Groundwater drainage into a tunnel in fractured rock mass (Flysch): Numerical modeling to predict maximum rate of water drainage

Tunnels and underground excavations can induce groundwater drainage, depending on the hydrogeological properties of the involved medium. Groundwater drainage, especially when unforeseen during the design phase, can represent a major issue during drilling operations, precluding the safety conditions. This study presents the results of a three-dimensional numerical model with the code FEFLOW®, which aimed to predict the tunnel inflows induced by a road tunnel drilled in a fractured rock mass (a turbidite, i.e. Flysch di Belluno Formation, Eocene). The simulations were implemented by means of the hydrogeological monitoring data collected during and after the drilling phase and were used to design the final drainage system of the tunnel. Simulations results allowed estimating the maximum groundwater drainage rate during the extreme hydrological events (rainfall with return period of 500 years).