According to Swedish guidelines for high speed railways on embankment, the total settlement is limited to 20 mm over a track length of 10 m during the construction service life. The main objective of this thesis was to investigate the deformation in the subgrade (unbound layer) in a slab track, since there are very few studies related to high speed railways on high earth structure, discussing particularly the unbound layer. This thesis examined the unbound layer consisting of granular material by using the discrete element method (DEM) software PFC. There was a focus on the material compaction and deformations due to traffic loading. DEM has the benefit to be able to model deformation with due consideration of processes at microscale level. Two different particle shapes were tested: balls and clumps. The results showed that the settlements were small, possibly associated to the well compacted material and the simplifications in the model, such as the shape of the particles, absence of particle breakage and the applied traffic load. The clump simulations resulted in less settlements and permanent strains compared to the ball simulations. The higher the embankment the more settlements but less strains were produced for all the three simulations. One interesting parameter to study for the balls simulation was the friction between the particles. Increased friction contributed to less settlement. The maximum height of the embankment was limited to around 3,2 m due to time restraints. Simulations for higher embankments are needed to be performed in order to better understand the effect of the embankment height on settlements.

The previous E4 Råneå road embankment was prone to flooding. Risk of flooding in combination with settlements of the road due to the weak underlying sulphide soil was problematic. The Swedish Transportation Administration improved the length section E4 Råneå by rebuilding the road using the method light embankment piling. The centre-to-centre pile distance was chosen to 1.1 m and embankment height 1.8 m. The embankment is reinforced with geosynthetic reinforcements resting on timber piles, which were installed on till stratum. Two geosynthetics were installed, with their strength properties in opposite direction from each other. Their purpose was to stiffen the soil and reduce loading on the weak sulphide subsoil. Field monitoring equipment were placed in the road to measure the behavior before and after consolidation. To validify the results, used instruments in this thesis concerns: pressure cells, extensometers, piezometers and a hydrostatic profile gauge. The Swedish Transport administration wants to evaluate if an increase in piling distance is possible. From the conventional practice of maximum 1.2 m to 1.4 m. It is also interesting if the increased pile distance holds for a taller embankment of 2.5 m. Answering this would aid in increasing the cost-effectiveness of light embankment piling. The performed investigation has been done in the finite element analysis program Plaxis 3D 2021 by simulating half of an embankment with supplementary load model.  To capture field behavior, PLAXIS SoilTest has been used to calibrate the compressive material parameters obtained in oedometer testing. The geosynthetics have been modeled with regards to creep and their stiffness increase on surrounding soil due to interlocking of soil particles. Guaranteeing the reliability of the numerical analysis was made by a comparison of the base model to field monitoring equipment before conducting the parametric study. The base numerical model was reliable in capturing the result of field monitoring equipment. Deviations in pile loads was observed beneath the light trafficked road lane. Conducting the parametric study, the results indicated an increase in pile head loading, total settlements, differential settlements, and deformations in the geosynthetic reinforcement when pile distance and embankment height increased. With a taller embankment of 2.5 m and increased pile distance of 1.4 m numerical simulated pile head loads were in sizes of the design pile strength. Tensile stress in the geosynthetic reinforcement was below long-term design strength. The ratio pile efficacy, that is how effective the structure is at reducing sub soil load has been evaluated in the parametric study at three unit cells. A logarithmic growth is observed when reducing the pile distance at the middle of the road with consistent behavior between embankment height. When studying cells beneath the heavy trafficked lane a linear relationship could be seen instead. This study suggests it is possible to perform the increase in pile distance of 1.4 m for the current embankment height 1.8 m, but needs to be investigated further for the 2.5 m high embankment.