Geophysical methods can be applied on tunnel applications during the design stage to find out the mechanical properties of the ground (with seismic profiles or borehole methods); during the construction stage as a quality assurance and hazard mitigation method (eg, void detection or grout thickness control) or even during the service stage of the tunnel.
Tunnel engineers demand a wide range of inspection procedures for routine tunnel lining monitoring. The primary inspection method is visual identification of anomalies. After an area requiring attention has been identified more advanced methods are usually required. GPR is very often the selected tool for locating subsurface defects as well as detecting and locating moisture in concrete that facilitates reinforcement corrosion. The high-speed nature of GPR and its sensitivity to changes in surface dielectric makes it an ideal candidate to use with laser scanning.
GPR is used to determine number of layers of reinforcing, reinforcing spacing and depth of cover, areas of moisture/water entrapment, as well liner thicknesses and behind the liner voiding and moisture concentration.