Under-ballast Mats Research Articles

Ageing tests with high temperatures of prototype vibroacoustic isolators – under ballast mats (UBMs) based on recycled materials

This study focuses on experimental ageing tests with high temperatures carried out on prototype under ballast mats (UBMs) according to the procedure of the European standard EN 17282, which was introduced in late 2020. Most of the analysed UBM samples are based on recycled elastomeric materials, such as styrene-butadiene rubber (SBR) granules and fibres from end-of-life car tyres and rebond polyurethane waste from polyurethane product manufacturing plants. Additionally, two variations of the mat based on mineral wool are analysed. Significant differences in the values of static and dynamic characteristics measured before and after the ageing tests are demonstrated, and permanent deformations of the recycled polyurethane-based mats are shown. Moreover, it is proved that the elastomeric mats based on recycled rubber materials (SBR granules and fibres) have high durability and are an effective solution in terms of protection against vibration and structure-borne noise from railway traffic. In addition, the authors propose criteria for assessing the results of the ageing test with high temperatures of UBMs tested according to the procedure of EN 17282.

Fatigue characteristics of rubber isolation layers used in high speed railway ballastless tracks: A theoretical and experimental study

In order to enhance operational conditions for high-speed trains on long-span bridges, the utilization of rubber under-ballast mats were taken as a novel isolation layer in ballastless tracks. Due to the increased train axle load, it is imperative to conduct investigation into the fatigue properties of the rubber isolation layer, so as to ensure the smoothness of the railways. In this work, a series of fatigue tests were performed on the rubber isolation layer to obtain the influence of stress amplitude and loading cycles on its fatigue performance. The results demonstrate a pronounced non-linear load–displacement relationship of the rubber isolation layer with increasing load, while the fatigue endurance significantly decreases with escalating the stress amplitude. Moreover, considering the hyperelastic properties of rubber material, the Mooney-Rivlin constitutive model was employed in finite element simulations. Subsequently, the stress distributions of the rubber isolation layer were analyzed, and the fatigue life was evaluated based on Luo stress method. The rubber isolation layer exhibits localized damage that does not affect the overall performance after 32 years of normal service condition. Therefore, the fatigue performance of the rubber isolation layer remains reliability, providing assurance for the application in ballastless tracks on high-speed railway long-span bridges.

Mitigation of Railway-Induced Ground Vibration by Soft Support Elements and a Higher Bending Stiffness of the Track

The mitigation of train-induced ground vibrations by track solutions is investigated by calculations and measurements. The calculation by a wavenumber domain method includes the correct vehicle–track interaction and the correct track–soil interaction. Some theoretical results for elastic elements and an increased bending stiffness of the track are presented where the force transfer of the track and the vehicle–track interaction are calculated for the high-frequency dynamic mitigation, and the force distribution along the track is calculated for the low-frequency mitigation which is due to the smoother impulses from the passing static loads. Measurement results for the ground vibration near isolated and un-isolated tracks are given for several under-sleeper pads, for under-ballast mats, and for several under-ballast plates and ballast troughs. The elastic elements yield a resonance frequency of the vehicle–track–soil system and a high-frequency reduction of the dynamic axle loads which depends mainly on the softness of the pads or mats and which can be improved by a higher sleeper mass. In addition, all troughs and most of the soft elements show a low-frequency reduction which is attributed to the scattered impulses of the static axle loads. Besides this main contribution of the article, the problem of a soft reference section on a different soil is discussed and recommendations for better ground vibration measurements of mitigation effects are given.

Modeling Railway Track Mechanical Behavior with Under Tie Pads and Under Ballast Mats

Under tie pads (UTPs) and under ballast mats (UBMs) are used in rail track construction to reduce track stiffness and increase the size of the ballast contact area where it meets track support elements either above the ballast (ties) or below the ballast (e.g., subgrade, bridge deck, or tunnel invert). Limited information on the in-track performance of UTPs or UBMs is available. The University of Florida recently completed a research program to monitor track performance and track support changes using a UBM and UTPs in the Virginia Avenue Tunnel in Washington, DC. Researchers measured parameters to estimate the effect of a UBM and UTPs on tunnel invert pressure, tie bottom pressure, and track deflection. A control condition without a UBM or UTPs was not available for testing; however, Volpe developed a finite element track model for estimating track behavior without a UBM or UTPs. The modeling results highlighted the individual contributions of both a UBM and UTPs to stress reduction on the ties and tunnel floor. The model included features that simulate the load chains that can develop in clean granular materials leading to high peak stresses and ballast and structure degradation. The modeling results revealed that a UBM and UTPs might reduce the average stress on the ties and tunnel floor by 5% to 20% when the ballast was well compacted. The peak stresses from ballast load chains, or edges, may be reduced by 50% or more.

