© Daniel Clements

High speed rail: analysis of track embankment behaviour

Proper engineering design of high-speed rail (HSR) embankments is a key component for effective construction and operation of high-speed trains (HST) and minimizing  maintenance costs. The  operational criteria required for the HSTs at their maximum travel speed dictates very small tolerance for settlement and deformations of HSR track roadbed and embankments. Achieving this necessitates stiff embankment fills, use of ground improvement of any soft soils present under embankments or at-grade sections, and consideration of long-term heave or settlement in at-grade and cut sections.

One of the main challenges in development of HST projects is to determine the track embankment design criteria to meet the acceptable deformation limits. This study aimed at analyzing the behaviour of HSTs over embankments with problematic soil conditions such as soft and/or expansive soils. The objective was to evaluate the deformations under dynamic train loads and the required stiffness (deformation modulus) for track embankments and subgrade to meet the maximum acceptable displacements. This study provided better understanding of various analytical methods for design of trackbeds for HSRs, particularly the assumptions and limitations. It also revealed that these methods are conservative in comparison with embankment behaviour simulated using a finite element model.

Existing analytical design methods for rail embankments have been shown to produce variable results in terms of the required design thickness for the top granular layer. For most of such methods, the results obtained are highly dependent on how the dynamic effects of train loading are taken into account.

The finite element (FE) analyses used in this study confirmed that the existing analytical design methods are conservative: with similar design parameters, the FE-calculated induced deformation is less than the threshold limits considered in existing design methods. The hysteretic constitutive soil model implemented in LS-DYNA successfully captured the nonlinearity in the soil behaviour under dynamic loading, relative to conventional elasto-plastic models (Mohr-Coulomb).

Results of this study contribute to Arup’s work in high-speed rail projects, including the HSR projects in Texas (Dallas to Houston) and California (Los Angeles to San Francisco).

The developed LS-DYNA model can be applied by Arup engineers for analysis of similar track embankment for other rail projects.  The proposed advanced soil constitutive model incorporated into the LS-DYNA model can be used by geotechnical engineers in rail applications to capture the dynamic loading effects in the subgrade soil.

The results of this study support Arup engineers in enhancing the design of high-speed rail embankments, including selecting the most suitable method for trackbed design, selecting embankment material properties to achieve the performance requirements, as well as developing a finite element (FE) model to analyse the track embankment response under cyclic loading.