Modelling ballast via a non-linear lattice to assess its compaction behaviour at railway transition zones

Locations in railway tracks where significant variations of the track properties occur are subject to increased track deterioration. To successfully mitigate this, the mechanisms leading to the increased deterioration need to be understood. To this end, this work presents a non-linear constitutive law for a lattice model able to describe the compaction behaviour of railway ballast. The parameters of the non-linear connections are tuned against lab experiments of cyclic loading tests and direct shear tests. The tuned lattice can be used with different foundation properties provided that the ballast in the track is equivalent to that of the tests. The non-linear lattice model is applied to the case of railway transitions, for which ballast compaction under train loading is studied as a cause of geometry degradation. It is observed that for the studied cases of a culvert crossing and of a ballast-slab transition, the operation-induced compaction converges monotonously to a stable situation, without leading to significant changes in the vehicle-track interaction. Ballast compaction is therefore insufficient as a stand-alone mechanism to explain a process of progressive degradation of the track geometry. Other mechanisms like autonomous differential settlement at the foundation level must be taken into account in such cases.