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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Propagation of Vibrations from a Railway Track Lying on a Semi-Infinite Soft Ground
B. Picoux+, G. Lefeuve-Mesgouez* and D. Le Houédec+
+Mechanics and Materials Laboratory, École Centrale de Nantes, France
B. Picoux, G. Lefeuve-Mesgouez, D. Le Houédec, "Propagation of Vibrations from a Railway Track Lying on a Semi-Infinite Soft Ground", in B.H.V. Topping, (Editor), "Proceedings of the Eighth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 115, 2001. doi:10.4203/ccp.73.115
Keywords: wave propagation, railway track, moving load, Fourier transform, soft soil, critical speed.
Near railroads, high displacements of the ground have been observed particularly in Sweden and France. Such vibrations can cause discomfort and damage on environment. The speed and the weight of the train have a great influence on this phenomenon: the speed of the Rayleigh wave appears to be a critical speed. The problem of critical speed has been studied by many authors and recent researches are directly concerned with the study of the track and the interaction between track and ground. Experimental validation projects of theoretical models are started but require the understanding of railway system.
In this paper we extend a semi-analytical wave propagation model introduced by Lefeuve-Mesgouez et. al.. This model is based on a calculation method of the dynamic stiffness matrix of the ground owing to Fourier transform complying with a good numerical efficiency and giving quick solutions. We also take into account the effect of a simple model of railroad like the model introduced by Sheng and Al.  especially conceived for the study of low vibration frequencies.
The train is represented by a discrete system of mass, stiffness and damping elements. Static and dynamic mechanisms contribute in different areas of frequency : quasi - static (3 - 15 Hz) and dynamic (30 - 60 Hz). The study of this complex system is not included in the framework of these works. Then the whole model is submitted to a set of harmonical moving load.
For the ground, we are first interested in a semi – infinite and homogeneous soil to study the analytical response of the ground in the wave number domain. Then, we consider the effect of a visco-elastic soft layer on this same soil.
The study of a simple two-dimensional model allows us to determine resonance frequencies of the system. A parametric analysis of the track components and the ground parameters is presented. Effects of intrinsic parameters modification of the track (strengthening of the platform by supplementary rail addition, reduction of the space between sleepers) are discussed. The presence of Mach cones can be found near the track for "super-Rayleigh" speeds and the model shows that, for these ranges of frequency, the track has for effect to increase the amplitude of vibrations and to distribute the sollicitation along the displacement axis of the load.
The main objective of this work consists in forecast simulation and experimentation methods in order to apprehend decently the phenomenon of wave propagation from vehicles (especially trains) moving at constant speed. Further works will include results of experimental measurements. The constitution of a significant basis of data and comparison with semi-analytical model will better characterise the response of the environment of tracks and appreciate the importance of the speed on the behaviour of structure.
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