Sub-ballast Layer Research Articles

Use of a bituminous mixture layer in high-speed line trackbeds

Feedback on the use of a sub-ballast bituminous layer in a ballasted high-speed line (HSL) is evaluated in this paper. Conclusions are drawn on the relation between the thermo-viscoelastic behaviour of bituminous materials and the improvement in maintenance needs observed on the studied HSL. The case study is a 3km long experimental zone of the French East-European high-speed line (EE HSL) built with a bituminous mixture sub-ballast layer. In service since 2007, the EE HSL test section has required fewer maintenance operations compared to adjacent sections with conventional structure. Thermo-viscoelastic properties of a bituminous mixture were characterized to understand and explain the contribution of a bituminous sub-ballast layer to the track behaviour. Improved 3D complex modulus tests were performed on laboratory-compacted cylindrical specimens using sinusoidal loading. A constitutive model, called 2S2P1D (2 Springs, 2 Parabolic, 1 Dashpot), developed in the LTDS/ENTPE Laboratory, University of Lyon, was used to simulate the linear viscoelastic (LVE) behaviour of the tested material. The results were analysed in the light of the working conditions and typical loadings of the EE HSL. The results of this study contribute to the definition of an optimal design method for ballasted HSL tracks with bituminous sub-ballast layers.

DETERMINATION AND EVALUATION OF RAILWAY AGGREGATE SUB-BALLAST GRADATION AND OTHER PROPERTIES VARIATION

The use of high quality aggregate mixture, proper technological parameters of laying and compacting as well as the required thickness of the mixture ensures railway sub-ballast layer properties. In this paper gradation, den­sity and water permeability of 49 produced random samples of aggregate sub-ballast (ASB) mixture which were taken from the stockpile in the plant and from the uncompacted railway layer are investigated. The statistical parameters of the ASB mixture quality indicators and histograms with theoretical curves of normal distribution are presented as well. Their conformity to normal distribution was tested through the use of skewness, kurtosis, Pearson, Shapiro-Wilk and Kolmogorov-Smirnov methods. Regression analysis was employed to determine the dependence of standard deviation of percent passing on the mean percent passing through the sieves. In this paper a new method of determining the homo­geneity of ASB according to the variation of gradation using the maximum standard deviation value of this dependence was employed. The minimum size n was calculated with the permissible relative error of 5%, 10%, 15% and 20%. The obtained research findings revealed that due to segregation processes the homogeneity of ASB mixture from the point of its production to application has decreased by about 38%.

A laboratory investigation to assess the functioning of railway ballast with and without geogrids

Abstract Understanding the complex load transfer mechanism and the subsequent accumulation of deformation in ballast and subballast layers under repeated wheel loading is essential to design resilient rail tracks. Large-scale cyclic tests have been conducted on railroad ballast instrumented with optical based fibre Bragg grating (FBG) sensors, LVDTs, pressure plates and the settlement pegs to explore the role of geogrid and its interaction with ballast in improving the track performance. Latite basalt and geogrids with different aperture sizes were used for the investigations. The laboratory experimental results indicate that the geogrid inclusions enhance track performance by arresting the lateral spreading of ballast and thereby significantly reducing the extent of its vertical settlement. In contrast, the reinforcement of ballast with geogrid has only a marginal effect on reducing the settlement in the subballast layer. The results also show that geogrid minimises the amount of particle breakage, the effectiveness of which is governed by its placement position, with lowest breakage occurring when the geogrid is placed at a location 130 mm above the subballast. In addition, geogrids also reduce the extent of vertical stress in the subgrade soil. The laboratory test results establish beyond doubt the effectiveness of FBG sensing system in capturing the ballast movement under cyclic loading.

Cyclic and Postcyclic Triaxial Testing of Ballast and Subballast

AbstractBallast and subballast are two layers of noncohesive, granular materials with different grain sizes filled in the track substructure to bear traffic loadings from train passages. The performance and sustainability of a track substructure depend on both of these two granular layers. This paper presents the experimental results of cyclic triaxial testing and postcyclic triaxial testing on individual ballast and subballast materials. Also, to investigate mechanical behavior at the ballast/subballast interface, cyclic tests were performed on composite specimens consisting of the upper half ballast layer and underlying half subballast layer. These cyclic tests indicated that the existence of the ballast/subballast interface depresses the development of permanent axial or volumetric deformation at the interface zone, and this effect vanishes as the initial compaction and confining pressure increase. Postcyclic failure tests show that the ballast/subballast interface zone’s strength is close to that of t...

