Micro-pressure waves from tunnel portals in cuttings

The influence of cuttings on the propagation of micro-pressure waves (MPWs) from tunnel portals is investigated, focussing on MPWs above ground level - i.e. not at track level within the cuttings themselves. Contrary to initial expectations, it is found that MPWs from cuttings are not necessarily smaller than those from over-ground portals. Indeed, close to the edges of the cuttings, they can be considerably larger. This is a consequence of the constraining influence of the cuttings on the expansion of pressure waves in three-dimensional space. The attenuation of pressure amplitudes in directions parallel to the track is found to be much smaller than that in directions normal to the track. The work reported herein consists of a preliminary study designed to infer general behaviour before narrowing the focus of future work on the development of simple, but meaningful, guidelines for design engineers, especially in the early stages of the design process. Nevertheless, the findings also provide initial design guidance in their own right.