CDT-SIS-302: Loading of railway earthworks: Expressing a link between the applied loading due to railway traffic and the capacity of an earthwork in terms of the load models that are used for route classification in the INF TSI

Earthworks are required to resist the loading from railway traffic following requirements set out in the relevant standard (INF TSI - COMMISSION REGULATION (EU) No 1299/2014). This sets out the requirements for calculation of appropriate loads, and their consideration for new and existing infrastructure.

Railway traffic loading tends to be a little overlooked, as the critical design load cases for earth embankments and cuttings are normally the gravity loads associated with the self-weight of the soil, which have the most influence on the stability of the resultant slopes. However, for embankments, shallow failure of the top part of the embankment supporting the track is possible if critical shear stresses are allowed to develop on potential failure planes due to traffic loading, e.g. as observed by Li & Selig (see RSSB research project T679) and also at Mottingham on Network Rail’s network. Failures of the ground supporting the railway are akin to those that would occur for any structural foundation. At-grade situations have better lateral support to resist shallow failures of the ground supporting the track, and are therefore probably less critical than embankments. Railway traffic does not usually govern the design of earthworks at the Ultimate Limit State (ULS) (e.g. slope failure), however, Serviceability Limit State (SLS) failures, where differential settlements exceed predefined limits, can be problematic and trigger the need for maintenance of the track alignment. Section 4.2.8 of the Infrastructure TSI sets a limit for track deformation over a pre-defined length, in order to reduce the risk of derailment posed by short wavelength ‘isolated’ track geometry defects. In effect, this becomes a failure limit, which can be significantly influenced by the loading from railway traffic. In summary, railway earthworks need to have sufficient strength and stiffness to resist railway traffic, in addition to the gravity loads that are present, to avoid excessive settlement, particularly differential settlement along the track.

It is important to be able to predict when the failure limits for earthworks might be reached, taking account of planned changes in railway traffic operation, and then to be able to plan and prioritise any track maintenance that is necessary as a consequence. Research within project T679 examined the models needed for prediction of the earthwork response to load, but did not consider the influence of different traffic models to replicate the response to real traffic, and how different load models could be used to represent the traffic load carrying capacity in accordance with the INF TSI.

Proposals for research:

In order to be able to express a link between the applied loading due to railway traffic and the capacity of an earthwork in terms of the load models that are used for route classification in the INF TSI (e.g. EN15528 and GE/RT8006 in GB), the following research is proposed:

i. Analysis of data from Network Rail embankment(s) instrumented to consider traffic loading effects as part of a proposed follow-on from T679.
ii. Development of a suitable analytical or numerical modelling approach to incorporate the models of earthwork response to traffic loading identified in T679, and the results from the Network Rail field trial described above. The model will be validated using the initial observed field behaviour.
iii. Application of the load methodology developed in T679 to a range of embankment geometries (e.g. slope angle, height of embankment, distance from centreline of track to top of slope.
iv. Application of different load models (i.e. EN15528 and GE/RT8006) and selected real traffic patterns.
v. Consideration of variation in embankment material properties (e.g. from medium/high plasticity clays which are often found in existing embankments, to new embankments constructed with selected fill with optimised engineering properties).
vi. Consider the influence of different earthwork scenarios (e.g. at-grade and cutting situations).

As well as building on earlier work carried out by RSSB in this area, the project would link to the current EPSRC iSMART and Track to the Future projects.

This project is being run in participation with the EPSRC Centre for Doctoral Training in Sustainable Infrastructure Systems. For details of our 4 Year PhD programme and further projects, please see http://www.cdt-sis.soton.ac.uk/