RESEARCH OF RAILWAY AGGREGATE SUB-BALLAST MIXTURE PROPERTIES / GELEŽINKELIO POBALASTINIO SLUOKSNIO IŠ UŽPILDŲ MIŠINIO SAVYBIŲ TYRIMAS

Railway sub-ballast layer is a connecting layer between the ballast and other track layers. This layer must ensure even load distribution into the subgrade, drainage surface water, prevent materials of layers mixing. Ensuring these properties allows to extend the explotation of all railway road construction and geometrical parameter of road maintenance. This article analyzes investigated railway sub-ballast mixture properties from large sample sizes (n = 49): gradation, water permeability and density statistical parameters. The regression equation showing the relationship between the fully percent passing through sieves averages and standard deviations is presented. Her coefficient of determination (R2 = 0.909). The minimum number of representative samples of these three properties research is determined. Irregularity in most cases of water permeability (ν = 88,3%), depending on the laboratory tests methodology errors, is indicated. Geležinkelio pobalastinis sluoksnis yra jungiamasis sluoksnis tarp balasto ir grunto sluoksnių. Šis sluoksnis turi užtikrinti tolygų apkrovos perdavimą į sankasą, nudrenuoti paviršinį vandenį, neleisti maišytis sluoksnių medžiagoms. Šių savybių užtikrinimas leidžia prailginti visos kelio konstrukcijos eksploataciją ir geometrinių kelio parametrų išlaikymą. Šiame straipsnyje yra analizuojamos užpildų mišinio, naudojamo geležinkelio pobalastiniame sluoksnyje, iš didelės imties (n = 49) gautos savybės: granuliometrinės sudėties, pralaidumo vandeniui ir tankio statistiniai parametrai. Pateikta granuliometrinės sudėties regresijos lygtis, rodanti sąsają tarp pilnutinių išbirų per sietus vidurkių ir standartinių nuokrypių. Jos determinacijos koeficientas R2 = 0.909. Apskaičiuoti reprezentatyvūs minimalūs bandinių skaičiai šioms trims savybėms tirti. Gauta, kad pralaidumo vandeniui savybė yra labiausiai nevienoda dėl laboratorinių tyrimų metodikos paklaidų, variacijos koeficientas (88,3 %).

An Evaluation of the Resilient Modulus and Permanent Deformation of Unbound Mixtures of Granular Materials and Rubber Particles from Scrap Tyres to be Used in Subballast Layers

Over the last years rubber from scrap tyres has been reused in different civil works such as road embankments and railway platforms due to its resilient properties, low degradation and vibration attenuation. Unfortunately, this issue is still scarce. For instance, in Spain about 175.000 tonnes of scrap tyres were collected in 2014, of which only 0.6% were reused in civil works. Aiming to contribute to the reutilisation of large quantities of this waste material, this paper focuses on the analysis of unbound mixtures of granular materials with different percentages of rubber particles to be used as subballast layers. Mixtures are tested under cyclic triaxial tests so as to obtain their resilient modulus and evaluate their permanent deformations. It is found that as the rubber content increases, the resilient modulus decreases and the permanent deformation increases. Taking into account the usual loads transmitted to the subballast layer, the optimum rubber content that does not compromise the behaviour of the mixture is set in a range between 2.5% and 5% in terms of weight.

Evaluating the Correlation Between the Geotechnical Index and the Electromagnetic Properties of Fouled Ballasted Track by a Full-Scale Laboratory Model

To determine the correlation between water content or fouling of a railroad track and ground-penetrating radar (GPR) signals, a full-scale railway track model was designed and constructed at the University of Massachusetts Amherst. A track section was simulated by creating a model consisting of rails and ties placed on a ballast layer on a subballast layer. The model was constructed three times with three fouling percentages. Each model was tested with water-content conditions of dry, field capacity, and two points in between these extremes. Frequency antennae of 450 MHz and 2 GHz were used to evaluate the conditions. The design and construction of the full-scale track in addition to the GPR data analysis and interpretation are presented in this paper. The results show that the dielectric permittivity and frequency spectrum can be used as an indicator of percentage of fouling and water content of a track. In addition, a linear correlation was observed between the percentage of fouling and the water content under field capacity conditions.

