Simulation of track settlement in railway turnouts-An iterative approach

Licentiate thesis, 2014

In this thesis, a methodology for simulation of track settlement in railway turnouts (switches and crossings, S&C) is presented. The methodology predicts the accumulated settlement for a given set of traffic loads using an iterative and cross-disciplinary procedure. The different modules of the procedure include (I) simulation of dynamic vehicle–track interaction in a turnout applying a validated software for multibody vehicle dynamics considering space-dependent track properties, (II) calculation of load distribution and sleeper–ballast contact pressure using a detailed finite element model of a turnout that includes all of the rails (stock rails, switch rails, closure rails, crossing nose, wing rails and check rails), rail pads, baseplates and sleepers on ballast, (III) prediction of track settlement for a given number of load cycles and (IV) calculation of accumulated track settlement at each sleeper and the resulting vertical track irregularity along the turnout which is used as input in the next step of the iteration. The iteration scheme is demonstrated by calculating track settlement at the crossing when the studied turnout is exposed to freight traffic in the facing move of the through route. Due to the design of the crossing panel and the character of the impact loads induced by the transfer of wheels between wing rail and crossing nose, the load transferred into the track bed is not uniform and the resulting spatially varying (differential) settlement leads to vertical irregularities in track geometry. In the first paper of this work, track settlement is predicted using a discrete empirical model that gives the local vertical settlement based on the number of load cycles and the prescribed distribution of sleeper–ballast contact pressure. Such a model does not account for the material behaviour in, or the interaction between, different regions of the track substructure. In the second paper, a computationally efficient method for predicting long-term settlement in generic configurations of track (various combinations of track superstructure and substructure and nonsymmetric loading conditions) using a constitutive model accounting for the multi-axial stressstrain condition in the track substructure is developed. A key ingredient of the method is that the selected constitutive model evaluates track settlement in the cycle domain. The constitutive model is adopted for both the ballast and the sub-ballast but with different parameter sets. The dimensions of the three-dimensional finite element model of the track substructure and the number of sleepers that need to be included to obtain accurate settlement at the crossing are investigated. To predict a reliable distribution of settlement under a given sleeper, it is concluded that the loading on at least two sleepers on either side of the sleeper must also be included in the model.