Assessing the risk of critical velocity effects at railway sites using site investigation and advanced laboratory testing

As train speeds increase on existing rail networks, and new high-speed routes are constructed, the likelihood of a line experiencing the large track displacements commonly termed critical velocity effects increases. The phenomenon occurs as the train speed approaches the speed of surface (Rayleigh) waves in the underlying ground. At a certain proportion of this speed the track deflections begin to increase above those for static loading, slowly at first and then more rapidly, reaching a maximum at a “critical” velocity. Larger trackbed deflections may cause increased rates of track deterioration, maintenance needs and in the worst cases risks to safety. Therefore, it is important to be able to predict which sites are susceptible to critical velocity effects and to determine threshold speeds below which they are not influential. Where these thresholds are exceeded mitigation measures can be considered. Reliable prediction of critical velocity effects using numerical modelling and other analytical tools require selection of a representative ground model and parameters. This paper describes a programme of site measurements, sampling and advanced laboratory testing to create a ground model for a site on the UK railway network known to experience critical velocity effects.