GPR Investigations to Assess the State of Damage of a Concrete Water Tunnel Diego Arosio 1 , Stefano Munda 1 , Luigi Zanzi 1 , Laura Longoni 2 and Monica Papini 2 1 Dip. di Ing. Strutturale, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy 2 Dip. di Ing. Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ABSTRACT 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 cross- section 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 investiga- tion 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 concrete- rock 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. Introduction Tunnels are important underground structures used in the transportation of vehicles, water, electricity, and other items. Many of these structures have been built decades earlier, and it is of paramount importance to assess the integrity of these underground construc- tions to ensure safety and long-term viability. Towards this end, several empirical (Schmidt, 1974; Attwell, 1978), analytical (Verruijt and Booker, 1996; Bobet, 2001) and numerical analyses (Leca and Clough, 1992; Augarde and Burd, 2001; Menguid and Dang, 2009) have been conducted to evaluate tunnel stability. In particular, Meguid and Dang (2009) used numerical models to evaluate the negative effect of erosional voids developing in close vicinity of tunnels. They studied the effects of voids on the circumferential stresses in the lining and the change in lining response caused by the introduction of voids behind lateral walls (i.e., at the springline) and below the paving (i.e., at the invert). While they assumed a homogeneous hosting medium and a theoretical 2-D geometry of the voids, their work helped to improve the knowledge of the stress state of these structures. In this work, we attempt to identify the erosional voids in a water-filled tunnel using geophysical methods, particularly ground penetrating radar (GPR). The geophysical investigation was part of a broader study focused on understanding the hydrogeological condi- tions that potentially could contribute to the deteriora- tion of the water-carrying infrastructure along a national road that runs through a narrow valley. The road is periodically affected by service interruptions caused by small landslides or partial road collapses induced by water erosion and intense precipitation events. The risk of road collapses increases where the road runs close to the river or where the river crosses the road by flowing into concrete-lined tunnels. The con- crete tunnels were built at the beginning of the 20th century and were not properly maintained over the past decades. Rapid deterioration induced by water erosion and lack of maintenance are a major safety concern. For example, a large sinkhole triggered by a partial collapse 159 JEEG, September 2012, Volume 17, Issue 3, pp. 159–169