Abstract
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.
Highlights
A large number of tunnels have been in service for the past decades all over the world
The results showed that the fractal dimension can characterize the distribution of cracks, and especially identify the intersection of different cracks, which is closely related with the health condition of tunnel lining
A new method was proposed for the health assessment of tunnel lining, which evaluates the lining states according to the fractal dimension of cracks
Summary
A large number of tunnels have been in service for the past decades all over the world. The type of concrete lining can be divided into three categories, including shotcrete, prefab and cast-in-place lining. The cast-in-place concrete lining was the most commonly used one and it was studied in this paper. The persistent ageing and seismic activity lead to many problems to the cast-in-place concrete lining, such as cracking, corrosion and leakage [3,4,5]. Damage to the tunnel lining will decrease its integrity and subsequently increase the risk of tunnel lining collapse [6]. In order to deal with this situation, there is an urgent requirement for accurate tunnel lining health assessment methods [7,8,9,10]
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