Ballasted Track Structure Research Articles

Elastic discrete element analysis of natural vibration characteristics and settlement behavior of railway ballasted track

An elastic multi-body model of a ballasted track, consisting of an aggregate of ballast gravel and a single prestressed mono-block concrete sleeper, is constructed. Field-measured traffic load waveforms from an operational existing railway track are then applied to the left and right rail positions on the upper surface of the model using elastic discrete element method. The numerical results are subjected to frequency analysis to examine the relation between track settlement and natural vibration phenomena in ballasted tracks. The relative error between the measured and analyzed frequency values for the natural vibration modes of the ballasted track under 500 Hz is 2.23%, indicating that the current ballasted track model accurately reproduces the natural vibration characteristics of existing ballasted track structures. The analysis results reveal that the phenomenon of plastic deformation within the ballast layer occurs primarily in the upper part of the ballast layer, just below the sleeper. Additionally, for the bounce mode of the ballast layer in the frequency range of generally 100 Hz and below, it has been identified as the primary factor promoting significant vertical motion of the sleeper, along with the associated reverse U-shaped deformation of the sleeper, which accelerates the track structure degradation. Furthermore, when loosening occurs, the vibrational energy of this bounce mode increases, contributing directly to ballast degradation.

Railway slab vs ballasted track: A comparison of track geometry degradation

Concrete slab railway tracks typically have higher initial capital costs yet lower operational costs in comparison to ballasted tracks. One reason for the elevated operational costs on ballasted lines is the need for more frequent maintenance (e.g. tamping to maintain track geometry). Therefore on higher linespeed routes where maintenance windows are limited, concrete slab track is a common choice of trackform. In contrast, on lower linespeed routes it’s common to opt for ballasted track structures. However, the speed ranges at which the two different trackforms are most suitable has received limited scientific analysis. Therefore this paper aims to investigate the train-induced differential settlement of ballast and non-ballasted tracks, considering typical modern intercity (200 km/h) and high speed (300 km/h) lines. Vertical railway track geometry is the metric used to analyse maintenance requirements. This is because it defines the longitudinal level of railway tracks and is a common metric used for the scheduling of track maintenance. First a novel numerical model is presented capable of computing track geometry deterioration due to repeated load passages. The model is then validated for both ballast and concrete slab tracks. Finally it is used to study the differential settlement of both track structures at the two train speeds. It is shown that the ballasted track exhibits higher train-induced differential settlement compared to the slab and at higher linespeeds the degradation of track geometry is increasingly pronounced for the ballasted track.

Influence of Guardrails on Track–Bridge Interaction with a Longitudinal Resistance Test of the Fastener

The guardrail is an indispensable part of ballasted track structures on bridges. In order to reveal its influence on the track–bridge interaction of continuous welded rail (CWR), the longitudinal resistance model of the guardrail fastener and its influential factors are established through tests. By taking a continuous girder bridge (CGB) for railways as an example, a stock rail-guardrail-sleeper-bridge-pier integrated simulation model is developed. The effects of the guardrails, installation torque of the guardrail fastener, and joint resistance of the guardrail under typical conditions are carefully examined. The research results indicate that the nominal longitudinal resistance of the guardrail fastener and the elastic longitudinal displacement of the rail prior to sliding approximately grow linearly with the growth of the installation torque. The presence of a guardrail can alleviate the track–bridge interaction in the range of the CGB, but exacerbate the interaction near the abutment with moveable bearings. This fact enables the abutment position to be considered as a new control point for the design of CWR on bridges. Considering the changing rules of the rail longitudinal force and rail gap, it is recommended that the installation torques of the guardrail fastener and guardrail joint are 40–60 N·m and 700–800 N·m, respectively. The recommended maximum longitudinal stiffness of piers for CGBs is evaluated. When the longitudinal stiffness of the pier for a CGB is lower than the recommended value, the influence of the guardrail can be neglected in the design of the CWR.

