Ground Penetrating Radar Evaluation of Concrete Tunnel Linings
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Ground penetrating radar was used to map tunnel lining condition and locate concrete deterioration and voids in a continuous and non destructive manner. GPR surveying of Kapoor Water Supply Tunnel, Victoria, Canada was carried out with a 1000 MHz Sensors and Software Ltd., Conquest system mounted on a custom built cart. This 8.8 km long 2.3 m diameter concrete lined circular tunnel was surveyed in both directions in two days. During this time, major anomalies were drilled to verify interpretations of voids behind the liner. The 30 ns time window and 5 cm sampling interval used provided good data quality and allowed speedy data collection. The processed data provided a wealth of information on the condition of the tunnel lining. Five major types of anomalies were identified. These consisted of variations in water content, void spaces, embedded wood, faults and metallic objects. A guide to interpretation of these anomaly types is presented. The 17.58 km of GPR data taken in the tunnel showed that GPR continuously mapped concrete liner thickness, presence of reinforcement and delineated zones where mesh roof supports and construction support timbers are embedded in the liner, as well as the locations and orientations of faults that intersect the tunnel. Minor voids, honeycomb sections and areas of rock-liner separation were also detected. Radar responses to voids, zones of slight liner-rock separation, sharp rock pinnacles and hollows under the liner, and embedded wood all had slightly different characters, but were not always uniquely distinguishable from each other.