Improving the effect of air chambers on micro-pressure waves from tunnel portals: Moderate underdamping

This research explores the ability of multiple small air chambers, distributed along railway tunnels, to suppress wavefront steepening and aims to find an effective method to prevent the radiation of unacceptable pressure disturbances from tunnel portals. It does so by utilizing the existing idle space in the tunnel, such as under walkways, and investigates the influence of moderately underdamped air chambers on wavefront evolution. This complements previous work, in which the chambers are assumed to be overdamped. In the early part of the paper, the validity of the numerical method used in this paper is verified through various means, including experiments with specially constructed air chamber model. Next, differences in the indicative influence of the wavefront, between underdamped and overdamped chambers, are compared. The asymptotic state of the wavefronts, in tunnels equipped with underdamped chambers, is then considered, along with a corresponding sensitivity study, centred on a specific base case. After that, the reasons for the large wavefront gradient caused by the underdamped chamber are discussed. Finally, the effect of installing a check valve at the chamber connection on wavefront evolution is explored. It is found that a moderately underdamped chamber may have a greater potential to suppress wavefront steepening to that of an overdamped chamber, especially for wavefronts with amplitudes up to 4 kPa. However, the presence of significant inertia at the chamber connection could result in the wavefront continuing to steepen for a distance of 2–3 km. Therefore, it is advisable to exercise more caution when employing underdamped chambers in shorter tunnels. While the check valve may have a certain degree of mitigating effect on wavefront steepening, but its impact is not significant.