Experimental and numerical testing of prototypical under ballast mats (UBMs) produced from deconstructed tires – The effect of mat thickness

In this study, the authors analyse performance of prototypical under ballast mats (UBMs) made of recycled rubber granulate and fibres, produced from end-of-life tires. First, a series of experimental tests is performed, in order to identify static and dynamic elastic characteristics (static bedding modulus, low- and high-frequency dynamic bedding modulus) of four UBMs with the same density and different thicknesses. Additionally, fatigue strength of the mat with a thickness of 20 mm is analysed. Then, the values of static and dynamic bedding moduli are used as an input in the finite element model of the ballasted track structure with UBM. The main aim of the numerical simulations is to compare mechanical behaviour of the reference system and systems with four different mats. It is proved that the thickness of UBMs has a significant effect on the values of static and dynamic bedding moduli, and on the effectiveness of vibration isolation. The insertion loss values vary between 62 % for the thickness 15 mm and 75 % for the thickness 30 mm (at the frequency 31.5 Hz, that is vibration), and between 37 % for the thickness 15 mm and 54 % for the thickness 30 mm (at the frequency 125 Hz, that is structure-borne noise).

Influence of the attenuation of subgrade elastic modulus caused by precipitation on ballasted track structure

• Developed numerical model of ballasted track-subgrade structure considering precipitation based on FEM/EMLT. • Quantitatively revealed the influence of the subgrade elastic modulus for ballasted track-subgrade structure. • Systematically presented latest materials to improve subgrade waterproofing and drainage performance. Precipitation will cause subgrade elastic modulus ( E s ) to decay as subgrade are composed of fine-grained soil with water sensitivity, resulting in adverse mechanical effects on the ballasted track structure. A numerical model of the ballasted track-subgrade structure was established using the KENTRACK program combining the finite element method (FEM) and elastic multi-layer theory (EMLT), taking into account the non-linearity of the ballast and sub-ballast. The reliability of the developed numerical model was verified macroscopically by a laboratory testing based on vertical compressive stress distribution at the top of subgrade. The mechanical response of the track-subgrade structure and the damage law of the subgrade based on permanent deformation were quantitatively analyzed due to the attenuation of E s . The latest materials for the selection to improve subgrade waterproofing and drainage performance were more systematically presented. The results show that the subgrade surface occurs the stress concentration phenomenon as E s becomes larger, and the subgrade elastic deformation is not sensitive to E s with increasing it to 80 MPa, which the impact mainly appears in the region surrounded by two-axle load and the two rails. In addition, E s decay due to precipitation under constant applied load conditions directly reduces the service life of the subgrade, and the relationship between the allowable number of load repetitions ( N a ) at the top of subgrade and E s can be expressed as a power function. The laying of wicking geotextiles at the top of subgrade, asphalt mixture/PP-modified gravel mixture waterproofing layer, etc. can be utilized to improve the elastic modulus of subgrade due to their excellent waterproofing and drainage performance.

Determination of settlement parameters of highway embankment and base consolidation time depending on soil characteristics

The paper proposes and implements a calculation and forecasting principle for designing a subgrade of fast-speed and high-speed highways on a weak foundation, and it is also proposed, taking into account a significant increase in the predicted values ​​of total deformations, to consider embankments with a height of more than 6 m on high-speed highways as individual design areas. It has been proven that in case of strengthening the subgrade foundation with CFG piles, the deformation characteristics of the soft soil of the space between the piles do not affect the amount of settlement and the time of consolidation. It is shown that the theoretical positions are confirmed by the results of numerical simulation. The necessity of keeping the embankments up to the construction of the track superstructure, even with the strengthening of weak foundations with composite structures, has been proved. Based on the results of the performed calculations, the area of ​​effective application of composite base structures with CFG piles for sections with a ballastless superstructure of the track, which provide the minimum allowable residual deformation during operation, was determined. Sand piles and Jet-Grouting piles can be effectively applied in composite subgrade structures of fast-speed and high-speed highways with a ballasted track structure. The influence of physical and mechanical characteristics and filtration properties of weak soil of the inter-pile space on the total amount of settlement and consolidation time was determined.

