Forests as natural metamaterial barriers for urban railway-induced vibration attenuation.

Noise and vibrations generated by railway traffic can seriously affect the adjacent buildings and their residents. Different mitigation methods have been proposed in the past decades to tackle this challenge. Despite many mitigation measures presented in the literature, some of these measures have shown limitations in their application, while for others their carbon footprint does not justify their implementation in real projects. This study introduces the concept of forests as natural metamaterials to attenuate the vibrations generated at the wheel-rail interaction. In particular, a group of natural metamaterials, in the form of a forest, is introduced into a vehicle/track/soil validated model based on the two-step approach. The ideal tree/soil unit-cell constituting the forest is obtained through a parametric investigation of the geometrical and material properties in order to have the first band-gap within the main range of frequencies generated by railway traffic in urban areas. The vibration attenuation levels obtained by the introduction of the natural metamaterial are then evaluated by considering a range of operational velocities for the T2000 Brussels tram LRV (Light Rail Vehicle). Finally, some insights on the attenuation efficiency of the selected forest towards vibrations generated by HSTs (High-Speed Trains) are given by considering a mono-wheel model with a higher range of vehicle speeds.

Smart Railway Traffic Monitoring Using Fiber Bragg Grating Strain Gauges.

There is today ample evidence that fiber Bragg gratings (FBGs) distributed along a railway track can provide robust axle counting and bring numerous assets compared to competing technologies in this practical environment. This work brings two relevant originalities with respect to the state-of-the-art solutions. First, a study of the strain distribution in the rail cross-section is performed to determine the sensitivity according to the charge and the position on the rail. Secondly, the technology is deployed along the rail track as a smart object where the sensor head is composed of four FBG wavelength-division-multiplexed in a single telecommunication-grade optical fiber and interrogated by a miniaturized read-out device. Two FBGs ensure the detection of the train direction and another two bring the required redundancy to reach a safety integrity level (SIL) 4. The read-out unit has been specifically developed for the application and contains a vertical-cavity surface-emitting laser (VCSEL) and a photodiode driven by a high-speed microprocessor unit that processes the data and communicates the useful information, i.e., the number of axles. On-field tests confirm that the proposed approach makes the installation process easier while it democratizes the technology.

Prediction and mitigation of train-induced vibrations of large-scale building constructed on subway tunnel.

Among all the recent improvements in the railway industry, ground vibration remains an important showstopper in metropolitan cities. In some particular cases, significant levels of vibration are felt by residents. The role of engineers is to propose mitigation solutions and to ensure that they are efficient in the long-term. This paper presents a numerical study of a large-scale building close to underground networks. A two-step time-frequency prediction method for train-induced vibrations of a superstructure is proposed in this work. In the first step, the spatial train-track coupled dynamic model in time domain is established and then simulated to obtain the vertical and lateral rail supporting forces (fastener forces). In the second step, the discrete Fourier Transform (DFT) of fastener forces are taken as the external loads of a finite element (FE) model of the track-tunnel-soil-building system to solve the building vibrations. On this basis, train-induced vibrations of the large-scale building are predicted under different train operation conditions, and two relevant standards are adopted to evaluate the building vibrations. Further, a base isolation measure, that consists in installing steel springs between the superstructure and the base, is employed to mitigate excessive building vibration. Results show that the underground train and track interaction could result in over-limit building vibrations. The train moving with a higher speed will deteriorate track vibration level and leads to more serious extent of over-limit vibrations of the larger-scale building. The base isolation measure can effectively reduce the excessive building vibrations, and also ensures the train-induced vibrations of the building to satisfy the relevant standard requirements under the worst train operation conditions.

Assessing the ground vibrations produced by a heavy vehicle traversing a traffic obstacle.

Despite advancements in alternative transport networks, road transport remains the dominant mode in many modern and developing countries. The ground-borne motions produced by the passage of a heavy vehicle over a geometric obstacle (e.g. speed hump, train tracks) pose a fundamental problem in transport annoyance in urban areas. In order to predict the ground vibrations generated by the passage of a heavy vehicle over a geometric obstacle, a two-step numerical model is developed. The first step involves simulating the dynamic loads generated by the heavy vehicle using a multibody approach, which includes the tyre-obstacle-ground interaction. The second step involves the simulation of the ground wave propagation using a three dimensional finite element model. The simulation is able to be decoupled due to the large difference in stiffness between the vehicle's tyres and the road. First, the two-step model is validated using an experimental case study available in the literature. A sensitivity analysis is then presented, examining the influence of various factors on the generated ground vibrations. Factors investigated include obstacle shape, obstacle dimensions, vehicle speed, and tyre stiffness. The developed model can be used as a tool in the early planning stages to predict the ground vibrations generated by the passage of a heavy vehicle over an obstacle in urban areas.

