In-situ stress amplification due to geological factors in tunnels: The case of Pajares tunnels, Spain

摘要

Geological and geomechanical anisotropies can significantly increase the magnitude of in-situ stress in a rock mass excavated for tunnelling purposes. Four tunnels for the new high-speed railway lines in Pajares in the Cantabrian Mountains, N. Spain, were analysed and significant deformations was found to have occurred in forty specific zones of these tunnels during excavation, requiring much more support than envisaged before construction. Local factors influencing in-situ stress have been identified in these zones of the tunnels, related to geological structures of high compressive tectonic stress regimes including thrust faults, folds with steep flank dip, or with geomechanical anisotropies in contact zones between rock formations of different strengths. A methodological procedure was applied to four tunnels to assess the influence of the geological anisotropies on the in-situ stress magnitudes. This procedure is based on the analysis of tunnel deformations and the support pressure needed to stabilise the excavations affected by geological anisotropies. The increase of in-situ stress due to local factors is expressed by the Stress Amplification Factor (SAF) defined by the ratio between the K(ó_H/ó_v)_(local) value estimated in a particular rock mass tunnel zone and the mean K(ó_H/ó_v)_(regional) value representative of the whole rock mass tunnel. K_(regional) was estimated from hydrofracture tests and from the TSI index.