Investigation of the impact of environmental conditions on the vibration mitigation performance of typical commercial under ballast mat materials

The performance of under ballast mats (UBMs) in mitigating vibrations caused by regular rail traffic has been the subject of numerous studies. However, most of these studies have focused on the impact of design and material properties on the vibration attenuation characteristics of UBMs, with little attention paid to the influence of environmental factors such as temperature, freeze/thaw cycling, and weathering. In this paper, we present the results of an investigation into the effect of environmental conditions on the vibration mitigation performance of typical commercial UBM materials including rubber and closed cell polyurethane elastomer. We conducted controlled laboratory experiments in which we exposed UBM samples to a range of simulated environmental conditions. The results of these experiments were compared to the results of tests conducted under standard laboratory conditions to evaluate the impact of the environmental conditions on the vibration attenuation performance of the UBMs. The results of this investigation have important implications for the design and selection of UBMs for use in railway systems, particularly in regions with extreme environmental conditions.

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).

Modeling the Influence of Under-Sleeper Pads on the Vibration Emissions from Railway Traffic

One of the aims of the European research project FINE-2 is to contribute to reducing noise and vibrations caused by railway traffic. In this context, the development of models describing the influence of relevant parameters on the excitation of train-induced vibration emissions were investigated in WP 8 "Ground Vibration" which is led by DB Systemtechnik. Here, resilient track components and especially under-sleeper pads were considered by a new modelling approach. Dependent on the dynamic stiffness of the relevant superstructure components, the model is based on the German standard DIN 45673-4, which describes an approach to compute the insertion loss of under-ballast mats by using an impedance model. The paper will show a general description of the model, the steps taken to adapt the existing model to consider under-sleeper pads instead of under-ballast mats, as well as results validated by using existing measurements.

GRIDMAT—A Sustainable Material Combining Mat and Geogrid Concept for Ballasted Railways

Under ballast mats (UBM) have demonstrated their capacity to reduce section stiffness and ballast degradation. However, UBM can cause ballast destabilisation under some circumstances due to excessive vertical track deflections, requiring the installation of geogrids over the mat which increases costs and time. As alternative to this solution, this paper shows the design of GridMat: a sustainable technology for ballasted railways that combines the concepts of geogrids and under ballast mats (UBM) manufactured from recycled crumb rubber. This aims to provide damping capacity while limiting the oscillations and settlement of ballast layer. To obtain the optimal GridMat design, five different configurations varying the aperture size and void areas were assessed through laboratory box tests reproducing the track section including the GridMat. Results showed that the optimal Gridmat was of 55 mm aperture seize and 25% void area. To evaluate the sustainability of this design, the expected number of conservation and renewal operations were calculated from full-scale laboratory tests and a life-cycle assessment and life-cycle cost analysis were undertaken. GridMat showed long-term reduction in ballast degradation and track settlement, reducing need for maintenance and renewal operations in comparison with standard mats.

Ballast loading plate design and mechanical behavior of under ballast mats

Geometry of loading plate which simulates the ballast contacting interaction is the critical factor for mechanical properties and durability test of the geotextile for railway track. In this study, a ballast indentation test with 3D laser scanning method was performed to study the actual contact characteristics of the bottom of the ballast. Based on this, the existing problems of geometric-ballast-plate (GBP) of the EN standard were presented. Therefore, a new ballast loading plate, Actual Ballast Plate (ABP), was designed and verified. The mechanical behavior of under ballast mats (UBMs) with different stiffnesses were studied by applying different ballast loading plates (the ABP, GBP, and flat plate). Results demonstrated that for the GBP, the geometric characteristics of sharp corners at the bottom of the ballast were largely overlooked, resulting in smaller contact stress in the fatigue test. The ABP resulted in a better simulation of the contact characteristics and load transfer relationship of the ballast. The stiffness test with the flat plate (FP) and GBP resulted in a higher static modulus for the hard UBM because of the overestimation of the contact area between the UBM and the ballast. It is recommended that the EN standards adopt the ABP to replace the GBP for the fatigue tests of the UBMs.

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.

Analysis of the Influence of Fatigue Strength of Prototype Under Ballast Mats (UBMs) on the Effectiveness of Protection against Vibration Caused by Railway Traffic.

