Performance of ballast mats on passenger railroads: Measurement vs. projections

Ballast mats have been installed on urban railway systems throughout the world to provide isolation of ground-borne vibrations from trains. In general, the performance has been found to be satisfactory. However, often there is a variance between the claims of the suppliers of ballast mats and the actual performance of the product in the real world. The classic case involves an infinite terminal impedance applicable to a tunnel configuration. However, a ballast mat installation outdoors on surface track with sub-grade slabs may not have the same performance as a tunnel base where sides are stiffened by walls. In order to represent this situation, Kimura developed a simplified prediction procedure based on an original Wettschureck/Kurze model, with a finite termination impedance based on a flat beam model. This prediction procedure has been tested against measurements on at-grade installations on light rail transit and commuter railway installations in Baltimore and Boston. In both cases, the model showed good agreement with measured values for the resonant frequency dip and the mid-frequency insertion loss. At higher frequencies, however, the model over-predicted the insertion loss, as do many of the models used by ballast mat suppliers. Suggestions are made to account for the discrepancies between predicted and measured values.