Detection of Sub-surface Delamination and Moisture Penetration in Unlined Rock Tunnels Using Passive Thermography and Tapping

An unlined rock-blast tunnel constructed by the drill and blast method is susceptible to frequent deterioration, including sagging beds, water ingress and loose rock masses. Thus, assessing potential damages above the roadway that may impact the safety of tunnel users is of utmost importance. While tapping, a conventional nondestructive method, is typically used to predict sub-surface delamination, water ingress can be identified with the naked eye. In this research, we utilized a modern-technology approach called passive thermography, together with a high-resolution digital camera and tapping. The study encountered two primary challenges; namely, the absence of direct solar radiation during the inspection and the groove-exposed surface of the unlined tunnel. The results demonstrated that passive thermography was able to detect delaminated areas and water infiltration on the tunnel's walls and ceiling, even without direct sunlight. Large delaminated areas can be detected with just a 2°C change in atmospheric temperature twelve hours before testing. Additionally, the results of image post-processing significantly contributed to enhancing the results of passive thermography. The thermal image was processed into a grayscale image prior to HE processing, which enhances contrast by over 50%. The combination of tapping, digital camera and passive thermography was proven to be effective in periodically inspecting unlined rock tunnels, while significantly reducing time and cost. KEYWORDS: Tunnel safety, Passive thermography, Old rock-cut tunnel, Hammer sounding, Non-destructive evaluation, Image post-processing.