Ballasted Rail Tracks Research Articles

Experimental and numerical settlement analysis of railway track over geogrid reinforced ballast

PurposeThe main objective of the present research is to investigate the benefits of using geogrid reinforcement in minimizing the rate of deterioration of ballasted rail track geometry resting on soft clay and to explore the effect of load amplitude, load frequency, presence of geogrid layer in ballast layer and ballast layer thickness on the behavior of track system. These variables are studied both experimentally and numerically. This paper examines the effect of geogrid reinforced ballast laying on a layer of clayey soil as a subgrade layer, where a half full scale railway tests are conducted as well as a theoretical analysis is performed.Design/methodology/approachThe experimental tests work consists of laboratory model tests to investigate the reduction in the compressibility and stress distribution induced in soft clay under a ballast railway reinforced by geogrid reinforcement subjected to dynamic load. Experimental model based on an approximate half scale for general rail track engineering practice is adopted in this study which is used in Iraqi railways. The investigated parameters are load amplitude, load frequency and presence of geogrid reinforcement layer. A half full-scale railway was constructed for carrying out the tests, which consists of two rails 800 mm in length with three wooden sleepers (900 mm × 90 mm × 90 mm). The ballast was overlying 500 mm thick clay layer. The tests were carried out with and without geogrid reinforcement, the tests were carried out in a well tied steel box of 1.5 m length × 1 m width × 1 m height. A series of laboratory tests were conducted to investigate the response of the ballast and the clay layers where the ballast was reinforced by a geogrid. Settlement in ballast and clay, was measured in reinforced and unreinforced ballast cases. In addition to the laboratory tests, the application of numerical analysis was made by using the finite element program PLAXIS 3D 2013.FindingsIt was concluded that the settlement increased with increasing the simulated train load amplitude, there is a sharp increase in settlement up to the cycle 500 and after that, there is a gradual increase to level out between, 2,500 and 4,500 cycles depending on the load frequency. There is a little increase in the induced settlement when the load amplitude increased from 0.5 to 1 ton, but it is higher when the load amplitude increased to 2 ton, the increase in settlement depends on the geogrid existence and the other studied parameters. Both experimental and numerical results showed the same behavior. The effect of load frequency on the settlement ratio is almost constant after 500 cycles. In general, for reinforced cases, the effect of load frequency on the settlement ratio is very small ranging between 0.5 and 2% compared with the unreinforced case.Originality/valueIncreasing the ballast layer thickness from 20 cm to 30 cm leads to decrease the settlement by about 50%. This ascertains the efficiency of ballast in spreading the waves induced by the track.

A comparative study of geosynthetically reinforced earth foundations in multi-utility transportation infrastructure for high-speed railways

A high-quality railway track resting on an excellent foundation is required to support high-speed railway transportation. The foundations of high-speed railway tracks are generally constructed on the lifted embankment with the improved ground using different reinforcement agents like geosynthetics and rigid lateral support. The present study performed dynamic finite element simulations on a ballasted rail track laid over a geosynthetically reinforced embankment with and without facing wall support. Three foundation geometries were analyzed to examine the effect of facing wall support and geosynthetics on the lateral resistance of the foundation. An area loaded with a constant pressure was moved at a constant speed, causing the load motion at different speeds in the 90–360 km/h range. Different parameters were calculated at node paths to help understand the lateral effect of moving load. The results showed that the lateral resistance based on nodal acceleration and velocity increased with facing wall support in the range of 40%–57%. Any increment over the minimum facing wall thickness of 300 mm does not significantly increase lateral resistance. Geosynthetics provided a vital function in the foundations with a less bulk volume of soil and increased the lateral resistance by 10%.

