It is known that the tunnel micro-pressure wave, one of the environmental problems of high-speed railways, tends to attenuate in a long ballast-track tunnel. The RTRI analyzed the mechanism of the attenuation of the compression wave in a ballast track tunnel, and clarified the effect of the ballast layer in reducing the micro-pressure wave.

According to theoretical analysis, increased friction on the ballast layer surface and the inflow of air into the ballast layer are dominant causes of attenuating the compression wave. The RTRI has proved that this effect can be mathematically included in a single term of attenuation. The results of the numerical calculation based on this approximation are in good agreement with the data obtained from field tests (Fig. 1). It therefore enables to calculate the effect of reducing the micro-pressure wave in case of ballasting slab track (thickness of 0.7 m) in a tunnel. When a high-speed train enters a tunnel at 350 km/h, for example, the micro-pressure wave will be reduced by half if a 4-m-wide ballast layer (equivalent to the width of a single track in a double-track section) is installed over a length of about 2 km (Fig. 2). For reference, the magnitude of a micro-pressure wave is in proportion to the maximum pressure gradient. The magnitude of the compression wave is so small relative to those of the atmospheric pressure that the basic equation can be simplified. It allows to cut the calculation time to 1/100 or less without compromising the accuracy of the calculation.

Based on the results of this study, it is now possible to predict the distortion of the compression wave propagating through a ballast track tunnel as well as through a slab track tunnel. The RTRI plans to incorporate these findings into a technique to predict the magnitude of the tunnel micro-pressure wave and the effect on reducing it.