HeliRail: A railway-tube transportation system concept

Helirail is an energy efficient mass transit transportation system concept, which combines developments in low-pressure tube transport with existing high-speed railway infrastructure. It addresses the problem that, currently at low speeds, steel wheel railways are an energy efficient transport mode, however at high speeds, >80% of energy is used overcoming drag. This means minimising these resistances presents a high-impact opportunity for reducing railway energy consumption. To reduce resistance, HeliRail consists of an airtight tube-track structure that allows existing steel-wheel trains to travel on existing railway corridors where slab-track is suitable, with minimal drag. The running environment is low-density heliox gas, held inside lightweight tubes, slightly below atmospheric pressure to minimise species transport. HeliRail captures this energy saving as an operational reduction, thus improving the energy efficiency of high speed rail by 60%. On a high capacity route, annually this could save enough energy to power 140,000 homes. Deploying Helirail on an existing line does not increase train cruising speeds, however a secondary benefit is journey time reduction, achieved using a small part of the energy saving for improved train acceleration. Unlike previous evacuated tube transportation embodiments, the system is interoperable with traditional rail lines/trains meaning vehicles can pass through HeliRail sections and onto traditional steel-rail networks. This also reduces land-purchase requirements. Further benefits include improved safety compared to vacuum transportation and fewer service disruptions compared to rail. Capital cost is low compared to a new rail or pressurised transportation line, and is recovered after a period competitive with renewable energy technologies

The effect of railway local irregularities on ground vibration

The environmental effects of ground-borne vibrations generated due to localised railway defects is a growing concern in urban areas. Frequency domain modelling approaches are well suited for predicting vibration levels on standard railway lines due to track periodicity. However, when considering individual, non-periodic, localised defects (e.g. a rail joint), frequency domain modelling becomes challenging. Therefore in this study, a previously validated, time domain, three-dimensional ground vibration prediction model is modified to analyse such defects. A range of different local (discontinuous) rail and wheel irregularity are mathematically modelled, including: rail joints, switches, crossings and wheel flats. Each is investigated using a sensitivity analysis, where defect size and vehicle speed is varied. To quantify the effect on railroad ground-borne vibration levels, a variety of exposure–response relationships are analysed, including: peak particle velocity, maximum weighted time-averaged velocity and weighted decibel velocity. It is shown that local irregularities cause a significant increase in vibration in comparison to a smooth track, and that the vibrations can propagate to greater distances from the line. Furthermore, the results show that step-down joints generate the highest levels of vibration, whereas wheel flats generate much lower levels. It is also found that defect size influences vibration levels, and larger defects cause greater vibration. Lastly, it is shown that for different defect types, train speed effects are complex, and may cause either an increase or decrease in vibration levels

Modelling the Environmental Effects of Railway Vibrations from Different Types of Rolling Stock: A Numerical Study

This paper analyses the influence of rolling stock dynamics on ground-borne vibration levels. Four vehicle types (Thalys, German ICE, Eurostar, and Belgian freight trains) are investigated using a multibody approach. First, a numerical model is constructed using a flexible track on which the vehicles traverse at constant speed. A two-step approach is used to simulate ground wave propagation which is analysed at various distances from the track. This approach offers a new insight because the train and track are fully coupled. Therefore rail unevenness or other irregularity on the rail/wheel surface can be accurately modelled. Vehicle speed is analysed and the frequency spectrums of track and soil responses are also assessed to investigate different excitation mechanisms, such as carriage periodicities. To efficiently quantify train effects, a new (normalised) metric, defined as the ratio between the peak particle velocity and the nominal axle load, is introduced for a comparison of dynamic excitation. It is concluded that rolling stock dynamics have a significant influence on the free field vibrations at low frequencies, whereas high frequencies are dominated by the presence of track unevenness

Settlement behaviour of hybrid asphalt-ballast railway tracks

The use of structural asphalt layers inside ballasted railway tracks is attractive because it can increase track bending stiffness. Therefore, for the first time, this paper investigates the long-term settlement characteristics of asphaltic track in the presence of a subgrade stiffness transition zone. Phased load cyclic compression laboratory tests are performed on a large-scale hybrid asphalt-ballast track, supported by subgrade with varying stiffness. It is found that an asphaltic layer acts as a bridge to shield the subgrade from high stresses. It is also found that the asphalt reduces track settlement, and is particularly effective when subgrade stiffness is low

Geodynamics of very high speed transport systems

This work reveals the existence of a new dynamic load amplification mechanism due to ground surface loads. It is caused by the interaction between a moving vehicle's axle configuration and the vibration characteristics of the underlying soil-guideway system. It is more dominant than the traditionally considered ‘critical velocity’ dynamic amplification mechanism of the guideway-ground structure, and is of relevance to very high speed transport systems such as high speed rail. To demonstrate the new amplification mechanism, first a numerical model is developed, capable of simulating ground-wave propagation in the presence of a series of discrete high speed loads moving on a soil-guideway system. The model couples analytical equations for the transportation system guideway with the thin-layer element method for ground simulation. As a practical example, it is validated using high speed railroad field data and then used to analyse the response of a generalised single moving load at high speed. Next the effect of multiple discrete vehicle-guideway contact points is studied and it is shown that dynamic amplification is highly sensitive to load spacing when the speed is greater than the critical velocity. In particular, large resonant effects occur when the axle/magnet loading frequency and the propagating wave vibration frequency of the soil-guideway structure are equivalent. As an example, it is shown that for an individual case, although critical velocity might increase displacements by 50–100%, for the same scenario, axle configuration can increase displacements by 400%. It is also shown that resonance is sensitive to the total number of loading points and the individual frequencies excited by various spacings. The findings are important for current (e.g. high speed railway) and potential future (e.g. hyperloop) transport systems required to operate at speeds either close-to, or greater than the critical velocity of their supporting guideway-soil structure. In such situations, it is important to design the vehicle and supporting structure(s) as a combined system, rather than in isolation

