ABSTRACT: Evaluation of the reliability in reference hydrogeological models (RHM) represents an essential part to be defined during design and/or construction of deep tunnels, in order to correctly evaluate technical and financial risks as well as environmental hazards due to the potential interaction between underground works and surface water resources (springs, lakes, etc). Deep tunnels in alpine-like mountain belts, generally crosscut a low primary permeability bedrock so that the main groundwater flow is normally related to brittle tectonic structures like faults, cataclastic shear zones, thrusts and joint systems. Therefore, the RHM reliability directly depends on the Reference Geological Model (RGM) and particularly on the quality and reliability of the geological data which are at the basis of the RGM.

The reliability of such data depends on A) the detail and reliability of geological, structural and hydrogeological surveys carried out on the surface, B) the outcrop percentage, the availability of key-outcrops and the structural complexity, C) the persistence and the degree of evolution of the existing brittle structures (faults, thrusts, joint sets, etc.), D) the amount and type of available investigations, E) the depth of the underground excavations, F) the possibility of projecting geological borehole data to the working level and the difficulties in the interpretation, G) the type and amount of borehole hydraulic tests and H) the possibility of determining the recharge area and the surface hydraulic balance.