Deformation behavior of railway embankment materials under repeated loading

This is a literature survey of the topic “Granular materials under repeated loading” for a research project that deals with railway embankments width and slope. The survey was done in the Laboratory of Foundation and Earth Structures at Tampere University of Technology by MSc Fabrizio Brecciaroli under the supervision of prof. Pauli Kolisoja. The survey is the continuation of Brecciaroli’s earlier research, the results of which were reported in the Publications of Finnish Rail Administration A 8/2004 (“Railway embankments with critical stability, preliminary study”) and A 9/2004 (“Railway embankments width and slope, preliminary study”). The research project is going to continue with laboratory experiments, field measurements, and modelling of railway structures that supplement the study. When the project ends the results will be reported in the form of a doctoral thesis. Railway embankments are made of unbound granular materials. An unbound granular material consists of a conglomeration, often inhomogeneous and non-isotropic, of a large number of individual, solid, and macroscopic particles in contact. One consequence of having a granular nature is that these materials, when untreated, have no inherent strength as a continuum and are unable to withstand any tension. On the other hand, they can support (small) shear stresses indefinitely. Placed in a layer and well compacted such materials possess the ability to carry traffic loads and distribute them onto the underlying layers or the sub grade. The deformation resistance of an unbound granular material depends on the applied stresses. The behaviour of unbound granular materials under compressive stresses is highly complex because of the existence of both resilient and permanent strains even at small levels of stress. The resilient strain response is important for the load-carrying ability of the embankment, while the permanent strain response characterizes the long-term deformation behaviour of the embankment. The state-of-the-art modelling of unbound granular materials requires the use of constitutive models for all the materials in the embankment and in the sub grade. The models are divided into continuum mechanics models and particulate mechanics models. The continuum mechanics approach does not consider the granular nature of the material. Instead, a fictitious element of granular material isolated from the surrounding soil mass is assumed to have a homogenous composition. The stresses in the fictitious element are assumed to be continuously distributed, whereas the forces acting at the contact points between the particles are not taken into account. In the particulate mechanics modelling, on the other end, the interactions between the distinct particles are explicitly investigated. The basic idea of the approach is that, when the number of particles is sufficiently large, the behaviour of the group of particles can be extended to describe the macroscopic behaviour of the actual material.