The influence of longitudinal crack on mechanical behavior of shield tunnel lining in soft-hard composite strata

Concrete cracking is a common structural defect of segmentally lined tunnels. In this article, the tunnel between Shangteng station and Dadao station of Fuzhou metro line one, which mainly passes through the two-layered (sand and medium-weathered granite) composite strata, was taken as an example to study this issue. First, the cracking situation of segmental linings was investigated from on-site inspection and the cracking mechanism is revealed through numerical simulation. Then, similarity model tests were conducted to study the mechanical response of lining with a non-through longitudinal crack. Finally, a new numerical approach for cracked segment was put forward to analyze the mechanical behavior of lining with multi-cracks. The results show that: (1) Longitudinal cracks and corner cracks are the main cracking types of segment, and the difference in jacking thrusts determines the cracking patterns. (2) The influence of crack on inner force distribution along tunnel perimeter shows joint truncation effect and amplification effect of heterogeneous formation, and the influencing degree can be classified into three levels: serious effect, general effect and weak effect. (3) The failure process of lining has four distinct stages: elastic stage, initial damage stage, ultimate limit stage, and unstable stage. In addition, the pre-existing crack will develop along the longitudinal direction and be crushed under the action of external load in most cases. (4) The influence of multi-cracks on the inner forces of lining shows no coupling effect. In addition, the asymmetrically mechanical response of lining is affected by two factors: the difference in mechanical parameters of layered strata and the difference in stiffness of cracked and non-cracked sections. (5) The damage degree of crack, the depth of surcharged soil, and the proportion of soft soil in the tunnel section are the three main factors influencing the safety of lining.