment indicator for sand content. Utilizing the discrete element method (DEM) and fully considering the complex interactions between ballast and sand particles, three-dimensional (3D) multi-scale analysis models of sandy ballast beds with different wind-blown sand contents are established and validated through field experiments. The effects of varying wind-blown sand content on the microscopic contact distribution and macroscopic mechanical behavior (such as resistance and support stiffness) of ballast beds are carefully analyzed. The results show that with the increase in sand content, the average contact force and coordination number between ballast particles gradually decrease, and the disparity in contact forces between different layers of the ballast bed diminishes. The longitudinal and lateral resistance of the ballast bed initially decreases and then increases, with a critical point at 10% sand content. At 15% sand content, the lateral resistance is mainly shared by the ballast shoulder. The longitudinal resistance sharing ratio is always the largest on the sleeper side, followed by that at the sleeper bottom, and the smallest on the ballast shoulder. When the sand content exceeds 10%, the contribution of sand particles to stiffness significantly increases, leading to an accelerated growth rate of the overall support stiffness of the ballast bed, which is highly detrimental to the long-term service performance of the ballast bed. In conclusion, it is recommended that maintenance and repair operations should be promptly conducted when the sand content of the ballast bed reaches or exceeds 10%.

Observing and identifying fouled ballast bed: On-site testing with infrared thermography (IRT) and uncovering thermodynamic transfer mechanisms within the ballast bed

The ballast bed serves as the foundation of the ballasted track, and its performance is maintained through periodic ballast cleaning. Early detection of fouled ballast bed significantly reduces maintenance workload and capital investment. Some scholars have studied the feasibility of utilizing infrared thermography (IRT) for detecting fouled ballast bed (DBF) and have made some progress. Existing studies have predominantly employed simulated boxes to simulate the ballast bed. To better reflect real-world conditions, this study established two sections of ballast bed on a newly constructed line: one with clean ballast and the other fouled, with a volumetric fouling rate (VFR) of 27.6 % (FI ≈ 21.5 %). Moreover, this paper takes a pivotal step in exploring the thermodynamic transfer mechanisms within the ballast bed, the influences of meteorological factors on the detection effectiveness of IRT, and other detection indicators that could be used for DBF.The results demonstrate that the different void fractions and composition substances of the clean and fouled ballast beds (CFB) contribute to their distinct thermodynamic properties. Furthermore, the high specific heat capacity of water exacerbates the thermodynamic property difference between the CFB. In terms of meteorological factors, both the solar radiation intensity (S) and air temperature (T) have a significant positive impact on the temperature of the ballasted structure (STT) and the temperature difference between the CFB (CF-S). Throughout the day, as the S and T increase, the ballast bed surface absorbs more solar heat than it loses, leading to an increase in its surface temperature. When it exceeds the soil temperature (S-S), heat is transferred downward. Since the poor heat conduction of the clean ballast bed, it has a higher surface temperature. As the S and T decrease, heat convection and conduction become dominant, leading to a decrease in the surface temperature of the ballast bed (BT-S). When the BT-S is lower than the S-S, heat is transferred upward, causing the surface temperature of the fouled ballast bed (F-S) to potentially exceed that of the clean ballast bed (C-S). Furthermore, the humidity (H) has a strong negative impact on the STT, while on sunny days following rain, it has a significantly positive impact on the CF-S. The effects of wind speed (W) on the STT and the CF-S are not prominently observed due to its low values during the experiment. Without considering rainfall, higher S and T, combined with reduced W, result in a greater CF-S and are more conducive to advancing fouling detection. Hence, the CF-S can reach up to about 3 °C on a sunny day and may even rise to about 5 °C after rainfall. Nonetheless, the CF-S is only around 0.71 °C on a cloudy day and 0.25 °C at night. Unexpectedly, there is a significant temperature difference between the sleeper and the ballast bed or the steel rail. These indicators could potentially be used for DBF on cloudy days. Overall, these findings demonstrate the feasibility of using IRT for DBF in the field, provding a broader theoretical support for its advancement and implementation.

