Condition assessment of shield tunnel using a new indicator: The tunnel serviceability index

The condition assessment of shield tunnels is one of the key issues in the structural maintenance during the service life of the tunnel. Traditionally, the shield tunnel condition has been graded based on certain types of tunnel distresses, such as convergence deformation, differential settlement, cracks, and water leakage. However, the overall condition of the tunnel cannot be so easily assessed, as it often depends on experts' decisions and evaluations. This paper proposes a comprehensive tunnel serviceability index (TSI) to evaluate the overall condition of a shield tunnel in soft ground. The TSI is a mathematical combination of measurable tunnel distresses that can be obtained by employing the following steps: (1) select the types of tunnel distresses to be included in the TSI, (2) prepare the tunnel samples with distresses, (3) have experts rate the tunnel serviceability, and (4) regress the TSI formula using the partial least squares (PLS) method. The TSI formula can then be applied to evaluate the serviceability of tunnels that are similar to the tunnel samples in step (2). Metro shield tunnels in Shanghai's soft ground are selected for a case study. Six common distresses, namely, relative settlement, differential settlement, convergence deformation, water leakage, cracking, and spalling, are selected for inclusion in the TSI. Tunnel serviceability ratings of 40 samples were conducted to regress the TSI formula. The TSI formula reveals that relative settlement, differential settlement, convergence deformation and water leakage are the four significant variables that impact the serviceability of Shanghai's metro shield tunnel with loading factors of −0.62, −0.13, −0.25 and −0.19, respectively. Cracking and spalling are two weak indicators with loading factors of −0.06 and −0.03, respectively. These conclusions, in general, are consistent with the maintenance experiences of the Shanghai metro group.