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Experimental and numerical determination of transfer functions along railway tracks

Ground borne vibration in buildings due to railway traffic is a major concern in densely built up areas. In practice, railway induced vibration is often predicted by means of empirical methods such as the Detailed Vibration Assessment developed by the U.S. Federal Railroad Administration (FRA). The vibration velocity level in the free field is predicted based on a separate characterization of the source and the wave propagation. They are characterized by a force density and a line transfer mobility, respectively. The line transfer mobility represents the energy transmitted through the soil due to a line source, such as a train on a track. The force density represents the power per unit length introduced into the soil by the source. While the force density is determined indirectly by subtracting the line transfer mobility from the vibration velocity due to a train passage, the line transfer mobility is determined directly based on wave propagation tests. In this paper, the determination of the line transfer mobility is investigated experimentally and numerically. The line transfer mobility depends on the dynamic track and soil characteristics, but is also influenced by the experimental setup used during the measurement. The location of the impact points, on the track or the soil, as well as the number of impact points and their spacing have an influence on the determination of the line transfer mobility. First, the line transfer mobility is investigated experimentally. Results are obtained from a measurement campaign where transfer functions from the track to the free field have been measured for several impact points. Second, a numerical prediction model is used to simulate the transfer functions from the track to the free field and predict the line transfer mobility numerically.