Modelling and measurements of critical train speed effects and associated track movements

As train speeds are increased, there is a greater likelihood of encountering critical velocity effects at sites with soft soil. These can lead to increased rates of track geometry degradation, poor ride quality and increased maintenance costs. These phenomena occur when the train speed approaches that of the Rayleigh waves of the underlying ground. A study of critical velocity effects is presented, based on a semi-analytical model of the vibration of the vehicle/track/ground system. Results from this model are compared with measured track deflections obtained at a site on a classic UK mainline at which large track displacements were observed following an increase in line speed from 160 km/h to 200 km/h. Geotechnical investigations have shown that the site is underlain by a layer of peat of low stiffness. By refining the parameters used in the model, assuming that the peat 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 results from the model in terms of maximum deflections and frequency spectra of rail displacements. As the water table has been observed to be quite high at the site, an additional investigation is presented into the effect of the water level on the critical speed effects. Three levels of saturation are considered for the peat layer, ranging from fully saturated to relatively dry. The results suggest that a reduction in the water levels at the site is unlikely to lead to a reduction in the critical velocity effects