Nonlinear dynamic responses of ballasted railway tracks using concrete sleepers incorporated with reinforced fibres and pre-treated crumb rubber

Abstract Damages on railway sleepers due to heavy impact loads induced by the movement of trains can be reduced by improving their impact resistance. Fibre-reinforced/pre-treated crumb rubber concrete sleepers (RCSs) have the potential to display significant impact resistance to withstand a high-magnitude impact load. The ideal proportions of pre-treated crumb rubber, steel fibres, and polypropylene fibres (PFs) can be identified based on the minimum cost-to-impact energy ratio after conducting a drop weight impact test on prisms. The numerical model developed to assess the behaviour of ballasted tracks has been validated using both simulation results and field measurements. Numerical studies have been conducted on ballasted rail tracks with steel and PF-reinforced/pre-treated RCSs using LS-DYNA software. Dynamic strain rate-dependent material parameters are introduced in the numerical simulations. The nonlinear effect of higher train speeds on dynamic track responses has been highlighted in this article. Although the static load-carrying capacity and modulus of elasticity of rubber concrete are low, their dynamic performance controls the track displacements from exceeding permissible limits. The outcome of this study will provide new insights into the effects of railway concrete sleepers incorporated with reinforced fibres and pre-treated crumb rubber on railway track performance in order to ensure safety and reliability before it is put into services.

Response Analysis of a Ballasted Rail Track Constructed on Soft Soil by 3D Modeling

The displacement, stress, and strain distributions of railway embankments on the soft deltaic deposit of the Ganges–Brahmaputra floodplain are investigated. A numerical model developed in general-purpose finite element software is used to simulate the design train load on a deltaic deposit for a 100 km/hr rail speed. The numerical analysis analogy is grounded in the spring model, where a beam under the unit load is modeled based on the Winkler foundation model concept. In the moving load simulation on soil, the static point load relating to the axle load is assigned in the form of a dynamic multiplier, determined using auxiliary software. The calculated shear force in terms of the influence line is applied as a dynamic multiplier. The numerical results demonstrate that under a dynamic train load, the loose ballast undergoes larger and more erratic displacement than the subballast. Comparative analysis between varying subballast stiffnesses shows that stiffer subballast yields smaller displacements. Moreover, a high subballast stiffness can counterbalance the potential of forming permanent deformation by generating lower strains. However, a stiffer subballast does not play a prominent role in reducing the displacement of ballast or vertical stresses. The subgrade is found to carry the maximum load, withstanding the maximum vertical stress; thus, the importance of using an improved subgrade with higher stiffness is also observed. A greater subgrade stiffness improves its load-carrying capacity but fails to reduce the tension responsible for the lateral spreading of the soft subsoil. To reduce the high radial strain, the effects of improving the stiffness properties of two immediately adjacent soft soil layers are numerically investigated. The improvement of subsoil alone is effective in reducing the radial strain, whereas the improvement of both subgrade and subsoil produces further reductions. The critical train speed generating the maximum displacement is identified as 120 km/hr, and the dynamic velocity amplitude decreases with depth. Finally, an allowable limit of rail embankment settlement on a soft deltaic deposit is observed.

Analysis of Dynamic Response of Ballasted Rail Track Under a Moving Load to Determine the Critical Speed of Motion

Analysis of Dynamic Response of Ballasted Rail Track Under a Moving Load to Determine the Critical Speed of Motion

Numerical analysis on combinations of geosynthetically reinforced earth foundations for high-speed rail transportation

High-speed trains, including freight transport, require a high-quality railway track resting on an excellent foundation to handle large deformations and vibrations. The tracks are generally laid on either cuts or fills. A geosynthetic reinforced embankment with or without a retaining wall is mainly used for fill sections. The present study attempts a moving load analysis on a ballasted rail track resting over different geometries of the geosynthetically reinforced embankment. Elastoplastic material models with material damping and infinite boundaries were applied to the model used in the analyses. A pressure-loaded areal patch was moved at a constant speed causing the load motion at semi-high and high-speed. The effect of reinforcing agents on fill section configurations, including their approximate construction cost, was analyzed. Dynamic responses of the moving load are analyzed along the lateral and the vertical direction from the application of the load. The study shows that the ground acceleration decreases by 50 % with geosynthetics and facing walls in the speed range of 100 to 400 km/h. It was also inferred that the bulk mass of earthen embankments is not enough to counter large vibrations.

