A challenging gpr investigation to assess the state of damage of a water tunnel

The GPR method was used to assess the conditions of a water tunnel built to force a river to underpass the road running along the Valganna valley, 60km north of Milan (Italy). The tunnel is concrete lined and has a semicircular section with a diameter that varies between 3 and 4m. The most damaged section of the tunnel runs parallel to the valley for about 300m before intersecting the road where the valley makes a pronounced left turn. The concrete structure is severely damaged because water erosion of the concrete floor has been creating a sequence of pools and waterfalls extending the erosive action below the lateral walls. Large fractures visibly affect the concrete ceiling especially where the water has eroded the floor and the soil below the wall shoulders. As a result, the risk of tunnel failure is really high and, after a partial collapse, a GPR investigation was planned to assess the extent of the erosive action behind the walls and below the concrete floor. Most of the GPR measurements had to be performed from inside the tunnel. The late summer was chosen to operate with a reduced rate of water flow. Here we discuss surveying method and data interpretation.