In situ impact testing of a light-rail ballasted track with tyre-derived aggregate subballast layer

In this study, the dynamic behaviour of ballasted track with and without tyre-derived aggregate (TDA) as subballast layer was assessed using impulse–response (IR) test. For this purpose, a prototype of a ballasted track was established and many IR tests were carried out. The obtained results were represented in three different kinds of velocity–time, velocity–distance and velocity–frequency curves. Processing of the obtained results revealed that TDA efficiency in terms of vibration reduction was in the range of 17–46 dB when the impact was applied to the ballast and it was in the ranges of 6–32 dB and 6–47 dB while impact locations were on the sleeper and rail, respectively. Overall, it was proven that TDA with the particle size of 5–50 mm and thickness of 200 mm reduced by 6–47 dB in vertical vibrations with the dominant frequency range of 32–63 Hz.

Performance of geosynthetic reinforcement on the ballasted railway track

Geosynthetics provide an important option to improve track support stabilization and thereby reduce the track maintenance costs and operation costs due to train delays. In railroad construction, geosynthetics may be installed within or beneath the ballast or subballast layers. In this present study, model tests were performed on model tracks laid at 1:3 scale to the prototype with adequate thickness of subballast layer and placed on soft subgrade soils. Model tracks were adequately instrumented to record induced stresses and displacements in the track. Model tracks were reinforced with geogrid or geotextile or both at suitable interfaces. Track condition after a heavy rainfall was simulated. In this present study, three dimensional finite element analyses of geosynthetic reinforced railway track sections have been carried out using MIDAS/GTS 2013 and compared with unreinforced sections. The result shows that the reinforcement can be used to improve the performance of railway tracks on clayey subgrade.

Characterisation of an unbound granular mixture with waste tyre rubber for subballast layers

Scrap tyres are a solid waste material produced in large quantities. One potential way of disposal is to use rubber particles from shredded tyres as a construction material. Within this context, this paper presents a comprehensive set of laboratory and field tests carried out to evaluate the characteristics of coarse aggregates mixed with rubber particles. The main objective is to assess whether these mixes could be used to form the subballast layer in new railway lines. All the technical features usually required for subballast were tested, including degradation, bearing capacity, density, resilient modulus, etc. The results show that adding between 1 and 10 % of rubber (in weight) improves resistance to degradation. On the other hand, bearing capacity is reduced, but still well over the usual range for common subballast if the rubber content is limited to <5 %. Moreover, the extension and compaction of these mixes can be done using conventional construction equipment.

Investigation of interlayer soil behaviour by field monitoring

As opposed to the new ballasted railway tracks where a sub-ballast layer is often emplaced, the conventional railway tracks were constructed with ballast directly emplaced on the natural sub-soils. Thereby, a layer of mixed materials, namely interlayer, was formed over time mainly by interpenetration between ballast and sub-soils. As this layer plays an important role in transmitting load to the sub-soils, its behaviour under the effects of dynamic loading and climate changes is of primary importance for the stability of tracks. In order to understand the behaviour of the material in such interlayers, field monitoring was performed at a selected site in France, namely Moulin Blanc. Firstly, site investigation was done by borehole, allowing the interlayer location to be identified. Secondly, the interlayer was instrumented with suction probes, temperature sensors and accelerometers at different depths. Two piezometers were also installed for water table monitoring, and a weather station was installed for air data monitoring. The data recorded allowed the suction changes with water evaporation and the behaviour of tracks under the effect of temperature to be analysed. Moreover, the recorded data of acceleration allowed assessing the double-integration method for different kinds of train. It was observed that the site is out of the hazards related to freeze/thaw and the double-integration method can be applied to determine the particle velocity and displacement provided that appropriate filters are used. Note however that further study is needed to confirm this point with comparison between the measured particle displacement and the calculated one.