Mechanical behavior and performance evolution of railway ballast track under dynamic stabiliser based on the hybrid MBD-DEM simulation

The uncertainty in local structure and time-variable compaction of ballast aggregates poses challenges in selecting suitable stabilization methods and parameters for ballast tracks in practical engineering, often resulting in suboptimal operational outcomes. To address this issue comprehensively, the spatial hybrid model of the double dynamic stabilisers-ballast track (MBD-DEM) is proposed in this paper. Subsequent numerical simulations of stabilisation operations are conducted to elucidate ballast vibration, migration behavior and the evolution of ballast track resistance under varying initial conditions of the ballast bed. Initially, leveraging multibody dynamics theory (MBD) and discrete element method (DEM), a MBD model of double stabilisers and a DEM ballast track model containing 6 sleepers were established respectively. Using the coupling interface, the real-time hybrid calculation of these two models is achieved through 6 sleepers in the middle of the ballast bed. Subsequently, the model's accuracy was validated by comparing test data from stabilisation vehicle operations on a Kunming Railway Bureau test line with simulation data from the dynamic double stabilisers-ballast track spatial hybrid model. Finally, utilizing the proposed hybrid model, the dynamic behavior, contact states, and movement rules of ballast particles during stabilisation operations on both loose and initially stable ballast beds (LBB and ISBB) were investigated at the mesoscopic scale. Concurrently, an in-depth analysis of lateral resistance changes in the ballast bed before and after the stabilisation operation is presented. The relevant findings effectively unveil the evolution of dynamic behavior in the ballast bed during stabilisation operations, concluding with practical suggestions for on-site operations. Simulation results illustrate that the rotation of ballast particles in the XY plane beneath each sleeper is predominantly characterized by harmonic rotation. With an increase in the depth of the ballast bed, the range of the rotation axis for combined angular velocity diminishes, leading to the phenomenon of rotation axis deflection. To ensure operational safety and efficiency, a minimum of two-times stabilization operations is recommended for loose ballast beds (LBB), with three-times operations considered optimal. For initially stable ballast beds (ISBB), one-time stabilization operation is basically sufficient.

Bonded particle model for dilated polyhedron considering fracture modes and its application to lateral resistance of ballast bed in cold regions

Considering the spatial arrangement characteristics of bonded irregular particle systems, this paper first constructs irregular dilated polyhedron elements based on the Voronoi cutting algorithm and Minkowski Sum theory. Subsequently, by extending the traditional sphere-based BPM (Bonded Particle Model) to irregular particle elements, a novel dilated polyhedron bonding-breaking model considering different fracture modes is developed under arbitrary contact modes. A collision example between two bonded square particles and a rigid plate is adopted to verify the stability of the model. Later, the difference between the tensile-shear fracture mode and the mixed fracture mode with damage process for contact bond is compared by the discrete element simulations of the tension-fracture behavior between two bonded square particles and the uniaxial compression simulation of frozen ballast aggregate. Finally, the influence of section size on the lateral resistance of the ballast bed structure at low temperatures is discussed using the dilated polyhedron BPM. The results showed that the developed dilated polyhedron BPM considering different fracture modes can realize mechanical behaviours between bonded irregular particles. Compared with the tensile-shear fracture mode, the mixed fracture mode not only considers the combined effect of normal and tangential stress on the bond but also realizes the damage deformation and stiffness attenuation of the bond, which can achieve the “delayed” fracture effect. Meanwhile, the model also shows good reliability in analyzing large-scale engineering problems, such as the mechanical performance of low-temperature ballast beds. The research effectively develops the bonding and breaking model of irregular elements and provides a new perspective for the contact-bond-fracture behavior simulation of irregular granular materials.

On the effect of subgrade strength on the performance of geogrid-reinforced railway ballast

This paper presents the results of a series of ballast box tests aimed at investigating the effectiveness of geogrid reinforcement in reducing track settlement in a 300mm-thick layer of railroad ballast supported by three different artificial subgrades. In each experiment, the ballast layer supports a model tie subjected to cyclic compressive loading applied at a frequency of 0.8Hz with stress extrema at the tie-ballast interface of 57kPa and 400kPa for a total of 40,000 cycles. The three artificial subgrades considered in this study have CBR readings of 25, 13, and 5. For each subgrade, four tests are performed whereby one corresponds to an unreinforced condition (i.e., no geogrid) and three are reinforced with a single geogrid placed at either 150mm, 200mm, and 250mm below the bottom of the tie. The results indicate that geogrids exhibit a superior ability to minimize the tie’s settlement when the ballast layer is supported by a weak subgrade. The experiments further allude to the fact that the influence of the geogrid’s placement depth is exacerbated by the subgrade’s strength. In ballast layers supported by competent subgrades, the geogrid placement depth wields a marginal influence on the resulting tie settlement. However, the geogrid’s location becomes a key factor in ballast beds underlain by soft subgrades, with geogrids placed closer to the bottom of the tie being the most effective at minimizing the tie settlement.

