Influence of ballast quantity on compression wavefront steepening in railway tunnels

The impulsive pressure wave (micro-pressure wave) emitted from a tunnel portal is one of the environmental problems associated with high-speed railways. The principle underlying the countermeasures for this micro-pressure wave is to reduce the pressure gradient of the compression wavefront at the tunnel portal. This study investigated the influence of the ballast layer on the rate at which compression waves steepen in railway tunnels. Field measurements in an actual Shinkansen tunnel with slab tracks were recorded before and after installing ballast along most of the length of the tunnel. Previously published results on the spread of the compression wavefront in a ballasted track tunnel are also presented for comparison. A rapid method using an empirical equation is shown to accurately estimate the variation in the maximum pressure gradient of the compression wavefront in certain conditions. A more widely applicable estimation method using a numerical analysis is also demonstrated, which describes the distortion of the compression waveform. The results show that both of these methods can estimate the influence of the ballast quantity on mitigating the compression wavefront steepening in Shinkansen tunnels.