The present paper focuses on laboratory tests of fatigue strength of prototype under ballast mats (UBMs), carried out according to the procedure described in the new European standard EN 17282, which was released in October 2020. The mineral wool-based mat revealed significant differences in the values of static and dynamic characteristics, measured before and after the fatigue tests. The elastomeric mats based on recycled materials (SBR granulate and fibers) turned out to have had sufficient durability and effectiveness of protection against vibration caused by railway traffic. The values of static and dynamic bedding moduli, determined before and after the fatigue tests, were used to develop a viscoelastic rheological model of the vibration isolator with the use of fractional derivatives. Using this original model of the ballasted track system with four degrees of freedom, a significant influence of cyclic loading on the level of vibration suppression (insertion loss factor) was demonstrated. The analytical model confirmed that the mats with variations of bedding moduli exceeding 10% should not be used as resilient elements.

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.

A transferable method for estimating the economic impacts of track interventions: Application to ground-borne noise reduction measures for whole sections of route

The environmental impacts of noise and vibration are becoming increasingly important in the assessment of new and upgraded railway routes. Vibration from railways propagates through the ground to nearby buildings where it may cause annoyance as feelable vibration or as re-radiated noise. To tackle the adverse effects of ground-borne noise a range of possible interventions are available, including softer rail pads, under-sleeper pads and under-ballast mats. The installation costs of such interventions are generally higher for the higher-performing track types. Additionally, there are potential effects on track maintenance costs which may be positive or negative, for example by reducing sleeper damage or increasing the need for ballast tamping. This study presents a socio-economic analysis of the effects of several interventions to reduce ground-borne noise. By selecting a whole route, the installation and whole-life costs are assessed using Network Rail’s Vehicle-Track Interaction Strategic Model (VTISM) and these are offset against benefits in terms of reduced social costs. Ground-borne noise is predicted at various distances from the alignment using the Modelling of Train Induced Vibration (MOTIV) model and the effect of track interventions is determined as insertion loss spectra. The re-radiated noise within a typical domestic building is then estimated using generic building transfer functions. Geographic Information System tools are used to estimate the population affected by ground-borne noise and their expected level of exposure. The methodology is illustrated using a case study route between Brighton and Portsmouth in the South of England.

Laboratory examination of ballast deformation and degradation under impact loads with synthetic inclusions

This paper presents a laboratory study on alleviating the deformation and degradation (breakage) of ballast subjected to impact loads using geogrids and rubber mats. A series of drop hammer impact tests are carried out to determine how well the geogrid, under-ballast mat (UBM) or under-sleeper pad (USP) can attenuate impact loads and mitigate ballast degradation. Geogrids to be placed at different locations in a ballast assembly, in combination either with a UBM or a USP are tested. Laboratory test results prove that the inclusion of rubber mats and geogrids decrease the dynamic impact loads transferred to the ballast aggregates and subsequently decrease the degradation (breakage) and deformation of ballast. The tensile strength of geogrids and subgrade stiffness are found to considerably influence the performance of geogrid-reinforced ballast under impact loading conditions. The measured impact forces and accelerations of ballast with and without an artificial inclusion show that rubber mats definitely reduce track vibration (acceleration) and the subsequent deformation and breakage of ballast. These inclusions will not only increase safety and passenger comfort they will also lead to more economical and efficient track designs.

Advances in ground improvement using waste materials for transportation infrastructure

Recycling waste materials for transport infrastructure such as coal wash (CW), steel furnace slag (SFS), fly ash (FA) and recycled tyre products is an efficient way of minimising the stockpiles of waste materials while offering significant economic and environmental benefits, as well as improving the stability and longevity of infrastructure foundations. This paper presents some of the most recent state-of-the-art studies undertaken at the University of Wollongong, Australia on the use of waste materials such as (a) CW-based granular mixtures (i.e. SFS + CW, CW + FA) for port reclamation and road base/subbase and (b) using recycled tyre products (i.e. rubber crumbs, tyre cell, under-sleeper pads and under-ballast mats) to increase track stability and reduce ballast degradation. Typical methods of applying these waste materials for different infrastructure conditions are described and the results of comprehensive laboratory and field tests are presented and discussed.

Stabilisation of Stiffer Rail Track Substructure Using Artificial Inclusion

The railway transport system in many countries plays a significant role in the passage of bulk freight and passengers. However, increased train speeds and higher freight loads (large dynamic wheel loads) accelerate the deterioration of rail track substructure. This problem is more critical in isolated rail track locations where the track substructure is much stiffer than the regular surface track assembly such as track at the bridges and tunnels. Ballast is a key track foundation material placed underneath the sleepers which provides structural support against high cyclic and impact stresses caused by moving trains. Inclusion of rubber mats called under ballast mats (UBMs) placed between the ballast and stiffer base layer is one of the measures to minimise the ballast deterioration. In this study, cyclic loads representing fast and heavy haul trains were simulated on stiffer track foundation condition using a large-scale process simulation prismoidal triaxial apparatus to investigate the mitigation of strain, stress and degradation characteristics of ballast stabilised with UBM. These UBMs were locally manufactured from recycled tyre wastes. The results show that ballast on a stiff foundation substructure stabilised with UBM experienced significantly less vertical and lateral deformation, ballast interface and inter-particle stresses and degradation. This study also confirmed that the recycled tyre UBMs used in this study had adequate damping to absorb the energy transmitted to the moving train to the track, thus preventing excessive plastic deformation and degradation of the ballast layer.

