Abstract: To investigate the cracking mechanism of concrete on the top of diaphragm walls, a case study is conducted on a largediameter shield tunnel section of the Nanjing metro. By field investigation and extended finite element analysis, the crack mode, distribution characteristics, and cracking process of the concrete on the top of the diaphragm wall in the tunnel are studied. The results reveal the following: (1) There are 96 cracks on the top of the diaphragm wall, and the crack modes are types Ⅰ, Ⅱ, Ⅲ, Y, and Z, among which types Y and Z cracks are subtypes of type Ⅰ cracks. (2) The cracks start at the opposite side of the horizontal differential displacement of the flue plate; the development curves of the length and end width of types Ⅰ and Ⅱ cracks show a threestage characteristic; the end width and length of the cracks are nonlinearly related; 250 and 225 mm are the critical crack lengths with the rapid growth of the end width of types Ⅰ and Ⅱ cracks, respectively. (3) In type Ⅱ cracks, the propagation curves of two cracks under the same working condition are consistent, and the space of plate joints significantly influences the length and slope of each stage of the curve; the first crack propagation process in type Ⅲ cracks is consistent with that in types Ⅰ and Ⅱ cracks, and the propagation of the other two cracks have obvious brittle characteristics.

Key words: largediameter shield tunnel, diaphragm wall, differential displacement, crack mode, propagation mechanism