An optimally designed entrance portal must be capable of minimizing the maximum growth rate of the compression wave generated when a high-speed train enters a tunnel. A theoretical and experimental investigation has been made to determine the changes in compression wave characteristics produced when the portal is ‘scarfed’ with tapering side walls. It is concluded that portal modifications of this type are unlikely to produce a significant reduction in the maximum compression wave growth rate. Small decreases in growth rate are possible (up to about 15%) for scarf walls extending a distance beyond the tunnel entrance of the order of the tunnel height, but little or no additional improvement is achieved with longer walls.
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