GPU-Based Simulation of Dynamic Characteristics of Ballasted Railway Track with Coupled Discrete-Finite Element Method

Considering the interaction between a sleeper, ballast layer, and substructure, a three-dimensional coupled discrete-finite element method for a ballasted railway track is proposed in this study. Ballast granules with irregular shapes are constructed using a clump model using the discrete element method. Meanwhile, concrete sleepers, embankments, and foundations are modelled using 20-node hexahedron solid elements using the finite element method. To improve computational efficiency, a GPU-based (Graphics Processing Unit) parallel framework is applied in the discrete element simulation. Additionally, an algorithm containing contact search and transfer parameters at the contact interface of discrete particles and finite elements is developed in the GPU parallel environment accordingly. A benchmark case is selected to verify the accuracy of the coupling algorithm. The dynamic response of the ballasted rail track is analysed under different train speeds and loads. Meanwhile, the dynamic stress on the substructure surface obtained by the established DEM-FEM model is compared with the <i>in situ</i> experimental results. Finally, stress and displacement contours in the cross-section of the model are constructed to further visualise the response of the ballasted railway. This proposed coupling model can provide important insights into high-performance coupling algorithms and the dynamic characteristics of full scale ballasted rail tracks.

The Use of Under Sleeper Pads to Improve the Performance of Rail Tracks

In recent years, with the growing demand for both passenger and freight mobility, faster and heavier rail traffic has been the norm rather than the exception in many countries. As a result, track geometry and the safety of ballasted rail tracks have been adversely affected, leading to exacerbated maintenance costs. Increased stresses in granular foundation induce progressive track degradation, which can result in excessive vertical and lateral deformation, ballast and subballast fouling and impeded drainage. These effects tend to be more severe at specific locations, such as bridges, level crossings and tunnels (i.e. over stiff subgrade). Finding an economical strategy to mitigate ballast degradation has been a challenging task for practitioners, and the inclusion of energy-absorbing rubber pads underneath the sleepers (under sleeper pads—USPs) to minimise track damage is an attractive solution. This paper presents a laboratory study conducted at the University of Wollongong to investigate the use of USPs as resilient elements in ballasted rail tracks involving a stiff subgrade. Test results have shown a significant improvement in track performance resulting from the use of USPs, whereby it is demonstrated that ballast damage induced by the applied cyclic loads can be reduced due to the favourable damping characteristics of these rubber pads. A significant attenuation in particle breakage was observed along with a reduction in both the vertical settlement and lateral movement of the ballast layer, thereby suggesting that USPs can be an effective means of improving the stability and serviceability of the track system.

Experimental Research on Fatigue Strength of Prototype under Sleeper Pads Used in the Ballasted Rail Track Systems

Experimental Research on Fatigue Strength of Prototype under Sleeper Pads Used in the Ballasted Rail Track Systems

Simulation of a Return Current System for AC Power Traction Network

The paper deals the problem of the reverse traction current’s asymmetry in the rail circuit under heavy haul traffic conditions for AC railways. The consequences and the possible causes of the negative influence of asymmetry of the return current system for AC power traction network are considered. For analyse the degree of influence of various factors on the reverse traction current’s asymmetry the return current system model is created in Multisim 12.0 software. The simulation model consists of the reverse traction current block-section. The model allows researching the influence of various factors including environmental conditions such as temperature and humidity, power and type of throttle-transformers, rail track ballast resistance on the conditions of the return current system breakdown. Particular attention is paid to the study of the breakdown voltage critical values on the spark gap in the return current circuit on the example of a railway section located in the Far East of Russia. Simulation results show that in addition to improving electrical characteristics, a positive effect of reducing asymmetry of the return current system for AC power traction network can be achieved by cleaning of the track ballast layer.

