The study investigates the cracking and damage of shield tunnel segmented lining via the field observations and measurements of a case in Shanghai metro system. The observations are carried out on local damage of segment, segment cracking and joint dislocation, respectively. Notable deformation can be observed on several lining rings. Two parameters, proportion of damaged rings and radius ovality (O_R), are introduced to describe to what extent the lining rings are damaged in a tunnel section and how large the transverse deformation of the lining ring is. The construction-relevant data, such as attitude of shield machine and assembled lining rings, are collected and analysed to explore the exact causes for tunnel sections with different characteristics of cracking and damage. Surrounding environment was checked and numerical results are also presented to validate our speculation.

Ground-penetrating radar (GPR) has been widely applied to the nondestructive inspection of concrete structures such as tunnel lining, bridge deck, and retaining wall, which are usually reinforced by steel bars. The scattering of electromagnetic (EM) waves caused by the dense steel rebar embedded in the concrete structures has a severe influence on the penetration capacity of GPR waves. In this letter, the scattering and penetration characteristics of EM waves propagating through rebar net are investigated via both numerical and laboratory experiments, with an aim to select the antenna nominal frequency for a different reinforcement density. The results show that the rebar, which is perpendicular to the polarization direction of GPR waves and has a very small diameter compared with the wavelength, is almost transparent to the impinged GPR waves. The scattering and interaction of GPR waves caused by the rebar that is parallel to the polarization direction result in a shielding effect, which is manifested as a blind band in the low-frequency range in the transmitted spectrum. This result violates the rule of thumb commonly used in the GPR community, i.e., the lower frequency has a deeper GPR penetration depth. In the end, a low cutoff frequency is recommended for selecting a GPR antenna with an appropriate nominal frequency when it is used in the detection of an anomaly inside and behind a reinforced concrete structure, in which the spacing of the rebar net is known.