BallastGAN: Random generation of ballast particle contour based on generative adversarial networks

Railway ballast particles exhibit unique variations in shape, and this diversity significantly impacts the mechanical performance of ballast beds. An intuitive approach involves defining corresponding morphological parameters through morphological analysis of particle contours to randomly generate ballast particle contours. This involves deriving the probability distribution of these parameters based on mathematical statistics of ballast particle samples. However, due to the empirical nature and underlying assumptions of these morphological parameters, the accuracy of the generated particle contours is often compromised. To generate highly realistic ballast particle images, we propose a novel model named BallastGAN for random ballast particle contour generation. This model employs fractal dimension regularization and optimizes the loss function of the discriminator within the framework of StyleGAN3. This optimization effectively controls the roughness characteristics of generated ballast particle images. Mutual transformation and style blending between different ballast particle contours are achieved by decoupling the latent space. The ballast particle image dataset (BPID) is curated to train the model. Experimental results demonstrate that the proposed model's randomly generated ballast particle contours exhibit a remarkably high degree of consistency with real contours. This indicates that the generated contours can be directly applied to numerical simulations in mechanics.

Research on fault location algorithm of TPSS based on PSOA.

It is extremely important to research traction power supply system (TPSS) protection technology in order to ensure the safe operation of urban rail transit. A TPSS includes rails, return cables, rail potential limiting devices, one-way conducting devices, drainage cabinets, ballast beds, and tunnel structural reinforcements. In urban rail transit, on the basis of the dynamic characteristics of the TPSS, a fault location algorithm based on particle swarm optimization algorithm (PSOA) is developed. An evaluation of multi-point monitoring data is proposed based on fuzzy processing of the average value of polarization potential forward deviation and multi-attribute decision-making. Monitoring points and standard comparison threshold values are determined by the distribution law of stray currents. In conjunction with the actual project, the model is trained using field measured data. Based on the results, TPSSOA is able to achieve optimal discharge current control, reduce network losses and improve power quality. Moreover, the reconstruction results demonstrate the high usability of the proposed method, which will provide guidance to design the TPSS in the future.

Real-time evaluation of mechanical qualities of ballast bed in railway tamping maintenance

Real-time evaluation of the mechanical qualities of ballast beds is essential for improving tamping effort in railway maintenance. This study aims at evaluating the mechanical qualities of ballast beds based on the impact force on tamper. A novel test device was newly developed to assess the impact force on tamper. This device has high accuracy and long service life. The mechanical qualities of ballast bed and the impact force on tamper were studied experimentally. Results indicated that tamping decreased the lateral resistance and support stiffness of ballast bed by 38.57%–54.08% and 46.92%–62.42%, respectively. Furthermore, a tamping-ballasted track coupling model was constructed and validated by comparing the mechanical qualities of ballast bed between the test and simulation. The dynamic characteristics of ballast and tamper were analyzed numerically. A mathematical expression was developed to invert the mechanical qualities of ballast bed based on the impact force on tamper according to the numerical results. Thus, an innovative real-time evaluation of the ballast bed was achieved by combining the proposed test method and mathematical expression. This study helps to conveniently collect health information of ballasted track at a low cost.

Experimental analysis of ballast bed state in newly constructed railways after tamping and stabilizing operation

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.

