Geomechanical characterisation of fault rocks in tunnelling: The Brenner Base Tunnel (Northern Italy)

• We aim at characterising rock faults involved in the tunnelling.
• We characterised them by means of drillings and geomechanical surveys.
• We carried out lab tests on specimens and analysed through thin sections.
• A relation exists between voids ratio at the micro-scale and rocks strength.
• We achieved good estimation of fault rock strength by using the Hoek–Brown method.

The goal of this study is to characterise the rock faults involved in the excavation works of the Brenner Base Tunnel, which crosses one of the main tectonic alignments of the alpine area, the Periadriatic fault. The characterisation was carried out at different scales from macro (through drillings and geomechanical surveys) to meso (lab tests on specimens) and micro-scale (through thin sections), in order to find a correlation between the different scales properties of the rock masses. Finally, a relation between the voids ratio at the micro-scale and the fault rocks conditions at the meso and macro-scale was pointed out. As a result, by means of the Hoek–Brown method, an estimation of the fault rock strength was achieved, tending to avoid quite a typical overestimation arising from the use of literature values.