Large air chambers for rail tunnel exit regions

The purpose of this paper is to present an assessment of the potential for using large air chambers inside enlarged exit regions of tunnels instead of using long extensions outside tunnel entrances. The target is to prevent the emission of unacceptable micro-pressure waves (MPWs) regardless of the steepness of pressure waves reaching the special exit region, even when no counter-measures are implemented further upstream. The study is complementary to previous work in which multiple small air chambers are assumed to be distributed along a tunnel, under walkways, etc. It is found that exit regions with large chambers could be effective for wavefronts longer than about two tunnel diameters, but probably not for wavefronts shorter than about one tunnel diameter. A possible way of incorporating large chambers in tunnel end regions is hypothesised.