GPR Investigations to Assess the State of Damage of a Concrete Water Tunnel

Erosional voids developing around concrete-lined tunnels can compromise the safety of the surrounding areas, as well as of the tunnels themselves. In this study, ground penetrating radar (GPR) was used to assess the condition of a water tunnel built to channel a river under a mountain road. The tunnel is lined with 60–80 cm thick concrete and has a semicircular crosssection with a diameter that varies between 3 m and 4 m. The concrete structure has been damaged from erosion beneath the concrete floor, creating a sequence of pools and waterfalls, which further extend the erosive action below the floor and side walls. After the collapse of a section of the tunnel running below a nearby parking lot, a GPR investigation was initiated to assess the extent of the erosive action behind the tunnel walls and below the concrete floor. Most GPR measurements were performed from inside the tunnel with a 200-MHz antenna, which was selected as the best trade-off between penetration and resolution. GPR results, integrated with a priori information and geological investigations, indicated a highly permeable soil consisting of a thin layer of alluvial sediments that covers an altered limestone layer strongly affected by erosion and karst phenomena. Fortunately, GPR inspections on the parking lot surface were able to exclude the presence of large cavities above the tunnel vault. On the contrary, GPR inspections performed inside the tunnel detected many voids forming behind the walls, especially near the concreterock contact. GPR inspections performed on the tunnel floor confirm that water erosion is active below the concrete paving. Overall, the survey was useful for identifying the damaged tunnel segments where repair interventions are most urgent.