A New Health Assessment Index of Tunnel Lining Based on the Digital Inspection of Surface Cracks

The stability assessment of aged tunnel linings are mainly evaluated based on the visual inspection, and the Tunnel-lining Crack Index (TCI) is one of the most widely used tunnel lining health assessment indexes in Japan. However, the intersection and distribution of cracks, which could influence the stability of tunnel lining greatly, were not considered in TCI. A new method was proposed for the health assessment of tunnel lining, which evaluated the lining states according to the fractal dimension of cracks. Based on the machine vision-based method, the crack image could be extracted efficiently. The fractal dimension of lining cracks in one span could be obtained in a few minutes. A series of comparative tests and field tests were conducted to evaluate the validity of this new method. The comparative tests confirmed that fractal dimension was able to characterize the density, width, and distribution of cracks. The intersection of cracks, which would increase the risk of lining collapse, could also increase the fractal dimension. The fractal dimensions of tunnel lining cracks were obtained according to the digital inspection test of Hidake Tunnel in Japan for all the 65 spans. Moreover, the TCI was obtained through statistical methods. The correlation between fractal dimension and TCI of tunnel lining was studied. The significance of the new evaluation index is that it can identify the unusual spans of tunnel lining and provide a basis for further internal testing. As a complement to the conventional visual inspection method, the fractal dimension of the cracks is a promising health assessment index.