Keywords: finite element modelling, subgrade improvements, bearing capacity, geotextile, geogrid.

This paper demonstrates the possibility of a combination of finite element modelling and laboratory testing to investigate the reinforcing of the geogrids used for optimal track design. Deformation characteristics of different designs of the subgrade system with or without the reinforcing geogrid together with characterization of the bearing capacity of these designs are the main objectives of the study. A methodology for the evaluation of innovative subgrade retrofitting solutions based on numerical and physical modelling is drawn from the results. An experimental model is used to study the influence of ballast bed construction on the bearing capacity.

According to the experimental setup, two sets of finite element (FE) models were prepared: (i) a bearing capacity test using the circular loading plate (two-dimensional and three-dimensional) and (ii) models with the concrete sleeper loaded by an axle load of 22.5 t (three-dimensional only). Together with these two sets of FE models, axisymmetric FE model representing the in-situ bearing capacity test using the circular loading plate is used. The effects of proper contact modelling between the individual layers is thoroughly investigated. A wide range of boundary conditions, including cases where no reinforcement is considered, was tested while important parameters, such as the geotextile length, thickness of individual layers and frictional model, were varied. Results from the parametric study were analysed to establish the qualitative relationship between the individual superstructure design and the resulting increase of bearing capacity. The first group of FE models are plain strain models. The second group comprises three-dimensional FE models.

The initial stress condition is established first by applying the gravity acceleration with the geogrid reinforcement in place. The loading of the circular plate was set according to the standard bearing capacity test from which the foundation modulus is determined from the unloading curve. The loading of the model with the instrumented sleeper is provided by point load set according to a given axle load (22.5 t to 30 t). In the first group of FE models without the reinforcing geosynthetics used, general design recommendations are drawn in form of design nomograms, where for different types of ballast bed construction and different materials used, minimal thickness of individual layers can be determined. The nomograms are constructed for different elastic modulae of the construction material used. In the second group of improvement methods, geogrid parameters for maximum reinforcing effects are suggested on the basis of the laboratory tests and numerical modelling. The second group of FE models was used to evaluate the effect of geogrid inclusion on the increase of bearing capacity. The results indicate that the bearing capacity can be significantly increased by the inclusion of layers of geogrid in the soil-ballast interface, and that the magnitude of bearing capacity increase depends significantly on the type of geogrid used. More importantly, it has been found, that proper interaction between the geogrid and the ballast is more significant for the reinforcing effect then the material properties of the geogrid itself. On the basis of the results of the laboratory model tests and the finite element analyses, critical values of the geogrid parameters for maximum reinforcing effect are suggested.

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