Evaluation of 3D in situ stresses for the Brenner Base Tunnel project.
The article present results of in situ stress analyses in the area of three exploration boreholes for the Brenner Base Tunnel. This includes direct determinations of the complete 3D stresses with Racos (Rock anisotropy characterisation on rock samples) and the minimum stress component with hydraulic fracs plus additional analytical and numerical calculations. The determined local stress at the northern part of the tunnel illustrates a strikeslip tectonic state with the maximum stress in N‐S orientation. In the central tunnel area, the in situ stresses (markedly different from vertical‐horizontal) correspond to a transition between tectonic strikeslip and normal fault conditions with a strong N‐S compression. Further to the south, a tectonic reverse fault configuration with an approximately verticalhorizontal stress orientation and NS aligned maximum component was determined. The orientations of all maximum horizontal stresses correlate to a largescale tectonic stress field with an N‐S to NNW‐SSE orientation perpendicular to the Alpine arc (corresponding to the World Stress Map). The small differences between total and effective in situ stresses indicate negligible mechanical pore pressure effects in the analyzed areas. Overall, the in situ stresses determined by different methods show (with accurate consideration of the respective spatial orientations) a good agreement.