Evaluation of tunnels presents a challenging problem. Roadway and rail tunnels by their very design are subject to the constant presence of moisture, even in relatively dry climates. This can lead to a number of problems such as corrosion of embedded reinforcement and delamination, voids between the liner and subbase material, and water flow through and behind the liner, which sometimes appears in locations remote to the water source. Tunnels that are faced with tile or other surface materials can become debonded or mask deeper flaws in the concrete liner as well as the source of the underlying problem. Manually intensive and destructive methods exist, however, because tunnels tend to be in high use and generally experience high traffic density, closures are undesirable, costly and in many cases impractical. Some tunnels have been in service for decades and because evaluating deterioration in these structures can be difficult, very little is sometimes known about their true condition. Due to the limited methods of assessment and limited accessibility for inspection, this can lead to potential safety concerns and liability issues with motoring traffic. This paper discussed the investigation and use of ground penetrating radar (GPR) in evaluation of tunnel liners, based on SHRP2 Project R06G research. A new and innovative method of using GPR for tunnel evaluation was developed and demonstrated on two tunnels in Pittsburgh, PA. An articulating structure was developed and utilized to position an air-coupled antenna in close proximity to the liner surface, permitting high-speed inspection of all parts of the tunnel liner. GPR was able to detect delaminated shotcrete and debonded tile facing, areas of water saturated concrete liner, and identify possible areas of concrete deterioration and voids behind the liner.

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