الفهرس 
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Abstract
This research presented new solution form of railway trackbed is bituminous sub-ballast. This kind of railway trackbed has been used in many countries and, in some cases, can be a costeffective solution. This solution is normally used in combination with traditional granular layers in various configurations. Effectively, from the point of view of trackbed design, the use of a bituminous sub-ballast may improve the geometric performance of the railway infrastructure and contribute to an effective reduction of track maintenance needs.
To develop this study the work was conducted by designing asphalt mixture using the Marshal method in the laboratory and using the ANSYS software to evaluate the best thickness of both the ballast and asphalt layer to minimize vertical deformations and corresponding stresses.. Usual assumptions in such models were adopted, which are described below: - Concerning the material properties: for soil and granular layers, Drucker-Prager elasto-plastic constitutive laws were used, while both elastic and visco-elastic model were used for the
bituminous subballast. An elastic railpad stiffness of 100 Kn/mm was considered. - Concerning model geometry: the track model geometry considers 11 sleepers, a width of 9 m
considering the base of landfill and 3 m of subgrade depth with QS3 quality is adopted. Usual symmetries on vertical plans are assumed as usual boundary conditions. - Concerning track load: an equivalent dynamic axle load was obtained by considering
Eissenmann’s criterium for current high-speed vehicles. - Concerning finite elements type: 20-nodes hexahedral elements (SOLID95).
- Concerning contact elements: contact interface were considered for the interaction between sleeper and ballast, so as to substitute rigid nodes between these two surfaces of contact: double nodes were considered and displacements on the surface plan were allowed while restricted on the perpendicular direction. The results obtained performed for track show that the railway track cross-section was
provided with a bituminous sub-ballast layer laid under the ballast. The main objectives of using this layer were to increacing the bearing capacity of the subgrade layer, reduce the
vertical deformations of the trackbed, thus the structural behavior of the cross-section will be enhanced, and the maintenance can be minimized. Also, nonlinear analyses have been carried out to get the minimum thickness of the bituminous layer, which causes minimum vertical
deformation and stresses, especially under sleepers by using the ANSYS program. Experimental work was conducted on five samples of asphalt mixtures to obtain the most
suitable parameters to be used in the simulation process. Based on the conducted analysis and discussions. This theoretical analysis proved that a track that consists of 250 mm ballast and 120 mm bituminous su sub-ballast layers enhances the structural behavior of the high-speed
ballasted tracks.