The influence of the soil on wave propagation, track vibration, and on the evaluation of mitigation effects

Train-induced ground and building vibrations are sometimes annoying. Mitigating railway tracks have been developed with specific elements such as rail pads, under sleeper pads, under ballast mats, under ballast plates and troughs. There is a high-frequency reduction of the dynamic axle loads by the elastically supported mass of the track. There is also a low-frequency reduction, which stems from a wider load distribution and therefore softer axle impulses. The mitigation effect is often evaluated by comparing the ground vibrations of two nearby track sections which gives acceptable results if the soils are similar. The track and ground vibrations are calculated by wavenumber integration and by FEBEM. The low frequency wave propagation is influenced by the soil layering where the low frequency amplitudes and wave velocities are determined by the deep soil. The high frequency wave propagation is ruled by the upper soil layer and its material damping. Railway tracks are mainly influenced at low frequencies where the stiffness varies clearly with the soil. Layered soils give reduced damping and stiffness at certain frequency ranges and resonances of the track are possible for layered soils. Methods are shown for identifying the soil from wave velocities (SASW, MASW, frequency wavespeed transform, SPAC), wave fields or track vibrations. Finally, measurement examples show the low-frequency mitigation and the evaluation errors due to different soils at the isolating and non-isolating track which could be identified by the different methods.