HIGH-SPEED RAILWAY LINES ON SOFT GROUND: DYNAMIC BEHAVIOUR AT CRITICAL TRAIN SPEED

Results from instrumented test runs with a high-speed train on a soft soil site in Sweden are presented. It is shown that large dynamic amplifications appear in the dynamic response of the rail/embankment/ground system as the train speed approaches an apparently critical value. The measured dynamic response is analyzed in detail, and it is shown that the critical speed is controlled by the minimum phase velocity of the first Rayleigh mode of the soil and embankment profile at the site. Moreover, it is shown that the critical speed and the amount of dynamic amplification also depend on a coincidence between characteristic wavelengths for the site and the distances between bogies and axles in the train. The displacement response is found to consist of a speed-independent portion in quasi-static equilibrium with the train loads and a dynamic portion representing freely propagating Rayleigh waves. An efficient computer code for the prediction of ground response to high-speed trains has been developed and its ability to reproduce the observed behaviour is demonstrated.