Properties of rubber under-ballast mat used as ballastless track isolation layer in high-speed railway

Rubber under-ballast mats (UBM) commonly used in metro traffic in China are applied in high-speed railway ballastless tracks as the isolation layer to improve the operating conditions of high-speed trains. Due to the different operating conditions between the high-speed railway and the metro traffic, the rubber UBM, used as the isolation layer in ballastless tracks, needs to be investigated furtherly. In this paper, a finite element analysis of the China Railway Track System (CRTS) III ballastless track was initially carried out, and the stress state and distribution of the rubber UBM used as the isolation layer were obtained at the service condition. Furthermore, the static, dynamic and fatigue tests of the rubber UBM samples were designed and carried out based on the analysis results. The main conclusions are as follows: under normal service conditions, the stress level of the rubber UBM is low, and it can be considered that the rubber UBM is still in the stage of linear elasticity. The material properties of the rubber UBM can be characterized by the equivalent elastic modulus, which has been determined to be 0.6 MPa by the experimental research. Furthermore, the dynamic characteristic of the rubber isolation layer exhibits a hysteresis phenomenon. The equivalent dynamic stiffness is larger than the equivalent static stiffness due to the viscoelastic characteristics of rubber material. The rubber UBM has large internal friction, high energy absorption rate and good damping performance characterized by its large loss factor. In addition, following the application of 3,000,000 repeated load, which is 1.5 times of a train axle load, the equivalent static stiffness only changes by 2%, indicating that the rubber UBM used as an isolation layer can maintain good mechanical performance under repeated loading. As such, the appropriate selection of rubber UBMs for the isolation layer in ballastless tracks can be made following the analysis and investigation of its performance characteristics.

Analysis of the Vibration Mitigation Characteristics of the Ballasted Ladder Track with Elastic Elements

Despite the fact railways are seen as an environmentally friendly and sustainable form of transport, however, the train-induced vibration has been seen as a negative environmental consequence. The ballasted ladder track is one type of ballasted track with longitudinal sleepers. The elastic elements can not only protect the track structure but also control the vibration. To investigate the vibration mitigation effects of ballasted ladder track with elastic elements, a finite element - infinite element (FE-IFE) model was built considering the elastic elements of under-sleeper pads (USPs) and under-ballast mats (UBMs). This model was validated by a laboratory test. Then, the moving train load was obtained based on the multi-body dynamics (MBD)-finite elements method (FEM) analysis. The vibration mitigation effects of the ballasted ladder track with different types of elastic elements were calculated compared with the ballasted tracks without elastic elements. The results indicate that: (1) the ballasted ladder track has the advantage of vibration reduction at low frequencies, with a maximum vibration attenuation of 25.2 dB and an averaged vibration attenuation of 19.0 dB between 5 and 20 Hz through the ballast. (2) The ballasted ladder track with USPs or UBMs can provide better vibration attenuation between 30 and 100 Hz, but it induces a vibration amplification between 5 and 30 Hz. (3) The ballasted ladder track with elastic elements in different cases can provide different vibration mitigation effects. The ballasted ladder track with both USPs and UBMs can provide the best mitigation effect with an average vibration mitigation of approximately 15 dB and a maximum vibration mitigation of 30 dB between 30 and 100 Hz.

Laboratory fatigue performance of under-ballast mats under varying loads and support conditions

Primarily being employed in ballasted bridge decks and tunnels, under-ballast mat applications have seen growth in the North American market as a solution to reduce the stress state of the ballast ...

Mechanical modelling and application of vibroacoustic isolators in railway tracks

The paper presents systematization and description of vibroacoustic isolators used in railway tracks (due to track structure type), with special attention paid to resilient mats. As in the second part of the paper the state-space mechanical model of a system with Under-Ballast Mat is formulated. Also some numerical problems arising from the mass matrix singularity are discussed. The poles of the system were calculated by using Matlab. Moreover, the influence of various parameters on the system’s insertion loss and its transmissibility was visualized in figures.