Transition zone design for reduced track settlements ̶ Field measurements and numerical simulations

Doctoral thesis, 2025

In a transition zone between two different railway track forms, the discontinuity in structural design leads to a gradient in track stiffness. This gradient, combined with differences in loading and support conditions along the interface between superstructure and substructure on either side of the transition, often leads to differential settlement and irregularities in vertical track geometry shortly after construction due to ballast densification, subsoil compaction and consolidation. These irregularities amplify dynamic traffic loads, exacerbating long-term track degradation due to differential settlement along the transition zone.

The research in this thesis aims to enhance the understanding of vehicle–track–soil dynamic interactions and the mechanisms driving differential settlement in transition zones, ultimately supporting improved track design and management strategies. A comprehensive suite of numerical models has been developed, including both linear and nonlinear two-dimensional (2D) and three-dimensional (3D) finite element (FE) models of ballasted track, a 2D slab track model, a 2D vehicle model, and a coupled 3D FEM–discrete element method (DEM) model.

The model of short-term dynamic vehicle–track interaction in the time domain provides sleeper–ballast contact forces that are used in an empirical model of long-term settlement, which is used as input in the next simulation of dynamics, etc. Using this iterative approach, these models effectively simulate long-term differential settlement in the ballast and subgrade layers, capturing key phenomena such as sleeper voiding, rail seat load redistribution, and the evolution of vertical track geometry irregularities near transitions. In contrast, the integrated FEM–DEM approach directly computes dynamic responses and long-term differential settlements without relying on empirical settlement models.

Using this iterative methodology to analyse various scenarios, including transitions between ballasted track and 3MB slab track under heavy haul traffic, and transitions from ballasted track to slab track in tunnels, the analyses address stiffness gradients, variability in track support, rail level misalignments, and ballast particle rearrangements induced by dynamic loads. Further, transition zone designs, such as with the use of under sleeper pads, rail pads with different stiffness, transition wedges, wider sleeper bases, and reduced sleeper spacing, are evaluated.

In parallel, an advanced monitoring setup, employing fibre Bragg grating sensors, was developed and deployed for long-term, in-situ measurement of track bed degradation under harsh environmental conditions. The instrumentation included optical strain gauges, accelerometers, and displacement transducers, supported by geotechnical site investigations and total station measurements. This comprehensive set of field test data has been key for model verification and calibration.