Cracking Characteristics and Mechanism of Internal Components of Large-Span Shield Tunnel

With the ever-increasing number of large-span shield tunneling projects and the extended operational time, their distinctive internal component issues are becoming more pronounced and exhibiting unique characteristics. In order to reveal the cracking mechanism of concrete on the top of the middle partition wall, based on a large-span shield tunnel section of Nanjing Metro, the crack mode, distribution characteristics, and cracking process of concrete inside tunnel structure were studied by combined field investigation and extended finite element analysis. The results showed that: (1) there were 96 cracks of concrete on the top of the middle partition wall in the interval. Based on the propagation path, number, and shape of cracks, they could be divided into type I, II, III, Y, and Z, with type-Y and type-Z being subtypes of type-I. (2) The crack started at the opposite side of horizontal differential displacement of the flue plate. The development curves of the length and end width of type-I and type-II cracks showed the significant characteristics of three stages. The relationship between crack end width and length was nonlinear, showing that 250 and 225 mm, respectively, were the critical crack lengths growing with the rapid growth of the end width of type-I and type-II cracks. (3) In type-II cracks, the propagation curves of two cracks under the same working condition were consistent, and the spacing of plate joints greatly influenced the length and slope of each stage of the curve. For type-III cracks, the first crack propagation process was basically consistent with type-I and type-II cracks, and the propagation of the other two cracks had obvious brittle characteristics.