Abstract: Based on the field measuring data of metro line 1 of a city in soft soil area, the disease state and crack patterns of the whole line and a typical section in soft soil area are statistically analyzed. The statistical results show that the distribution of the cracks is demonstrably related to the convergence displacement of the tunnel, and the longitudinal cracks are the main disease in metro section, while they is densely distributed on the tunnel crown and related to the rotation angle of the segment ring. On this basis, a layerstructure numerical model is established using nonlinear numerical analysis method of fracture mechanics so as to analyze the adverse effects of weak ground conditions on tunnel rings. The smeared crack model and simplified reinforcement grid elements are used in model segment, while the longitudinal joint is simulated by the gap contact model considering the joint opening. The calculating results illustrate the cracking characteristics of the segment and its influencing factors. The results show the following. (1) The tunnel damage area increases significantly under insitu stress condition with decreasing elastic modulus and lateral pressure coefficient of surrounding soil. (2) The depth of current crack zone and predicted crack zone further expands on the original basis with increasing burial depth, resulting in an increase in overall crack rate. (3) The parameter analysis reveals that the main cause for the crack pattern on the tunnel crown is the position of the key segment.

Key words: shield tunnel, disease, segment crack, structural nonlinear