Improved analysis of ground vibrations produced by man-made sources.

Man-made sources of ground vibration must be carefully monitored in urban areas in order to ensure that structural damage and discomfort to residents is prevented or minimised. The research presented in this paper provides a comparative evaluation of various methods used to analyse a series of tri-axial ground vibration measurements generated by rail, road, and explosive blasting. The first part of the study is focused on comparing various techniques to estimate the dominant frequency, including time-frequency analysis. The comparative evaluation of the various methods to estimate the dominant frequency revealed that, depending on the method used, there can be significant variation in the estimates obtained. A new and improved analysis approach using the continuous wavelet transform was also presented, using the time-frequency distribution to estimate the localised dominant frequency and peak particle velocity. The technique can be used to accurately identify the level and frequency content of a ground vibration signal as it varies with time, and identify the number of times the threshold limits of damage are exceeded.

Railway cuttings and embankments: Experimental and numerical studies of ground vibration.

Railway track support conditions affect ground-borne vibration generation and propagation. Therefore this paper presents a combined experimental and numerical study into high speed rail vibrations for tracks on three types of support: a cutting, an embankment and an at grade section. Firstly, an experimental campaign is undertaken where vibrations and in-situ soil properties are measured at three Belgian rail sites. A finite element model is then developed to recreate the complex ground topology at each site. A validation is performed and it is found that although the at-grade and embankment cases show a correlation with the experimental results, the cutting case is more challenging to replicate. Despite this, each site is then analysed to determine the effect of earthworks profile on ground vibrations, with both the near and far fields being investigated. It is found that different earthwork profiles generate strongly differing ground-borne vibration characteristics, with the embankment profile generating lower vibration levels in comparison to the cutting and at-grade cases. Therefore it is concluded that it is important to consider earthwork profiles when undertaking vibration assessments.

The growth of railway ground vibration problems - A review.

Ground-borne noise and vibration from railway lines can cause human distress/annoyance, and also negatively affect real estate property values. Therefore this paper analyses a collection of technical ground-borne noise and vibration reports, detailing commercial vibration assessments undertaken at 1604 railway track sections, in 9 countries across the world. A wide range of rail projects are considered including light rail, tram lines, underground/tunnelled lines, freight, conventional rail and high speed rail. It documents the rise in ground-borne vibration problems and trends in the prediction industry, with the aim of informing the current research area. Firstly, the reports are analysed chronologically and it is found that railway vibration is a growing global concern, and as such, assessments have become more prevalent. International assessment metrics are benchmarked and it is found that velocity decibels (VdB), vibration dose value (VDV) and peak particle velocity (PPV) are the most commonly used methods of assessment. Furthermore, to predict vibration levels, the physical measurement of frequency transfer functions is preferential to numerical modelling. Results from the reports show that ground vibration limits are exceeded in 44% of assessments, and that ground-borne noise limits are exceeded in 31%. Moreover, mitigation measures were required on approximately 50% of projects, revealing that ground-borne noise and vibration is a widespread railroad engineering challenge. To solve these problems, the most commonly used abatement strategy is a modification of the railtrack structure (active mitigation), rather than the implementation of a more passive solution in the far-field.

Review of Trackside Monitoring Solutions: From Strain Gages to Optical Fibre Sensors.

A review of recent research on structural monitoring in railway industry is proposed in this paper, with a special focus on stress-based solutions. After a brief analysis of the mechanical behaviour of ballasted railway tracks, an overview of the most common monitoring techniques is presented. A special attention is paid on strain gages and accelerometers for which the accurate mounting position on the track is requisite. These types of solution are then compared to another modern approach based on the use of optical fibres. Besides, an in-depth discussion is made on the evolution of numerical models that investigate the interaction between railway vehicles and tracks. These models are used to validate experimental devices and to predict the best location(s) of the sensors. It is hoped that this review article will stimulate further research activities in this continuously expanding field.