Railway formation design by elasto-plastic FEM mechanistic-empirical approach

Abstract

A design method for conventional railway track formations to limit deformations in fine-grained subgrades is proposed. It is based on previous performance-based design methods with the important distinction that two-dimensional mechanisms are considered. Advanced hardening plasticity constitutive models are used for the subgrade to predict permanent deformation under a single loading directly, rather than by elastic analysis and approximate empirical transfer functions, before using empirical relationships to predict permanent deformation accumulation of the subgrade under successive loadings. The approach was validated by back-analysis of a laboratory test and full-scale trial. A parametric study identified a strong correlation between mobilised subgrade shear strength and permanent strain and between mobilised track bearing capacity and permanent subgrade deformation – correlations that could streamline design significantly. A strain rather than stress-based criterion for subgrade failure was found to be more appropriate since it takes account of the contribution of ballast strength at resisting permanent strain in the subgrade. The benefits of geogrids in granular layers on subgrade strains can be directly assessed using this method. Simple, approximate equations to determine permanent subgrade strain and deformation are derived that could be used for rapid non-FEM based optioneering prior to more rigorous, time consuming, analysis being performed.