Index for assessing spalling in tunnel lining based on displacement monitoring and crack mapping

Lining deformation, cracks, and spalling are the most crucial items to be checked during structural safety inspections of tunnels in operations. These three faults are often cited as indicators of structural safety and are used to determine the need for maintenance or repair operations. However, existing evaluation criteria do not account for how the quality of a tunnel’s lining structure varies over its life cycle. The present study examined the 30-year construction and repair records of a highway tunnel in southeastern Taiwan. This study also quantitatively analyzed the relationships among tunnel displacement, lining cracks, and concrete spalling. The results revealed that compared with the associated displacement, tunnel deformation after overall profile translation was compensated for was more representative of tunnel structural safety and lining spalling potential. Specially, when the cumulative tunnel deformation reaches 1 % equivalent radius and when the deformation rate reaches 40E-6/year, the potential for lining spalling is notable. Concrete spalling is common near closed, radial, acute-angle intersecting cracks in a tunnel and also in the vicinity of circumferential construction joints. If a tunnel intersects the potential sliding surface of a moving mass on a slope, the vicinity of this intersection should be evaluated for concrete spalling. The spalling potential of concrete increases with the crack density. For a tunnel in operations, crack densities of 1.0 and 1.5 m/m² and associated change rates reach 2.0 and 3.0 m/m²/year can be applied as thresholds for grading moderate and high spalling potential, respectively, in the tunnel lining.