Experimental Survey on Dry Asphalt Rubber Concrete for Sub-ballast Layers

This paper presents the results of an experimental survey on the potential application of DARC (dry asphalt rubber concrete) in rail superstructure, within sub-ballast layers by measuring its damping and mechanical properties. Based on the environmental friendly point of view the DARC has the significant advantage as the backfill material of sub-ballast layer because the rubber comes from the waste tires of truck and its usage can results a significant recycling of non-biodegradable wastes. After a preliminary mix-design of several DARCs, with different rubber content that confirmed by using the Marshall test, the stiffness modulus and damping ratio both of a standard bituminous mixture and of dry asphalt rubber concrete with a rubber content equal to 1.5% were determined using the four points bending device. The experimental results were compared and a numerical analysis by means of a 2D lumped mass model was developed in order to evaluate the different performance within the rail superstructure in terms both of the deflection and of the pressure on sub-grade. Both the results on the mechanical and dissipative properties of the DARC and the mechanical behavior of the correlate rail superstructure encourage the authors to continue the research on the application of such material for sub-ballast layers.

Vibration Induced by Rail Traffic: Evaluation of Attenuation Properties in a Bituminous Sub-ballast Layer

The attenuation properties of a Hot-Mix Asphalt track bed containing crumb rubber (particle size 2-5mm) are evaluated. A large number of vibration measurements have been carried out in a test section of a railway interconnection (Turin-Milan HS Railway) where two solutions of 12cm of sub-ballast layer have been realized (with and without rubber). The vibrations were measured using a set of piezoelectric accelerometers positioned at various distances from the track to measure their generation and propagation. The results show that the new hot mix asphalt layer can influence the train vibration transmission.

A Dynamic Model of Ballasted Rail Track with Bituminous Sub-Ballast Layer

The bituminous sub-ballast layer within the ballasted rail track allows some mechanical and environmental advantages. An analytical model of a ballasted rail track with sub-ballast layer has been proposed by considering the rail as viscous-elastic continuous beam discretely supported, at four levels of elasticity.The model was used to compare the mechanical performance of both ballasted track with and without bituminous sub-ballast layer. The results confirmed that the bituminous sub-ballast layer reduces the dynamic forces on the ground and achieves the technical objectives as reduction of ground borne vibrations and increase of the design life of the rail track.

Discrete element modelling of cyclic loads of geogrid-reinforced ballast under confined and unconfined conditions

The discrete element method has been used to simulate cyclic loading of geogrid-reinforced ballast under confined and unconfined conditions. For the confined condition, box tests have been simulated on unreinforced samples and reinforced samples with different geogrid positions and geogrid apertures. The response of the ballast layer reinforced with geogrid under repeated loading agrees with experimental results. It was found that the optimum location of geogrid is 100 mm depth from base, and the triaxial geogrid outperforms biaxial geogrid. For the unconfined condition, cyclic loading of a trough of ballast (the Composite Element Test (CET) devised by Brown et al. (2007)) has also been simulated, and the sample with the geogrid at 50 mm from the sub-ballast layer performs better than that at 100 mm or 150 mm from the sub-ballast. It was also found that the used of two geogrids at both 50 mm and 150 mm from the sub-ballast gave smaller settlement than using a single layer geogrid, or the unreinforced ballast. However the double-reinforced ballast performs only marginally better than the sample reinforced 50 mm above the sub-ballast. The geogrid reinforcement limits the lateral displacement in reinforced zone, which is approximately 50 mm above and below the geogrid.

Evaluation between Two Brazilian Railway Tracks

Most of Brazilian railways were built more than 100 years ago. Some of them were submitted to rebuilding processes while others were just overloaded by additional layers of ballast. Nowadays, Brazil is going through a new railway transport impulse in relation to the increase of load, despite of the necessary supply capacity. For this reason, there were developed evaluations from some Brazilian railways in order to determine their operational conditions. This work shows a comparative analysis of results from two parts of studied old railway, aiming to determine minimum features to enable them to accept higher load axis. One of these studied old railway parts did not have a sub-ballast layer in contrast to the other one. The strains and stresses of these old railway track parts were generated by the same locomotive.