Static and cyclic compressive mechanical characterization of polyurethane-reinforced ballast in a railway

Polyurethane reinforcement technology can improve the stiffness and the stability of ballast beds. In this study, the performances of the uniaxial compression and fatigue of polyurethane-reinforced ballast with different amplitudes of cyclic loading were studied. The corresponding simulation models were established by the discrete element method (DEM). Then those simulation models were compared and verified with the tests in the laboratory. The mesoscopic mechanical properties of polyurethane-reinforced ballast during compression were analyzed. The uniaxial compression results showed that during the compression process, the stiffness modulus of the samples increased from low to high (increased by 119.93%), and then decreased gradually and gently at the later stage of loading (the stiffness modulus at the turning point was approximately 2.86 MPa) until failure. The mesoscopic results based on the DEM showed that the polyurethane materials not only bore the normal force of the structure but also substantially bore the tangential force. The stiffness modulus of the polyurethane-reinforced ballast decreased with the increase of the loading times but gradually tended to be stable. The cyclic loading results showed that the deformation of the structure did not increase linearly with the increase of the cyclic loading amplitude. The DEM showed that the stiffness modulus of the polyurethane-reinforced ballast was small when the cyclic loading amplitude was large. However, as long as the cyclic loading did not exceed the bearing limit of the structure, the structure could recover to the initial deformation after the cyclic loading was removed.

Dynamic characteristics of the railway ballast bed under water-rich and low-temperature environments

Studying the dynamic characteristics and evolution laws of the ballast bed under low-temperature, rain and snow environments has practical significance for the driving stability of railways in alpine. In this paper, a full-scale ballasted track model was constructed in a programmable temperature control laboratory, and the frequency response function (FRF) curves of the ballast bed under different temperature and humidity conditions were measured. Then the vibration characteristics and the evolution laws of the ballast bed under different conditions were analyzed. The longitudinal transfer behavior and the dissipation of the vibration energy in the ballast bed under different humidity and temperature environments were discussed combined with the finite element method. The results show that the influence of temperature on the vibration characteristics of the ballast bed is not significant in the dry and water-rich environments, but the vibration characteristics of the ballast bed in the frozen environment change dramatically with the decrease of temperature. The vibration energy is harder to dissipate in the frozen ballast bed than in the dry and water-rich ballast beds, and the frozen ballast bed is more prone to be sudden damaged when a train passes due to the significant increase in its stiffness. Thus, the performance monitoring and emergency maintenance of the ballast bed in those environments should be strengthened.

In-situ performance assessment of track superstructure on fouled railroad

This paper assesses the performance of a class I railroad superstructure due to track fouling. Instrumentation comprised rails and cross-ties’ industry standard circuits in the crib and above the cross-tie to quantify vertical load, digital cameras, and ballast embedded moisture content probes. Normalized maximum rail-tie tensile force was employed to analyse uplift resistance. The longitudinal range of influence of distant wheels was then evaluated by correlating the distance of wheel load peaks on the induced rail-tie force. Moisture analysis showed that water-storage capacity of fouled ballast bed decreased for typical and rigid track stiffness. Results indicate that as the fouled track stiffness increased the influence range of wheels on superstructure increased, for concrete cross-tie supported railroad. The tie-load ratio may misrepresent the superstructure efficiency in fouled ballast beds. Alternatively, the range of influence of distant wheels along with the uplift ratio may be used to assess fouled railroad performance. For this purpose, empirical models are proposed to evaluate the longitudinal range of influence of wheels on the superstructure and uplift ratio.

Importance of examining particle movements in modelling ballast bed via discrete element method

ABSTRACT The discrete element method is widely adopted to investigate the micro-mechanism of degradation and reinforcement of ballast tracks. Generally, numerical parameters of ballast particles are examined by macro characteristics obtained in field or indoor tests, while particle-scale responses are rarely examined due to the difficulties in practical measurement. In this study, we demonstrate the importance of examining particle movements in modelling ballast bed via discrete element method. A previous laboratory model test is repeated via Particle Flow Code, where 25 sets of damping coefficients are assigned. The results show that the variation of damping coefficients has more significant influence on particle movements than the macroscopic behaviours of ballast and ballast beds. According to the three-dimensional motion characteristics of ballast particles obtained using a SmartRock sensor, the appropriate local damping constant and critical contact damping ratio are suggested for typical loading frequencies.

Experimental study and discrete element analysis on dynamic mechanical behaviour of railway ballast bed in windblown sand areas

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.

