Influence of local spalling and cracks in concrete linings on the mechanical properties of metro tunnel structures

Concrete metro tunnel linings are prone to typical defects such as cracking and spalling after long-term service, which seriously threaten structural safety. This study combines physical model tests and numerical simulation methods, considers the long-term strength attenuation effect of concrete, and systematically explores the ultimate bearing capacity, deformation characteristics, failure modes and mechanical behavior of tunnel linings under three typical spatial combinations of crack-spalling defects of overlapping, tangential and spaced. A coupled XFEM-CDP numerical method based on the concrete tensile damage parameter is proposed and verified. The results show that a single vault spalling defect has a limited impact on the ultimate bearing capacity of the lining. However, when this spalling defect overlaps with or is tangential to a crack that extends to a depth of half the lining thickness, the ultimate bearing capacity of the lining structure decreases by up to 28%. As the crack shifts away from the center of the spalling zone, the ultimate bearing capacity of the structure gradually recovers, and the failure mode transforms from symmetric to asymmetric bending failure. The combined effect of vault spalling and cracking leads to a decrease in bending moment in the defective area, inducing internal force redistribution and transferring the bending moment to vulnerable regions such as the invert and haunches, thus triggering global failure of the tunnel structure. In addition, the proposed XFEM-CDP method can accurately capture the crack propagation and damage evolution laws of tunnel structures, thereby providing an effective approach for analyzing the failure mechanisms of tunnel lining structures with multiple types of defects.