Hydrogeological monitoring and modelling in the S. Lorenzo road tunnel area (Passo della Morte, Udine) for the design of countermeasure works

In the eastern Italian Alps, a large deep-seated rock block slide affects the S. Lorenzo road tunnel (46°23’49” N, 12°42’51” E) along the National Road 52 “Carnica”, near to Passo della Morte (Udine Province); the tunnel was excavated through a fractured carbonatic rock mass overthrusted on clay-rich units. Previous studies of the unstable slope outlined the relationship between slope displacements and groundwater flow in the fractured rock mass. The conceptual geological model of the area, together with field observations, allowed specific investigation and monitoring activities to be defined in order to investigate this relationship. In particular, during the last years, six weirs were built and pressure transducers were installed in order to assess the total water discharge from the area. Moreover, 14 piezometers were drilled outside and inside the tunnel to monitor the hydraulic head. After 15 years of monitoring the groundwater system can be described in detail. In particular, results show a double flow system. The first is almost constant for all the year, while the second is influenced by rainfall events and is hosted in faults, fractures and joints in the western part of the tunnel, in the area of the larger and deep-seated rock slide. A three-dimensional groundwater flow model was built and calibrated along the total outflow from the road tunnel in order to better understand the interactions between groundwater flow and road infrastructure. In particular, the effects of countermeasures works, like the extension of the drainage adit in the carbonatic rock mass, was tested.