Keywords: wave propagation, mitigation of vibrations, two-and-a-half dimensional finite element-boundary element, railway traffic.

Environmental problems related to vibrations induced by railway traffic are becoming an important concern in recent years. Therefore, the nuisance for people who live or work in buildings near railway lines must be mitigated, demanding the invention and the development of efficient solutions in order to achieve that purpose.

The countermeasures against vibrations induced by traffic can be grouped in three distinct groups, depending of the location of installation: (i) in the source, (ii) along the wave propagation path and iii) in the receiver. The latter comprises the isolation of the buildings itself, without changing the pattern or the amplitude of the waves that propagate from the track to the receiver. On the other hand, it is also possible to proceed to the interruption of the path of the wave, i.e., preventing the waves from reaching the buildings.

Alternatively, countermeasures can be adopted at the source level, i.e., at the track. Generally, the objective is achieved by changing the dynamic behaviour of the track through the insertion of resilient elements. As a result of that, a low natural frequency in the railway track dynamic behaviour is induced, conditioned by the mass above the resilient element and by its stiffness (which is lower than that corresponding to the remaining elements), which should be as low as possible in order to reduce the energy transmitted from the track to the ground. In spite of the wide world existence of some applications of this countermeasure, there is a lack of research about this important topic. In this paper a parametric study is developed using a two and a half dimensonal finite element method - boundary element method approach for the simulation of the track-ground system. The rolling stock simulated by a multi-body approach [1]. Both models are integrated in a computational code developed in Matlab, which permits the train-track interaction of dynamic loads induced by the track unevenness to be taken into account. The parametric study is developed in order to better discern the influence of the mat's stiffness and its location at the cross-section of the track. Moreover, the influence of the introduction of the mats on the mechanisms of train-track interaction are also discussed.

From that parametric study it is found that the mass above the mat has an important influence on the efficiency of this countermeasure. For the same stiffness of mat, a higher efficiency can be achieved by the introduction of this element beneath the subballast layer instead of its installation beneath the ballast layer. Two main aspects contribute for that conclusion: (i) on the one hand, increasing the mass above the mat is favourable since it reduces the resonance frequency induced by the presence of the mat; (ii) one the other hand, the increase of the depth of the installation of the mat allows a better distribution of the load and, consequently, minimizes the displacements of the rail. Furthermore, the parametric study developed reveals that an advanced analysis of the efficiency ballast mats on the reduction of vibrations must include not only the typical analysis of mobility and of receptance but also the analysis of the influence of the ballast mats on the dynamic interaction mechanism between the rolling stock and the railway track.

1

P. Alves Costa, R. Calçada, A. Silva Cardoso, "Track-ground vibrations induced by railway traffic: In-situ measurements and validation of a 2.5D FEM-BEM model", Soil dynamics and Earthquake Engineering, 2011. doi:10.1016/j.soildyn.2011.09.002

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