The Rivas project (Railway Induced Vibration Abatement Solutions) is identifying and implementing mitigation methods to overcome this problem, as Lise Pesqueux, acoustics engineer at Alstom, and Estelle Bongini, project manager for acoustics and ground vibrations at French National Railways (SNCF), explain.
ANYONE who lives in the vicinity of a railway line will be all too aware of the impact of a passing train. Great strides have been made in recent times to reduce emissions and the unsightliness of railway infrastructure. However, the problem of excessive noise and vibration from railway vehicles remains.
With demand growing for housing in urban areas it is inevitable that more residential buildings will be developed close to railway lines. As a result research efforts are underway to identify the source of noise and vibration and to suggest infrastructure improvements that will mitigate its impact.
Coordinated by UIC the Rivas project, which is partially funded by the European Commission within its seventh Research and Development Framework Programme, and is supported by 26 partners including infrastructure managers, manufacturers, research institutions, and associations from nine countries, is one such initiative. The project is assessing the influence of track parameters on the generation of ground vibration, and based on this work will propose new and suggest optimised mitigation solutions.
Vibrations induced in the vicinity of track are caused by the interaction force created at the wheel-rail contact point when a train passes. This force causes track components to vibrate and transmit vibration energy into the soil which then propagates into the ground causing the foundations of the surrounding buildings to vibrate and potentially disturb residents.
By focusing exclusively on the track parameters (excluding the vehicle and the propagation path effects), two methods have been identified to reduce the energy transmitted to the sub-layers and the ground:
• reducing the excitation force at the contact point, and
• reducing the vibrations of the track components.
As far as railway infrastructure is concerned, the excitation force is driven by track defects and the track receptance (displacement for a unit load) at the contact point.
In Rivas, track defects are classified according to their impact on the generation of ground vibration. The project has assessed the effect of standard maintenance operations on these defects and the corresponding reduction in vibration.
In parallel, studies of track design have been carried out to identify means to control the track receptance at the contact point and track mobility in order to suggest ways to reduce the extent of vibrations transmitted to the sub-layers.
Contributions to railway-induced ground vibration are generated by both the quasi-static and dynamic components of vehicle excitation. The quasi-static excitation is determined by rolling stock characteristics such as the static component of axleloads and axle distances, and vehicle speed, while the dynamic excitation is induced by wheel/rail irregularities and irregularities in track support stiffness. It is the latter that dominates the vibration levels felt by the residents living close to a railway line.
The dynamic component of the vertical wheel-rail contact force is an important source of ground vibrations and ground-borne noise. This can be generated by irregularities in track geometry brought about by differences in longitudinal level, isolated defects, insulated joints, rail corrugation, switches and crossings; track stiffness caused by transition zones, hanging sleepers or culverts; and/or misshaped wheels. Each type of irregularity induces specific vibrations, depending on the type of vehicle, its speed, the soil conditions and a number of track dependent parameters.
The perceivable ground vibration has a frequency content ranging from a few Hz up to around 80Hz, while the ground-borne noise typically contains frequencies in the range of 30-250Hz.
At vehicle speed v, a periodic wheel/track irregularity with wavelength λ will generate a dynamic excitation at frequency f = v/λ, so when running at 80km/h, the irregularities between 0.09m and 5.6m have an influence on ground-borne noise and ground-borne vibrations, while when running at 300km/h, higher wavelengths are of interest, namely the range 0.3-21m. In addition impacts at rail joints or worn welds produce a high wheel-rail impact load with broad-band frequency content.
The longitudinal level corresponds to the standard deviation of vertical irregularities from the two rails identified by a track geometry recording car (Table 2). Initial vertical track irregularities may be induced by tolerances in the rail manufacturing process or by surface irregularities in the ballast bed after track construction or tamping takes place.
The longitudinal level may also deteriorate because of irregularities in track support stiffness such as hanging sleepers and transition zones. Differential settlement of the track embankment such as non-elastic deformations of the ballast and subgrade caused by repeated static and dynamic loads from passing trains may have an effect.
EU standards define a minimum track quality for safe operation of trains on high-speed and main lines. The wavelength range D1 corresponds to track irregularity wavelengths λ in the interval 3m < λ < 25m, and such wavelengths lead to low-frequency excitation of the train/track-system that may be important for ground vibration.
Welding of rails in the field may result in a cusp-like discontinuity. As is the case with rail joints, it is suggested that the discontinuities on each side are approximated by a quadratic function. Such discontinuities in the rail induce high-frequency vertical wheel-rail contact forces. Simulations carried out within Rivas show that for dipped rail, large contributions to the dynamic contact force in the frequency interval 50-100Hz are observed, and that the vibration level increases with an increasing dip depth.
The gap between rail joints is typically 4-20mm and the height difference (misalignment) between adjoining rails may be 0-2mm. In addition, on each side of the joint, there may be a dip in the rail. Such defects induce low-frequency vibrations, with wider gaps creating higher levels. The vertical irregularity for a loaded rail joint can be observed by measurement vehicles with a sampling distance of 5cm.
Track design optimisation
Many track defects have been presented above to show their impact on ground vibration generation. Once these defects are run by a wheel, the track design can also have an impact on track receptance at the contact point (the ratio between the excitation force produced at the top of the rail and the track displacement at this contact point) which partially drives the wheel-rail interaction force generation, and on track mobility as a whole so that less vibration is transmitted to the sub-layers.
The mitigation measures implemented on the track can therefore act either on interaction force generation, track mobility, or both, to reduce vibration transmitted to the sub-layers.
Rivas is going to test two mitigation measures for straight ballasted track. The graphs show the influence of the mitigation measures on different dynamic indicators of the track which have been derived from a numerical parametric study carried out within Rivas to optimise these mitigation solutions.
The first mitigation measure consists of installing soft or very soft pads under heavy sleepers. This system decouples the rail and sleepers in the upper part of the track from the ballast and sub-layers which now "capture" the energy from these upper layers while reducing mobility between the track and the free-field.
In this context, the heavy sleepers, with a larger contact area between the sleeper and the ballast, allow a reasonable rail deflexion at the contact point, even with very soft under-sleeper pads. Different combinations of soft or very soft under-sleeper pads and heavy sleepers are being tested on the Eiffage Rail Test Rig in Bochum, Germany, this spring.
The second mitigation measure consists of using a new rail fastening system that uses very soft railpads. In fact, a reduction of railpad stiffness implies an increase of the track receptance at the contact point. With a higher rail receptance, the resonance of the wheel-rail interaction force is shifted to the lower frequency ranges, reducing the interaction force for the frequencies above this resonance.
Rivas plans to test different rail fastening systems which use soft to very soft rail pads on a freight corridor in France. Effects on rail receptance, track mobility and ground vibrations generated by passing trains will be assessed during the trials which will place in August and September.
These different solutions will have an immediate impact on the ground vibrations generated by acting on the track mobility and/or on the wheel rail interaction force. Furthermore, they will also modify the interaction force distribution along the track which could also have an influence on the track defect generation and/or the track defect time evolution. In addition the results are likely to influence the track maintenance practices which the Rivas project is also studying.
After measuring the vibration reduction in the ground according to standard measurement procedures (ie at a distance of 8m from the track), Rivas' next step is to translate this mitigation effect to reduce vibration and ground borne noise exposure in buildings. This will be followed by a final study and evaluation of the corresponding reduction in residents' irritation. Appropriate procedures for these trials have been developed within Rivas and will be presented at the completion of the project at the end of this year.