Modeling of the Effect of Embankment Dimensions on the Mechanical Behavior of Railway Track

In the Northern areas the total thickness of structural layers in railway embankments is primarily governed by the design against harmful effect of seasonal frost. Because practically no frost heave can be allowed to take place on railway tracks with normal speed passenger traffic, the embankment must typically be built up to two or even two and a half meters thick. Meantime, the embankments have typically fairly steep slopes, for instance in Finland track embankments are normally built using a slope ratio of 1:1.5. Introduction of higher allowable axle loads and traffic speeds is, however, exposing the embankments structures to continuously increasing intensity of repeated loading which is also increasing the rate of permanent deformations accumulating into the embankment structure. In practical terms the embankment is widening as it deforms and the respective movements of the track must be compensated by more frequent maintenance actions. The most straightforward measures to increase the internal stability of a railway embankment are to make the embankment wider and/or to reduce the slope steepness of the embankment. Both of these actions mean, however, larger space requirement for the railway track and, above all, extensive increase in the use of high-quality non-frost-susceptible aggregate materials in connection with the embankment construction or renovation. Therefore, taking into account both the construction time costs on one hand and the maintenance costs of the track on the other hand, optimisation of the embankment dimensions and shape is an important issue regarding the life cycle costs of a railway line. In a research project going on at the Laboratory of Earth and Foundation Structures of the Tampere University of Technology the above mentioned problem is being studied by in-situ monitoring of a full scale railway track embankment having sections that are shaped in different embankment widths and slope angles. The long term deformations of the embankment have been monitored for about three years in addition to which also the short term responses of the embankment structure have been measured while trains are passing over the monitoring sections. In addition, model scale (1:4) test structures with different embankment widths and slope angles have been tested in laboratory using a loading system consisting of five hydraulic actuators operating consecutively so as to simulate the loading effect of a moving train. The results obtained so far indicate clearly that it is not only the embankment width and slope angle that are decisive concerning the permanent deformation behaviour of the railway embankment, but also the subgrade stiffness plays an important role in the overall performance of the embankment structure.