The magnitude of tunnel micro-pressure waves, an environmental issue in high-speed railways, depends on the tunnel exit waveform of the compression wave generated when the train enters the tunnel. To clarify the phenomenon of tunnel micro-pressure waves and discuss methods for their reduction, it is important to assess their deformation during the propagation process within the tunnel.
      To quantitatively assess this phenomenon, therefore, the RTRI measured the deformation characteristics of a compression wave propagating in a 26km-long slab track tunnel (currently the longest land tunnel in the world) and performed numerical calculation to develop a prediction method for the phenomenon. This calculation combines the analysis of one-dimensional compressive flow to consider the effects of the compression wave's non-linearity, wall surface friction and heat transfer, and acoustic analysis of the effect of branch shafts (i.e. shafts for equipment/materials and inclined shafts) in the tunnel.
      The results of measuring the maximum pressure gradient of a compression wave front on a revenue service line (Fig. 1) show that the front rises sharply to a peak at the initial stage of propagation and decreases gradually thereafter. The results of numerical calculation were in close agreement with the actual measurements taken, demonstrating the validity of the method.
      The RTRI plans to incorporate this numerical calculation technique into the method for predicting the magnitude of tunnel micro-pressure waves and apply it to the development of measures for the reduction of such waves.

Fig. 1 Changes in the maximum pressure gradient of a compression wave front
(comparison between measured and calculated values)

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