Tunnel Seismic Prediction in Stockholm Bypass

Abstract: Tunnel Seismic Prediction (TSP) is a geophysical investigation method used to predict the rock conditions ahead of the tunnel face. The method has been used in different types of rock and rock conditions. The Swedish Transport Administration, Trafikverket, has used the investigation method in multiple locations in the large infrastructure project E4 The Stockholm Bypass. The method is however rather new to Swedish rock conditions, and there is therefore a need to evaluate the method to assess its strengths and weaknesses. In this thesis, the TSP method is compared to other investigation methods used in the Stockholm Bypass project at the location Sätra-Kungshatt where the tunnels cross underneath Lake Mälaren. The investigation methods include geological mapping and Measurement While Drilling (MWD). The TSP results are also compared to the engineering geological prognosis. An evaluation of how the seismic primary and secondary waves, Vp and Vs, correlates to rock quality was carried out, and a linear regression analysis was performed to determine if the wave velocities found using the TSP method correlate with the Q value retrieved through the geological mapping. It was found that the TSP method is capable of detecting weaker zones of rock mass, however no correlation between the wave velocities and the Q value used to describe the quality of the rock mass was found. When comparing the TSP results to the MWD results, it was found that the methods could be used as complements to each other as the different methods sometimes detected weakness zones where the other method did not. As the geology in this location of Stockholm Bypass overall was found to be complex with rather poor rock mass quality, it could be concluded that the TSP method might be better suited for less complex geology where the contrast in rock quality is greater.