Dynamics of railway transition zones in soft soils

Zuada Coelho, B. E. (2011). Dynamics of railway transition zones in soft soils. TU Delft, Delft University of Technology.

Transition zones in railway tracks are locations of discontinuity in the support, such as at bridges, culverts and tunnels. These zones are of main concern to railway inframanagers, since often substantial additional maintenance is required to preserve line, level and ride quality. This extra maintenance increases the exploitation costs, and often causes delays. Despite the importance of transition zones, the fundamental causes of their poor performance are not fully understood.
This thesis aims to give new insight in the behaviour of transition zones. The first step consists of the development of a monitoring plan of a real transition zone, located in The Netherlands, where the track on an embankment passes over a culvert. The underground consists of soft soil layers and sand. The measurements have two components. The static behaviour of the structure is monitored during a period of about ten months. The dynamic response of the track and surrounding soil is assessed during regular train services. The main observation from the static measurements is that the settlement of the track is composed of two different stages. Initially, after maintenance has been performed, a significant densification of the ballast occurs, followed by a second stage related to ongoing settlement of the embankment and peat layers. This causes a differential settlement of the track across the culvert, which has a stiff foundation. The dynamic behaviour shows that a transition in stiffness between the free track and the culvert, is not observed for the track, but only on the ballast bed. This result at the track is related to the fact that the sleepers above the approach slab are hanging free, due to the settlement before and after the culvert. Since the hanging gap is significant, the majority of the displacement exhibited by the track is due to this gap. At the same time, an increase in the axle load occurs within the hanging region, and a constant slapping of the free sleepers into the ballast occurs, which increases the degradation rate. The embankment is found to be moving as a rigid body on top of the soft soil layer. The embankment response is quasi–static,
and the attenuation of the displacement within the embankment is much smaller than the expected from literature or analytical solutions. The second step consists on the assessment of the behaviour by means of a numerical model, based on the finite element method. A fair agreement between the experimental and numerical results is found. The numerical analysis showed that the presence of the slab combined with the fact that the sleepers are hanging causes a stress  redistribution at the free end of the approach slab, that aggravates the long–term deformation of the soil, and increases the differential settlement. The observations were performed under normal train speed. The validated numerical analysis allows analysis of the behaviour at critical train speed, and special recommendations are suggested. The maintenance is found to play a key role on the long–term performance of the transition zone. The standard maintenance procedure aggravates the ongoing settlement of the ground, particularly in the proximity of the approach slab. Application of slabs to smooth the transition is not recommended any longer, and the maintenance procedure at transition zones should be revised.