Numerical and experimental investigations of the effect of temperature and moisture on the repose angle of railway ballast

The angle of repose (AOR) is one of the key parameters used to comprehensively characterize the basic mechanical properties and stacking properties of granular materials. In this research, the influence of temperature and moisture on the stacking performance of railway ballast was studied through tests of the AOR. Additionally, the parameters of the discrete element method (DEM) for ballast beds with various temperatures and moisture levels were calibrated and combined with the response surface method. Then, the AORs of the ballast in different environments were simulated with the DEM, and the micromechanical properties of the ballast in dry, wet, low-temperature, and frozen environments were compared. The results showed that the AOR of the ballast in a frozen environment was smaller than that in a dry environment at the same low temperature (the difference could reach 8.48° at −30°C). The AOR of the ballast increased with the decrease of the temperature in a dry environment (18.44% increase between 20°C and −30°C). The AOR of the ballast decreased with the decrease of the temperature in a wet (frozen) environment (22.86% decrease between 20°C and −30°C). In addition, the validity tests of the AOR simulations of ballast proved that the obtained parameters could be used in the DEM models of ballast for the corresponding temperature and moisture. At the same time, it was found that with the change of temperature and moisture, the force chain network of the ballast would also change.

Comparative analysis of resistance characteristics of composite sleeper and concrete sleeper in ballast bed

A composite sleeper has a similar shape, stiffness, and weight to that of an existing wood sleeper with additional benefits including excellent durability, extended design life like a prestressed concrete sleeper, environmentally friendliness, and cost effectiveness. Therefore, in some cases, composite sleepers are considered as an alternative for wooden or concrete sleepers. In this study, the longitudinal- and lateral-displacement resistance changes of the fiber reinforced foamed urethane (FFU) composite sleepers and the concrete sleepers in ballast beds were tested. The application conditions of the composite sleeper in a ballast bed were analyzed. The results show that the resistance of the composite sleeper changed more sharply, and its value was greater than that of the concrete sleeper during the initial stage of their displacements. The longitudinal and lateral resistance of the composite sleeper was lower than that of the concrete sleeper with an increase in displacement (at 2 mm, longitudinal: 41.35% of the concrete sleeper, lateral: 35.64% of the concrete sleeper). Using the discrete element method (DEM), it was found that when the composite sleeper moves, the ballast around it may ‘rebound.’ The force and its transfer of the ballast near the displacement end of the concrete sleeper was an upward arc, while that of the composite sleeper was a downward arc. The ballast on the side of the concrete sleeper moved upward (tending to be horizontal) along the lateral-displacement direction of the sleeper, while that of the composite sleeper moved downward (tending to be vertical).

Influence of a tamping operation on the vibrational characteristics and resistance-evolution law of a ballast bed

A ballast bed provides a stable foundation and a sufficient vibration-damping performance for a railway line due to its loose and porous structure. Tamping operations change the compactness of ballast stacks and improve and maintain the state of ballast beds, which also affect the vibration-transfer characteristics and resistance performance of ballast beds. In the present study, impact excitation technology was used to test and analyze the influence of the number of tamping operations performed by a tamping car on the damping and stiffness of a ballast bed. Additionally, the influence of different tamping operations on the longitudinal vibration-transmission characteristics of the ballast bed was obtained by the methods of “single-point excitation and multi-point pick-up”. The mode of analysis of the railway structure was carried out by numerical simulation, and the resistance test on site was used to study the evolution law of the longitudinal and lateral resistance of the ballast bed during three tamping operations. The results showed that a large machine tamping operation changed the values of the two dominant resonances (140.75 Hz to 202.35 Hz and 381.25 Hz to 418.75 Hz) of track structure in the range of 0–600 Hz. The tamping operation also affected the resistance, damping, and stiffness of the ballast bed (the longitudinal and lateral resistances decreased by 17.39% and 20.34%, respectively, and the damping and stiffness increased by 204.07% and 148.80%, respectively), which suggests that the longitudinal/lateral resistances, damping, and stiffness of the ballast bed were strongly negatively correlated with one another. The modal shapes of the track structure caused by the first tamping and the second tamping were obviously different, and the positions and values of the maximum displacement changed (1.92 mm to 1.71 mm). The tamping also affected the longitudinal vibration transmission between the sleepers, and the maximum attenuation rate between adjacent sleepers was 81.25%. The change of the resistance work of the ballast bed with the displacement was more significant than the change of the resistance of the ballast bed with the displacement (when the ballast bed resistance decrement rates were 4.34%, 16.19%, and 20.34%, the track bed resistance work decrement rates were 13.14%, 28.15%, and 39.35%, respectively). Thus, the resistance work was more applicable when describing the variational degree of the stability of the ballast bed.