The existing legal framework in most developed countries includes provisions on the vibration impact of general activities, and activities related to infrastructure use in particular. The legislation has been established not so much because vibrations can cause structural damage, although this is also a factor, as because of the nuisance to people. People are affected either by vibrations themselves or by the resulting noise produced by structures. However, this problem is not easy to address, and a considerable number of conflicts arise and are resolved – or not resolved – using experience-based knowledge rather than science.
Vibration amplitude curves calculated for the preliminary phase of the project
The vibrations caused by a train passing depend on a large number of variables including: the weight, speed and suspension of the rolling stock; the roughness of the wheel and the rail; the characteristics of the track (for example, whether or not it is ballasted and the way the rail is secured); the kind of ground and the presence of different strata; the foundations of the building and the variables that define its structure. It is a complex problem. As a result, there is currently no agreed method for calculating the vibration impact of an infrastructure. The focus of the assessment must also depend on the stage of work; one example is the L9 metro line in Barcelona. This suburban railway line project is over 40 km long, crosses different terrain, and, above all, runs below innumerable dwellings, all of which are different. Therefore, a preliminary study is needed to identify points at which vibrations may be above legal limits, using a limited dataset and a rapid, but not basic, calculation method. Once these points have been located they should be analysed in detail, using all of the available information, to assess whether the regulations are breached. If they are, appropriate corrective measures should be proposed.
3D model for the detailed study
The Acoustic and Mechanical Engineering Laboratory (LEAM), which is a member of CIT UPC, has provided a solution for the preliminary stage and for the in-depth study in the project stage. For preliminary studies, an analytical model has been proposed to carry out highly efficient calculations. However, this model is not basic in terms of its conception or results. It can be used to assess track construction solutions that are adapted to the traffic and the terrain, and helps to minimize the number of buildings that are affected or to select the ideal solution, taking into account the costs and the vibration impact. A preliminary version of this model has been applied to the preliminary study of the L9, in collaboration with the GREVTAM research group of the UPC’s Department of Mechanical Engineering. The result was a map of the metro line that pinpoints affected buildings and the level of vibration on undeveloped land, represented in the form of curves of equal vibration amplitudes. This result is particularly useful for new building sites, as infrastructure managers can pass on the information to property developers, who will then be responsible for meeting requirements for the interior of the new building, thus freeing the manager from any responsibility for a new construction.
For the in-depth studies, LEAM UPC provides a hybrid solution that combines an analytical model of interaction between the rolling stock and the superstructure, and a numerical model that uses finite elements (finite element method, FEM) to analyse propagation in terrain and structures. Depending on the type of problem and its geometry, the FEM model could be the typical 3D model, with the resulting consumption of resources and costs, or the more innovative 2.5D model, which considerably reduces use of resources and costs, without affecting the accuracy of the results. The 2.5D model was used recently to study a corrective measure proposed by a supplier to an infrastructure manager to resolve a vibration problem in the area around a railroad switch. The supplier guaranteed that its solution would reduce the vibration in the surrounding environment by 7 dB, whilst the LEAM study found that it would cause an increase of 1 dB. In the end, the solution was installed and experiments verified that the level of vibration had increased by… 1 dB.
Jordi Romeu
Director of Laboratory of Acoustics and Mechanical Engineering (LEAM UPC)
CIT UPC Member