Resistance to severe environmental conditions of prototypical recycling-based under ballast mats (UBMs) used as vibration isolators in the ballasted track systems

The present paper focuses on laboratory tests of resistance to severe environmental conditions performed on prototypical under ballast mats (UBMs) according to the procedure described in the European standard EN 17282, which was introduced at the end of 2020. Most of the analyzed samples of UBMs were based on elastomeric materials from recycling, such as styrene butadiene rubber (SBR) granulate and fibers from recycled waste tires and rebond polyurethane from polyurethane waste, and additionally, for comparative reasons, one of the tested UBM samples was based on mineral wool. It was proved that in the case of mineral wool- and polyurethane-based samples, there are significant differences in static and dynamic characteristics determined before and after the resistance tests and permanent deformations are observed. Elastomeric mats based on recycled rubber exhibited good properties in the performed tests and thus, they should be regarded as effective and durable vibration isolators which can be used to protect people and the surrounding environment against structure-borne noise and vibration generated by railway traffic. The results of experimental tests were additionally used by the authors to create a viscoelastic model of the vibration isolator (UBM) with fractional derivatives. Using the original model of the ballasted track structure with four degrees of freedom, a significant influence of severe environmental conditions together with the dynamic cyclic loading on the level of vibration suppression (insertion loss factor) was demonstrated. Additionally, the authors proposed the assessment criteria for the resistance to severe environmental conditions of UBMs, tested according to the new standard EN 17282.

Simulation and experimental study on vibration and acoustic characteristics of a continuous supported embedded track

With urban rail transit developing fast, there is an increasing demand for riding comfort and reduction of vibration and noise. Metro being the main public transportation facility of urban rail transit in China, faces more severe challenges. Metro tracks use a range of different vibration isolating ballasted track structures. Environmental vibration along metro lines is controlled by using so-called vibration isolating track structures. In doing so, many problems arise in the train-track system of metro lines, such as strong vibration and excessive wear of wheel/rail systems, causing vibration fatigue and noise. Embedded track is used as an option to reduce vibration and noise, usually applied for tramways. When applied to metro lines, embedded track faces challenge due to high speed, large axle load and more complex line conditions. When applied to metro lines, the acoustic characteristics of embedded track need to be re-evaluated. These characteristics were investigated through a combined method of simulation and experiment. In this study, the wheel and track acoustic radiation of the embedded track designed for metro is predicted firstly. Then the tests of vibration and acoustic of the embedded track are carried out to verify the accuracy of the model on the one hand, and compare with the common fastener track to obtain more aspects of vibration and noise on the other hand. This study can provide a basis for the application of embedded track in metro systems.

Effect of Rail Irregularities and Rail Pad on Track Vibration and Noise

This paper focuses on the effect of short and long wavelength track irregularities and rail pads on train induced vibration and noise using a train-track interaction model. An Indian rail vehicle is modelled as a spring-mass-damper system, and a ballasted track structure is modelled as an infinite rail resting on the viscoelastic foundation. Long-wavelength track irregularity is obtained from measured field data; however, short-wavelength track irregularity is generated using Sato track spectrum. It is observed that vibration and noise levels are not only higher for short-wavelength compared to long-wavelength but also exceed the limits imposed by the Indian Railway standards. Thus, track properties are modified by varying stiffness and damping of rail pads to reduce vibration and noise levels. The stiffer pads reduced vibration level in the range of 7dB–24dB and noise level in the range of 8dBA–14dBA; hence, stiffer pads are recommended for the ballasted track. Moreover, track properties for short-wavelength also keep vibration and noise levels within the permissible limit for long-wavelength. Thus, modification of track properties based on only short-wavelength irregularity is required.