A 2.5D time-frequency domain model for railway induced soil-building vibration due to railway defects

A new hybrid time-frequency modelling methodology is proposed to simulate the generation of railway vibration caused by singular defects (e.g. joints, switches, crossings), and its propagation through the track, soil and into nearby buildings. To create the full source-to-received model, first the force density due to wheel-rail-defect interaction is calculated using a time domain finite element vehicle-track-soil model. Next, the frequency domain track-soil transfer function is calculated using a 2.5D boundary/finite element approach and coupled with the force densities to recover the free-field response. Finally, the soil-structure interaction of buildings close to the line is computed using a time domain approach. The effect of defect type, train speed and building type (4-storey office block and 8-storey apartment building) on a variety of commonly used international vibration metrics (one-third octaves, PPV, MTVV) is then investigated. It is found that train speed doesn't correlate with building vibration and different defect types have a complex relationship with vibration levels both in the ground and buildings. The 8-storey apartment building has a frequency response dominated by a narrow frequency range, whereas the modal contribution of the 4-storey office building is over a wider frequency band. This results in the 8-storey building having a higher response

Study of railway track stiffness modification by polyurethane reinforcement of the ballast

This paper presents the measured results of full-scale testing of railway track under laboratory conditions to examine the effect on the track stiffness when the ballast is reinforced using a urethane cross-linked polymer (polyurethane). The tests are performed in the GRAFT I (Geopavement and Railways Accelerated Fatigue Testing) facility and show that the track stiffness can be significantly enhanced by application of the polymer. The track stiffness is measured at various stages during cyclic loading and compared to the formation stiffness, which is determined prior to testing using plate load tests. The results indicate that the track stiffness increased by approximately 40–50% based on the measured results and from the previously published GRAFT I settlement model. The track stiffness was monitored during loading for a maximum of 500,000 load cycles. The paper concludes by presenting and commenting on, the application of the technique to a real site where the Falling Weight Deflectometer was used before and after polymer treatment to determine the dynamic sleeper support stiffness. The very challenging site conditions are highlighted, in particular the water logged nature of the site, and comment made on the effect of the water on polymer installation. The results of the FWD measurements indicate that a good increase in overall track stiffness was measured. These results are consistent with the laboratory tests which are performed on a different soil and use a different measurement technique and hence confirm that regardless of the soil and measurement system track stiffness increases are observed using this technique

Non-linear soil behaviour on high speed rail lines

This paper gives new insights into non-linear subgrade behaviour on high speed railway track dynamics. First, a novel semi-analytical model is developed which allows for soil stiffness and damping to dynamically change as a function of strain. The model uses analytical expressions for the railroad track, coupled to a thin-layer element formulation for the ground. Material non-linearity is accounted for using a ‘linear equivalent’ approach which iteratively updates the soil material properties. It is validated using published datasets and in-situ field data. Four case studies are used to investigate non-linear behaviour, each with contrasting subgrade characteristics. Considering an 18 tonne axle load, the critical velocity is significantly lower than the linear case, and rail deflections are up to 30% higher. Furthermore, at speeds close-to, but below the non-linear critical velocity, dynamic amplification is highly sensitive to small increases in train speed. These findings are dependent upon soil material properties, and are important for railway track-earthwork designers because often 70% of the linear critical velocity is used as a design limit. This work shows that designs close to this limit may be still at risk of high dynamic effects, particularly if line speed is increased in the future

Assessment of railway ground vibration in urban area using in-situ transfer mobilities and simulated vehicle-track interaction

This article proposes an alternative approach to the well-known Federal Railroad Administration method to evaluate ground vibrations induced by the passing of railway vehicles. The originality lies on the excitation mechanisms that occur in urban areas. A common source of railway-induced ground vibrations is local defects (rail joints, switches, and turnouts) which cause large amplitude excitations at isolated locations along the track. To analyse such situations, a combined numerical-experimental study is developed, based on the use of numerical train/track results and experimental mobility transfer functions. The influence of building foundation type, vehicle, defect type, and size and location is evaluated through experimental data collected in Brussels (Belgium). The results show that it is possible to assess vibrations from light rapid transit systems in the presence of local rail defects and unknown soil conditions

Rail trackbed and performance testing of stabilised sub-ballast in normal and high-speed environments

The ability of geogrids to preserve track alignment within a high-speed rail environment at close to critical velocity is somewhat uncertain; testing in a controlled environment can be problematic. This paper presents the results from a new ‘true triaxial’ test apparatus that overcomes some of these problems. In ‘normal’-speed rail environments, geogrids have been used for many years to stabilise and enhance the performance of sub-ballast to maintain both vertical and horizontal alignment and increase the interval between maintenance events. This has been reflected in controlled testing conducted in both the laboratory, in the field and under heavy loading. To look at this issue for high-speed rail and to make comparisons between track alignment preservation in normal and high-speed environments, a new ‘true triaxial’ test apparatus (GeoTT) has been developed at Heriot Watt University that can subject railway sub-ballast to forces in all 6 directions, mimicking the principle stress rotation that has been implicated in track alignment deterioration subjected to high speed train traffic. The use of this apparatus, where the rams are programmed using force-time histories developed from 3D finite element models, allows sub-ballast performance to be evaluated for the fraction of the time and cost that would be necessary for full scale testing. A comparison is made between existing testing results from ‘normal-speed’ testing and the new high speed simulations that indicate the continued potential for geogrids to continue to aid track performance in much more critical environments