Analyse numérique de la réponse dynamique de structures ferroviaires.Application à la réduction des désordres géométriques induits dans les couches de ballast des Lignes à Grande Vitesse.

摘要

The repeating train passages on High Speed Lines leads to progressive geometrical disorders of the ballast layers. These disorders are observed to increase in severity as a function of the train speed.Former studies have shown that a possible cause for these disorders was the development of high vertical accelerations in the ballast layer induced by High Speed Trains. These high accelerations may indeed disrupt the granular assembly. The objective of the present work is twofold: evaluate acceleration as a function of speed and of the railway track parameters and search for construction solutions that make it possible to mitigate the magnitude of acceleration.To accomplish this, a semi-analytical tool dedicated to the computation of the reversible dynamic response of railway structures was developed based on continuum mechanics (ViscoRail) and compared to in situ measurements. The time required to compute the acceleration fields is small using this tool. Therefore this tool is utilized in the sensitivity analysis of the response with respect to speed of the moving loads and to the numerous other railway track parameters. The negative effect of speed on acceleration is proved. This work also shows that an increase of the Young modulus of the sub-ballast layer and of the moment of inertia of the rails may be considered to decrease the accelerations computed at high speed to an “acceptable” level. A decrease of the rail-ballast contact stiffness may also be considered for this purpose. These results lead to the proposal of construction solutions able to reduce acceleration and thus possibly the geometrical disorders. An example of solution incorporating a bituminous sub-ballast layer is analyzed taking advantage of the modeling capability of ViscoRail to account for viscoelastic constitutive laws such as the Huet-Sayegh model.Triaxial tests and 2D discrete element simulations of granular specimens resting on flexible pads of different stiffness also showed that the reversible behavior, at the macroscopic scale, of such specimens could reasonably be modeled by a continuum elastic law (anisotropic). This law is nonetheless non intrinsic since its parameters depend on the flexible medium on which rest the specimens. These analyses validate a posteriori the continuum approach considered in ViscoRail.Finally, the detrimental effect of vertical downward acceleration on the stability of ballast layers is revisited based on Limit Analysis. However, this approach also suggests considering the change in the load distribution due to changes of the railway construction parameters. A more global conclusion on the efficiency of the proposed solutions would have to be done combining ViscoRail and Limit Analysis.