<p>Tunnels often develop various defects over long-term service periods. Assessing tunnel behavior with defects is hence crucial to guide tunnel design, optimize the construction process, or maintain in-service functionality. However, existing numerical models lack the ability to simulate and analyze the coupled behavior of multiple tunnel defects, particularly those with cascading effects. To bridge the gap, this study proposes a systematic modeling method for tunnels with any of three most critical defects, i.e., excessive deformation, cracks, and leakage, to quantitatively assess their in-service conditions. In particular, a finite element model with adaptive fidelity is developed to evaluate deformations and internal forces under excessive settlement, while concrete damaged plasticity and extended finite element methods are further applied to simulate the initiation and propagation of cracks and leakages at locations experiencing excessive deformations. The proposed approach is validated through a case study assessing the impact of new tunnel excavation on an existing tunnel in Singapore. The results indicate excessive displacement and stress levels in the existing tunnel lining during excavation, and cracks initiate predominantly in close proximity to bolt holes, some of which propagate and penetrate linings; in the meantime, the pore pressure surrounding the penetrating cracks gradually decreases and reaches a stable state, indicating a leakage process. This case study demonstrates the effectiveness of the proposed method for quantitative evaluation of complex tunnel behavior for design and construction guidance, potentially enabling digital twin of in-service tunnels.</p>