Analysis of the Irreversible Strain of an Underwater Shield Tunnel Based on SHM Data

Deterioration evaluation of tunnel structures is the prerequisite for the tunnel’s daily traffic safety management, maintenance planning, and service life prediction. Previous studies primarily rely on material degradation tests and theoretical analysis, often lacking the support of long-term field monitoring data. This study aims to address this gap by implementing a structural health monitoring system including 48 strain gauges installed in an underwater shield tunnel in 2013. More than 8 years of data were obtained, and an irreversible strain-based evaluation framework was proposed. This framework operates in two steps: first, it separates the irreversible strain and its damage-induced part from the total strain. Second, it assesses the segments’ damage status using the hypothesis test method. Results show that the mean irreversible strain of 41 monitoring points is 43 με over 8 years, agreeing well with the predictions based on the concrete shrinkage and creep theory. It is also found that the irreversible strain and elastic strain at different monitoring points vary significantly but they are highly correlated. Quantitative results from the hypothesis test show that the irreversible strain is induced mainly by the concrete shrinkage and creep, rather than segment damage. This study provides a method for early-stage performance diagnosis of the tunnel segment during its service life.