A framework for maintenance of shield tunnels with secondary linings using strength and life-cycle cost as indicators

Many shield tunnels have been constructed during the urban development of past decades, serving as important municipal infrastructures, such as for roads, railways, water supplies and sewers, etc. However, continued good performance of such urban utilities requires appropriate maintenance of the tunnel structure, especially when used beyond the designed working life. On the other hand, maintenance of extensive social infrastructure demands a substantial budget, so a rational stock management method is adopted by agencies and authorities. Although preventive maintenance has been realized in the most efficient and effective manner, based on the criteria for the life-cycle cost and performance of such structures, there is still some argument about the superiority of corrective and preventive maintenance, and predictive maintenance. In any event, how life-cycle performance should be predicted and evaluated must still be studied, as well as the selection of the most appropriate countermeasures for maintenance. This paper examines shield tunnels with secondary linings, and proposes a maintenance framework based on performance-based design. Strength is used as an indicator of structural performance, considering their features of having a secondary lining and being infrastructure utilities. A successful application of the framework to trunk sewer pipe is also presented. The sewage tunnel strengths under the designed static and seismic events were evaluated using numerical analysis, and the corresponding life-cycle costs were estimated for three assumed cases of steel corrosion and countermeasures. The rational countermeasures was then more easily selected by determining the minimum life-cycle cost per unit strength for a designed period.