Impact of Bituminous Subballast on Railroad Track Deformation Considering Atmospheric Actions

High-speed ballasted track design standards usually require the use of sand and gravel layers as subballast to fulfill an accurate protection of the formation layers, not only against traffic loads, but also against the effects of weather. Seasonal changes in soil water content, or suction changes, are responsible for cyclic volumetric strains on railroad trackbed layers, thus on the infrastructure. Being almost completely water-resistant when compared with granular-only layers, bituminous subballast offers a higher protection of the subgrade, consequently improving its behavior along the infrastructure life cycle. This question is investigated through the comparison of the performance of the track formation against atmospheric actions, taking into consideration the unsaturated state of the geomaterials. The method adopted consists of modeling the vertical displacements of both bituminous and granular subballast designs through a finite-element coupled thermo-hydro-mechanical (THM) analysis. The comparison of the two design solutions confirms that the adoption of a bituminous subballast layer might allow important reductions in seasonal vertical displacements.

New possibilities to reduce track costs on high-speed lines using a bituminous sub-ballast layer

The development of structural solutions for high-speed or very high-speed tracks that minimise total life-cycle costs of the system is a key issue to improve the operational profitability of new investments. In opposition to conventional ballasted tracks, slab track solutions can be a cost-effective solution, but only in the cases where the benefits due to the increase in track availability and the reduction of track maintenance offset its much higher construction costs. In the cases where such investment is not feasible, it is worth to evaluate possible structural improvements to ballasted track that allow reducing its maintenance needs without increasing too much its construction costs. This paper evaluates the design requirements and the impact of improving conventional high-speed ballasted tracks using a bituminous sub-ballast layer. It is divided into two main parts: first, the design requirements of the structural solutions with bituminous sub-ballast and its possible benefits on high-speed track deterioration and, second, the evaluation of the economic impact, in terms of construction costs, of using this structural solution material in future Spanish high-speed lines.

Stress changes in the ground below ballasted railway track during train passage

The design of railway track formations has traditionally been empirically rather than analytically based, with ballast and sub-ballast layer thicknesses specified mainly on the basis of previous practice. Recent design methods are more scientifically based, and for the most advanced design methods currently in use, input parameters are typically determined from cyclic triaxial testing. The changes in stress experienced by an element of soil below a railway track as a train passes are complex, involving (for example) a cyclic rotation of the principal stress directions. In these conditions, soil element testing in uniaxial compression may lead to the underestimation of vertical strains. Testing in a hollow cylinder apparatus, which can impose the rotations in principal stress direction likely to be experienced by a soil element in the field, may therefore be preferable to triaxial testing. However, there are as yet no data to guide the designer to a rational specification of a testing programme in this more complex apparatus. This article reports the results of finite element analyses carried out to investigate the stress changes experienced by an element of soil beneath a ballasted railway track during train passage. The effects of element location, the initial in situ stress state of the soil, and the elastic parameters used to characterize its behaviour are investigated, and the modelling of the stress paths in a cyclic hollow cylinder apparatus is discussed.

The EOS system: New perspectives for musculoskeletal biomechanics

Feedback on the use of a sub-ballast bituminous layer in a ballasted high-speed line (HSL) is evaluated in this paper. Conclusions are drawn on the relation between the thermo-viscoelastic behaviour of bituminous materials and the improvement in maintenance needs observed on the studied HSL. The case study is a 3 km long experimental zone of the French East-European high-speed line (EE HSL) built with a bituminous mixture sub-ballast layer. In service since 2007, the EE HSL test section has required fewer maintenance operations compared to adjacent sections with conventional structure. Thermo-viscoelastic properties of a bituminous mixture were characterized to understand and explain the contribution of a bituminous sub-ballast layer to the track behaviour. Improved 3D complex modulus tests were performed on laboratory-compacted cylindrical specimens using sinusoidal loading. A constitutive model, called 2S2P1D (2 Springs, 2 Parabolic, 1 Dashpot), developed in the LTDS/ENTPE Laboratory, University of Lyon, was used to simulate the linear viscoelastic (LVE) behaviour of the tested material. The results were analysed in the light of the working conditions and typical loadings of the EE HSL. The results of this study contribute to the definition of an optimal design method for ballasted HSL tracks with bituminous sub-ballast layers.