Propagation of Cracks in the Secondary Lining of Tunnels Subjected to Asymmetrical Pressure and a Safety Evaluation

This paper aims to study the way cracks propagate in the secondary lining of tunnels subjected to asymmetrical pressure and to determine the corresponding safety evaluation standards. To achieve this, a cracked part of the secondary lining of the Gucheng-Zhuxi expressway tunnel in Hubei, China was studied under asymmetrical pressure. Using fracture mechanics, the propagation laws and characteristics of the cracks on different scales are analyzed under different asymmetrical pressures and at different positions. The study is based on stress-intensity factor theory and uses ANSYS finite-element software. The results show that crack width (b) has a small effect on crack propagation in the lining structure. On the other hand, crack depth (h) has an obvious effect. The stress-intensity factors at asymmetrically-loaded positions are found to be clearly larger than those at symmetrically-loaded positions. That is, an asymmetrical pressure distribution promotes the propagation of cracks at asymmetrically-loaded positions (and inhibits that at symmetrically-loaded positions, to some extent). When the value of h falls in the range from 3 to 12 cm, the cracks propagate in a stable manner in asymmetrically-loaded positions; at a depth of 15 cm, the propagation becomes unstable. Thus, a crack depth of h = 12 cm can be regarded as the critical value signaling the onset of instability and failure of the secondary lining when considering the safety margin required of the lining structure. Combining the existing safety evaluation standards for cracks with a new evaluation index—the crack depth h—we propose a new safety evaluation standard that is suitable for cracks in the secondary lining of the Gucheng-Zhuxi expressway tunnel under asymmetrical pressure. In addition, suggestions are made for properly treating different types of crack.