Evaluation of lateral stability of railway tracks due to ballast degradation

Track lateral stability is one of the most critical considerations for safe and reliable railway infrastructures. With increasing exposures to high temperatures globally, a greater expansion in continuous welded rails can induce a higher risk of track buckling, especially when track defects exist. In ballasted track structures, ballast layer holds sleepers in place and provides lateral support and stiffness to the track. Note that there are several factors influencing the lateral resistance of ballasted railway tracks. However, the effects of the progressive degradation of the ballast on the track’s lateral resistance have thus far never been fully investigated. Note that the fouling conditions can be due to the accumulation of ballast breakage or external contamination, such as subgrade intrusion or coal dust, and difficult to inspect in the field. It is evidenced that track buckling can incur even if the railway track and ballast seem to be in a good condition by visual inspection. Therefore, this paper presents a more realistic model to study Single Sleeper (Tie) Push Test (STPT) conditions using the Discrete Element Method (DEM) with the objective to evaluate ballasted track lateral resistance considering different fouling scenarios. Note that coal dust, acting as a lubricant, is considered as a fouling agent. The lateral force–displacement curves of sleepers are analysed. The lateral force is derived from the sleeper-ballast contact forces obtained from three main components: sleeper bottom friction, sleeper side friction, and sleeper end force. The fouling conditions are employed by adapting appropriate model parameters to the ballast layer that represents the fouled ballast condition by coal dust in the DEM simulations. Note that the fouling layer is considered to start from the bottom of the ballast layer and is applied all the way to the top to represent the completely fouled ballast layer condition. The results indicate that fouled ballast can significantly undermine the lateral stability of ballasted tracks by more than about 50%. Track lateral stiffness may be reduced significantly due to fouled ballast layer conditions that cannot be inspected visually in the field. This may reduce track restraint and increase the likelihood of track buckling even though the degraded ballast does not have direct contact with the sleeper. Finally, the study will enrich the development of inspection criteria for ballast lateral resistance and support conditions, improve safety and reliability of rail network, and mitigate the risk of delays due to track buckling leading to unplanned maintenance.

Study on structural health monitoring of vertical vibration of ballasted track in high-speed railway

The ballasted track structure of a high-speed railway repeatedly bears the action of vehicle loads, and the vibration energy is gradually diluted and dissipated due to the ballasted bed. It is difficult to monitor the structural health of ballasted track. By embedding sensors into the ballast, a ballast sensor was obtained. It was positioned on the ballasted track to study the structural health monitoring (SHM) under an impact excitation and to verify the Discrete Element Method (DEM)–Multi-body Dynamics (MBD) co-simulation model. Therefore, the SHM of the ballasted track is realized. The results showed that distributed ballast sensors can realize the SHM status of the ballasted track. Under an impact excitation, the peak of the main vibration frequency of the ballast at the sleeper box, sleeper end and ballast shoulder occurred at a frequency of 153 Hz. At twice the frequency, a secondary peak appeared in the ballast vibration amplitude at the sleeper box, while the vibration amplitudes at the sleeper end and ballast shoulder were suppressed. In low-frequency regime, the vibration damping performance of the ballasted bed structure was better than that in high-frequency regime. When the ballasted track structure was excited by high-frequency vibrations, the surface layer of the ballast at the sleeper box tended to splash, which affected the structural health state. Under the moving wheel load, both sides of the ballasted bed form a “cyclone” shape.

Laboratory Tests and Analyses of the Level of Vibration Suppression of Prototype under Ballast Mats (UBM) in the Ballasted Track Systems.

The present paper is aimed at the analysis of under ballast mats (UBM) which are used in ballasted track structures as vibration isolators and to protect the ballast layer against fast degradation. The mats were tested in the laboratory and afterwards a novel 4-DoF mechanical model of the track structure with UBM was developed. The novelty of this study consists in the comparison of two testing procedures: a procedure based on the popular German standard DIN 45673-5 and a new European standard EN 17282, released in October 2020. Major discrepancies were demonstrated in the determined values of the static and dynamic characteristics using both approaches—especially in reference to the mats with higher stiffness.

