Numerical simulation and experimental study of the effects of retaining block structures on wavefront steepening in rail tunnels

The entry of a high-speed train into a tunnel triggers severe micro pressure wave (MPW), posing a major obstacle to the fast and environmentally friendly operation of trains. The installation of a retaining block structure in the tunnel can alter the propagation process of the compression waves, thereby mitigating the effects of MPWs. In this research, a two-dimensional-axisymmetric retaining block is taken as the research object. Based on CFD and an experimental device for generating initial wavefronts, we investigate the influence of the retaining block's radial, axial lengths and the installation location, on the mitigation of wavefront steepening. It is found that the results of numerical simulations compare favourably with those of the experimental device. In mitigating the steepening of the wavefront, there is a critical value for retaining block's axial length which is about 1/4 of the length of S-shaped initial wavefront, and the retaining block is more effective when the wavefront pressure gradient is higher. This implies that when considering the inertial effect of the wavefront inside medium to long tunnels, the retaining block should be placed as close as possible to the tunnel exit to minimize the maximum pressure gradient of the wavefront. The results of this paper may provide a new approach for improving the evolution of pressure waves in tunnels.