Development of a hybrid prediction method for railway induced vibration

The Detailed Vibration Assessment is an empirical procedure developed by the U.S. Federal Railroad Administration (FRA) for the prediction of railway induced vibrations and re-radiated noise. The vibration velocity level in the free field is predicted with a force density, characterizing the source, and a line transfer mobility, characterizing the transfer of vibrations due to a line load. The line transfer mobility is determined with in situ measurements of transfer functions. The force density is obtained by subtracting the line transfer mobility from the vibration velocity level due to a train passage. It is assumed that the resulting force density can be used to predict the vibration velocity level at other sites with similar train and track characteristics. In this paper, the force density is calculated numerically by simulating the FRA procedure with a numerical method. The force densities are obtained by subtracting the line transfer mobility from the vibration velocity level, computed at three sites with different soil characteristics. Next, each force density is used for a prediction of the vibration velocity level at another site. It is shown that the influence of the soil characteristics on the force density and the predicted vibration velocity level is relatively large. When a prediction is needed at a certain site, the force density should therefore be determined at a site with similar soil conditions. In case that no appropriate force density can be determined experimentally, the force density can be computed numerically by simulating the FRA procedure, as demonstrated in this paper. The numerical force density can be combined with an experimental line transfer mobility, resulting in a hybrid numerical–experimental prediction of the vibration velocity level.