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.

Finite Element Modeling of Ballasted Rail Track Capturing Effects of Geosynthetic Inclusions

This paper presents a two dimensional finite element (FE) approach to investigating beneficial aspects of geogrids in the railway track. The influences of different factors including the subgrade strength, the geogrid stiffness, the placement depth of geogrid, the effective width of geogrid, the strength of ballast-geogrid interface and the combination of double geogrid layers were investigated under the monotonic loading. The results indicated the role of geogrid reinforcement is more pronounced over the weak compressible subgrade. A stiffer geogrid reduces ballast settlement and produces a more uniform stress distribution along a track. The placement location of a geogrid is suggested at the ballast-sub-ballast interface to achieve better reinforcement results. Although the width of a geogrid layer should be sufficient to cover an entire loaded area, excessive width does not guarantee additional benefits. Higher interface strength between a ballast and a geogrid is beneficial for effective reinforcement. Increasing the number of geogrid layers is an effective way to reinforce the ballast over weak subgrades. The results of the limited cyclic FE simulations revealed the consistency of the reinforcement effect of the geogrids under monotonic and cyclic loads.

Discrete element modelling of under sleeper pads using a box test

It has recently been reported that under sleeper pads (USPs) could improve ballasted rail track by decreasing the sleeper settlement and reducing particle breakage. In order to find out what happens at the particle–pad interface, discrete element modelling (DEM) is used to provide micro mechanical insight. The same positive effects of USP are found in the DEM simulations. The evidence provided by DEM shows that application of a USP allows more particles to be in contact with the pad, and causes these particles to transfer a larger lateral load to the adjacent ballast but a smaller vertical load beneath the sleeper. This could be used to explain why the USP helps to reduce the track settlement. In terms of particle breakage, it is found that most breakage occurs at the particle–sleeper interface and along the main contact force chains between particles under the sleeper. The use of USPs could effectively reduce particle abrasion that occurs in both of these regions.

Improved performance of geosynthetics enhanced ballast: laboratory and numerical studies

Ballasted rail tracks form one of the most important worldwide transportation modes in terms of traffic tonnage, serving the needs of bulk freight and passenger movement. High impact and cyclic loads can cause a significant deformation leading to poor track geometry. In order to mitigate these problems, the concept of the inclusion of geosynthetics in rail tracks is introduced. This paper presents the current state-of-the-art knowledge of rail track geomechanics, including results obtained from laboratory testing, field investigations and numerical modelling to study the load–deformation behaviour of ballast improved by geosynthetics. The shear stress–strain and deformation behaviour of geosynthetic-reinforced ballast are investigated in the laboratory using a large-scale direct shear test device, a track process simulation apparatus and a drop-weight impact testing equipment. Computational modelling using the discrete-element method is employed to simulate geosynthetic-reinforced ballasted tracks, capturing the discrete nature of ballast aggregates when subjected to various types of loading and boundary conditions. Discrete-element modelling is also used to conduct micromechanical analysis at the interface between ballast and geogrid, providing further insight into the behaviour of ballast subjected to cyclic loadings. These results provide promising approaches to incorporate into existing track design routines catering for future high-speed trains and heavier heavy hauls.

Testing procedures of the Under Sleeper Pads applied in the ballasted rail track systems

The paper describes the essential functional and operational features of the Under Sleeper Pads (USPs), used in ballasted rail track systems. The overview of appropriate test methods for determining the values of relevant parameters for the purposes of using vibro-acoustic isolators was presented, with special attention drown to the changes resulting from the introduction of the PN-EN 16730 standard in this field. Due to the given testing procedures for USPs, selected examples of the normative testing methods of the USPs’ carried out at the Faculty of Civil Engineering of the Warsaw University of Technology with the results were presented.