Analysis on the track quality evolution law of polyurethane-reinforced ballasted track in high-speed railway

Polyurethane-reinforced ballasted track (PRBT) can improve the integrity of ballasted track structures and satisfy the high-stability requirements of high-speed railways. In this study, the quality evolution law of PRBT structures after being launched into train service was analyzed, and a reference for structural optimization and maintenance operation was provided. The track geometric state of the PRBT test section of a high-speed railway was measured and monitored for nearly one year after it was launched into operation, and the ballastless track of the adjacent section was selected as a reference. The geometric states of the tracks were evaluated and compared using various parameters, including sliding standard deviation, average standard deviation, and track irregularity spectrum density. Results show that the track quality indexes of the test section, which were in operation for nearly one year, were slightly over the limit. Moreover, the fastener can be finely adjusted for the high-value index sections to further improve the ride comfort. The application effect of PRBT in the subgrade fracture zone was good, which could satisfy the requirements of high-quality transportation as well as the normal operation and maintenance of high-speed railway.

Study of the characteristics of ballast bed resistance for different temperature and humidity conditions

A ballasted track structure is one of the most representative types of railway construction. Due to the influences of low-temperature rain and snow environments in alpine regions, an in-depth study of the resistance characteristics and evolution patterns of ballasted track beds is urgently needed. A full-scale ballasted track model was constructed in an insulated test chamber with a programmable temperature-controlled environment. The longitudinal and lateral resistances of the ballast bed were tested for different temperature and humidity conditions, and the performance evolution patterns were analyzed. After the failure of the internal ice layers, a resistance test was carried out for the ballast bed in a low-temperature freezing environment, and the influence of the ice destruction on the characteristics of the ballast bed resistance was analyzed. By simulating the low temperature rise environment, the variation patterns of the ballast bed resistance during the freeze–thaw process were tested and analyzed. The results showed the following. The ballast bed resistance in a dry and low-temperature environment was different from that at normal temperature, and the variation with temperature was greater than that at a normal temperature. The resistance of the ballast bed in a wet state decreased compared to that in a dry condition; as the humidity increased from 34% (dry) to 91% (wet), the longitudinal resistance decreased by 22% and the lateral resistance decreased by 17%. The freezing environment could increase the ballast bed resistance when the sleepers had small displacements. However, after the displacement exceeded a certain value (3.8 mm longitudinally and 2.9 mm laterally under the test conditions), the ballast bed resistance was reduced, with the average reduction rates of the longitudinal and lateral resistances reaching 4.55 kN/mm and 2.90 kN/mm, respectively. The rapid rise and fall of the ambient temperature had an enormous influence on the ballast bed resistance. When an ice-water mixture appeared inside the ballast bed at about 0 °C, the longitudinal and lateral resistances were reduced to 31.64% and 30.28%, respectively, of the resistances in a normal situation. The state dependence of the ballast bed resistance on the low-temperature rain and snow environment indicated that more attention is necessary for the complex service characteristics of the ballasted track under similar conditions.

Effect of distributed support of rail pad on vertical vehicle-track interactions

Rail pads are important components in railway tracks that greatly influence the vehicle-track interactions. A vehicle-track vertically coupled dynamics model that considers the distributed support of rail pad has been proposed in this paper. The rail is modelled as a Timoshenko beam with nonlinear distributed supports, which can reflect both the finite support length of the rail pad and the continuity of the rail. Based on the mode superposition method, the matrix form of the equations of motions is derived for iterative computation. The comparisons between the proposed model and the traditional model of discrete elastic point support are conducted through numerical simulations. The influence of rail pad support length on wheel-rail interaction and pad stress are investigated. The results indicate that the proposed model can provide a more precise result around the “pinned-pinned” frequency. Besides, the traditional model overestimates the rail deformation, and the dynamic responses are magnified in low-frequency vibration analysis. It also reveals that increasing the rail pad support length can mitigate the wheel-rail interactions and the vibration of the ballasted track structure. Furthermore, both ends of rail pad have been proved to suffer larger stress comparing with the middle area when the wheel is rolling over a sleeper.

A Novel Approach to the Analysis of Under Sleeper Pads (USP) Applied in the Ballasted Track Structures.

The present paper is dedicated to the analysis of under sleeper pads (USP), which are resilient elements used in ballasted track systems as vibration isolators. Four types of USP are considered. The authors present the results of laboratory tests, which are then used as input values for the finite element (FE) and mechanical model of the structure. A special focus is put on the description of an original four-degree-of-freedom (4DoF) mechanical model of the system that includes a fractional rheological model of USP. Using the proposed approaches, the dynamic characteristics of under sleeper pads are determined, and conclusions on vibration isolation effectiveness are drawn.

