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Critical train speeds and associated track movements – a case study

As train speeds are increased, ballasted railway tracks that have previously performed acceptably may experience large movements as a result of what are commonly termed critical velocity effects. These occur when the train speed approaches the speed of surface (Rayleigh) waves in the underlying ground, and can lead to increased rates of track geometry degradation, poor ride quality and in-creased maintenance costs. Critical velocity effects are also a potential concern for new high-speed lines. An improved understanding of the causes of the ground and track movements, through field instrumentation and modelling, will help to identify potentially problematic locations and to develop more cost-effective remediation methods. This paper presents the results of a study of the ability of a semi-analytical model (TGV) to calculate realistic ground movements at train speeds approaching the critical velocity. Several ground geometries and loading cases are considered, and a site on the classic railway network is used as a case study for validation purposes. The track at the study site experienced large displacements following an increase in line speed from 160 km/h to 200 km/h. Geotechnical investigations showed that the site is underlain by a horizon of peat of low stiffness. By refining the parameters used in the model, assuming that the peat horizon was the primary cause of the large track movements at high speed, it was possible to obtain reasonably close agreement between site measurements and the model in both quasi-static and dynamic analyses.