Effect of particle size distribution and subgrade condition on degradation of railway ballast under impact loads

Ballasted rail tracks are generally exposed to impact loads generated from abnormal wheel-rail interface as well as sudden variations in support rigidity. These induced impact loads can lead to railway ballast degradation by attrition of the angular edges of the aggregate and breakage of single particle into finer fragments. In the present study, the degradation of ballast particles under impact loads is investigated by considering various fouling and breakage indices. For this purpose, impact test is conducted on ballast aggregates obtained from different quarries (rock types of basalt, marl, dolomite and trachyte) by varying the gradation of ballast aggregates, impact energy and subgrade type. According to the obtained results, the degradation of ballast specimens under impact loading is less for more broadly-graded ballast. In addition, providing a flexible subgrade as support condition leads to reduction in ballast degradation resulted from diminishing impact energy. Furthermore, the axial strain of ballast specimens reduces with decrease in degradation of aggregates under repeated impact loads.

Use of seismic surface wave testing to assess track substructure condition

This paper describes the use of Spectral Analysis of Surface Wave (SASW) for characterizing rail track ballast and foundation layers. Surface wave testing on a rail track is more complicated than on soil sites or pavements due to the presence of large ballast void spaces, crossties, and rails as well as the complex variation of shear-wave velocity and Young’s modulus with depth. Young’s modulus is presented for both clean and fouled ballast in wet and dry conditions using the results of surface wave testing and SASW. Young’s modulus can be used to assess tie and track support at different locations, e.g. tie center and edge, under the applied loads. A portable surface wave system is described as well as its expansion to measure Young’s modulus under two ties in the same test.

Mathematical Model for Forecasting of Rail Track Ballast Bearing Capacity

This work contains results of theoretical and experimental research aimed at forecasting of rail track ballast bearing capacity under vibrodynamic load. The analysis of the existing techniques for determining ballast strength shows that most of these techniques have a number of shortcomings which results in significant calculation errors. While developing a mathematical model that describes the critical stress of rail track ballast, the authors proceeded from the limit equilibrium theory developed by V.V. Sokolovsky. The mathematical model was developed with consideration of the impact of inertial forces arising in ballast and in subgrade when the rolling stock is moving.

Stress Response Analyses of Ballasted Rail Tracks, Reinforced by Geosynthetics

This paper highlights recent field studies at Emperor Alexander I St. Petersburg State Transport University, which helped quantify the geosynthetics reinforcement performance in railway ballast. For this purpose, the laboratory of engineering-geological surveys was used to create calibrated load on the railway track. The experimental results indicate that the geosynthetics inclusions have improved track stability directly after it repairs and track doesn’t change their properties during the time.

Improved Performance of Ballasted Rail Tracks Using Plastics and Rubber Inclusions

Current railroads require significant upgrading to meet the challenges of heavier loads at higher speeds. Due to excessive track degradation, the Australian rail industry spends large amounts on frequent track repair and maintenance, as well as ground improvement prior to track construction where soft and saturated subgrade soils pose considerable difficulties in design and construction. Moreover, the degradation of ballast particles under impact loading seriously hampers the safety and efficiency of rail tracks, which leads to speed restrictions and more frequent track upgrading. Hence, there is a need for innovative design solutions that can extend the service life of tracks to cater for faster and heavier train traffic. The use of planar geosynthetics and recycled rubber mats placed at the interface of ballast and subballast layer has proven an effective approach to mitigate ballast degradation and improve track longevity. This paper presents the current state-of-the-art knowledge of rail track geomechanics conducted at the University of Wollongong (UOW) including topics relating to laboratory testing and computational modeling approaches. The load-deformation responses of rubber mat/geogrid-stabilised ballast are studied in the laboratory using a large-scale drop weight impact testing facility, and Track Process Simulation Apparatus (TPSA). Numerical modelling using discrete element methods (DEM) are used to model geogrid-reinforced ballasted tracks, capturing both the discrete nature of ballast subject to various types of loading and boundary conditions. These results provide promising approaches to incorporate into the existing track design routines catering for future high speed and heavy haul trains.