A comparison of trackbed design methodologies: a case study from a heavy haul freight railway

One of the major roles of railway trackbed layers is to reduce vehicle induced stresses applied to the underlying subgrade to a level that limits the progressive build up of permanent deformation. The ability of trackbed layers to satisfy this requirement is dependent upon the materials used for construction and their thickness. Numerous design methods, (both empirical and analytical), have been developed across the World to evaluate trackbed design thickness. However, where there is limited information or experience of previous trackbed design with the specific materials or site conditions under consideration, the choice of methodology becomes one of engineering judgment, in assessing the significance and reliability of the design input parame-ters. This paper describes a number of design methods which were assessed in a recent project to design a new heavy haul freight railway trackbed, founded on moisture sensitive subgrades, using locally available materials for the track support layers. The produced design thicknesses for each of the methods are compared for differing subgrade conditions. The results show considerable variation of thicknesses from each method with little consistent pattern to the variation. Reasons for these variations are suggested and the choice of the final design used for specific subgrade conditions are presented together with appropriate justification. Concluding on these issues, recommendations are made for a more considered approach to trackbed design