Spectral analysis of surface waves and seismic refraction for the inspection of tunnel walls

In many fields, e.g. civil engineering, mining engineering and underground disposal, it is very important to be able to quantify the extension of the rock mass zone disturbed by underground excavations. In multi-layered plan media a combination of Seismic Refraction (SR) and Spectral Analysis of Surface Waves (SASW) can be used to determine shear modulus and compression modulus profiles. Both techniques cannot be transferred to circular tunnels without questioning the limit of a plan geometry approximation. Indeed, in the following we show that the curvature of the tunnel cannot be neglected even for a thin damage zone compared to its radius. Special attention is given to the SASW for this technique will work even if the tunnel walls are covered with a concrete lining. Both experimental and numerical results are presented. The error introduced by the curvature on the SR interpretation is of the order of precision required for the investigation but its influence does not show clearly on the experimental data. For the SASW, the influence of the curvature on the experimental data cannot be ignored for the global trend of the dispersion curves is greatly changed. The experimental SASW dispersion curves are matched with numerical dispersion curves calculated with the finite element code CESAR_LCPC in an error and trial process.