Shakedown solutions for ballasted track structure under multiple uniform loads

Abstract Shakedown analysis method is attractive in calculating the bearing capacity of structures under repeated loads with a large number of cycles. This paper firstly introduces an equivalent Finite Element (FE) model to simulate the ballasted track structure, whose results of elastic stress field were calibrated against those obtained by the full three-dimensional (3D) model, so as to reach an optimum balance between the accuracy and consumption of the numerical simulation. By representing the wheel loads as equivalent uniform loads acting on the ballast layer, an analytical lower-bound shakedown solution is developed to determine the capacity of a half-space subjected to multiple uniform loads, and is further extent to shakedown analysis of ballasted track structure. The required elastic stresses in shakedown analysis are derived analytically for homogenous half-space, while are acquired numerically based on the equivalent FE models. The parametric study on the basis of the proposed shakedown solution is finally performed by introducing the envelope diagrams of shakedown limit, to investigate how the properties and thickness of the materials affect the shakedown limit of ballasted track structure under multiple uniform loads, whose conclusion may provide reference for the optimized design of ballasted track structure subjected to repeated moving train loads.

Shakedown analysis of ballasted track structure using three-dimensional finite element techniques

Shakedown analysis is an attractive method for determining the capacity of geostructures to sustain repeated loads involving a large number of cycles, e.g. rolling and sliding train wheel loads. Its main advantage is that it comes at a significantly reduced computational cost, compared to standard time-domain analyses. An essential component of shakedown analysis is the derivation of closed-form solutions to compute stresses due to the external repeated loads, a task that is not always feasible for complex problems as the ballasted rail track discussed herein. To tackle this, we present in this paper the use of finite element tools to obtain a quasi-lower-bound shakedown load numerically. The proposed method is based on the computation of the three-dimensional elastic stress field numerically, and the estimation of the shakedown load iteratively via an optimisation subroutine implemented in ABAQUS. Following a short presentation of this concept, we compare the elastic stress fields from models featuring varying degree of complexity, with the aim of identifying an optimal discretisation of the problem. This approach can be used for optimising the design of ballasted track structure, and this concept is briefly presented via a parametric study.

Numerical analysis of beam rail bridges with random track irregularities

The study focuses on dynamic analysis of composite bridge / track structure / train systems (BTT systems) with random vertical track irregularities taken into consideration. The paper presents the results of numerical analysis of advanced virtual models of series-of-types of single-span simply-supported railway steel-concrete bridges (SCB) with symmetric platforms, located on lines with the ballasted track structure adapted to traffic of high-speed trains.

The effects of under-sleeper pads on sleeper-ballast interaction

Under-sleeper pads (USPs), typically made from polyurethane, are used by railways in certain parts of the world to reduce ballast settlement and consequently lengthen the ballast tamping cycle. The rationale behind this relatively new addition to the conventional ballasted track structure is that the pad increases the contact area between the angular ballast particles and the underside of the concrete sleeper, with the effect that ballast breakdown and total track settlement are reduced. This paper describes two experiments on the effects of USPs on four aspects of sleeper-ballast interaction, namely contact area, contact pressure, ballast settlement and ballast breakdown. Static and dynamic tests up to 1 million loading cycles were performed under controlled laboratory conditions on concrete sleepers with and without USPs. Sophisticated pressure sensors revealed an increase in contact area from 12% to 35% for static loading tests, and from 8% to 20% for dynamic tests, with a resulting 70% reduction in contact pressure. In addition, a 44% reduction in ballast settlement and a 23% reduction in ballast breakdown were achieved by the introduction of USPs. In conclusion it is argued that the introduction of USPs specifically on heavy-haul lines would offer significant advantages with respect to ballast settlement and breakdown. These advantages are most likely to lengthen general ballast tamping and screening cycles, resulting in significant life cycle cost savings.