Investigating the interactions of acoustic waves with underground structures via the boundary element method

Abstract The interactions of acoustic waves with an underground structure have been investigated in this paper. For this study, an underground tunnel with a circular cross section of radius 1.5 m under the action of a nearby acoustic source was considered. Four different types of materials including loose sand, clay, limestone, and granite rock have been considered as the medium surrounding the tunnel. The circumference of the circular tunnel has been modelled by 20 boundary elements, and also 360 radially arranged points have been considered as measurement points around the tunnel. The Helmholtz equations for the boundary points have been numerically solved by employing the boundary element method. The computations have been performed for a frequency range of 0.6–9 Hz (with an increment of 0.6 Hz) and for all the boundary and measurement points and the four types of materials considered. In order to predict the behaviour of the examined structure at high frequencies, the computations have also been carried out for a frequency range of 0.1–106 Hz. Based on the obtained results, under similar conditions, a greater acoustic pressure is observed at the boundary and measurement points of the tunnel buried in granite rock. Also, the logarithmic pressure distribution diagram obtained for an arbitrary boundary point of a tunnel buried in different materials indicates that, for tunnels embedded in sand and clay, acoustic pressure increases at such boundary point up to a frequency of 100 Hz and then diminishes and approaches zero as the frequency increases further. Also, for tunnels embedded in limestone and granite, acoustic pressure at an arbitrary boundary point increases up to a frequency of 1000 Hz and then diminishes at higher frequencies.