ABSTRACT
With the introduction of high speed trains in Sweden, high vibration levels were encountered in low embankments founded on soft, cohesive soils. In these cases, it appeared that the maximum amplification occurred when the train speed exceeded a critical value. A series of field tests were therefore carried out at Ledsgård, a site in south western Sweden near Gothenburg, in order to investigate the problem. In these tests, an X2000 high speed train was run at speeds ranging from 10 km/h to 200 km/hr, and a substantial increase of the peak values of the particle displacement and velocity was observed at train speeds of 140-160 km/hr. The shear-wave velocities varied between 40m/s to 60m/s in the upper layers of the soil, which means that the tests at higher train speeds were performed with super seismic speeds in relation to the shear wave velocity of the soil. This paper presents theoretical considerations on ground motions in the track environment when a train runs at sub and super seismic speeds. Mathematical expressions are presented along with a conceptual model that illustrates the behaviour. The phenomenon of the "two sources" for super seismic train speeds is explained and displacement and velocity time histories from the model are discussed for various speeds. Recordings from the field measurements are also presented and compared with the results of the conceptual model.
