The surface defects on a shield subway tunnel can significantly affect the serviceability of the tunnel structure and may compromise operation safety. To effectively detect multiple surface defects, this study uses a tunnel inspection trolley (TIT) based on the mobile laser scanning technique. By conducting an inspection of the shield tunnel on a metro line section, various surface defects are identified with the TIT, including water leakage defects, dislocation, spalling, cross-section deformation, etc. To explore the root causes of the surface defects, association rules between different defects are calculated using an improved Apriori algorithm. The results show that: (i) there are significant differences in different association rules for various surface defects on the shield tunnel; (ii) the average confidence of the association rule “dislocation & spalling → water leakage” is as high as 57.78%, indicating that most of the water leakage defects are caused by dislocation and spalling of the shield tunnel in the sections being inspected; (iii) the weakest rule appears at “water leakage → spalling”, with an average confidence of 13%. The association analysis can be used for predicting the critical defects influencing structural reliability and operation safety, such as water leakage, and optimizing the construction and maintenance work for a shield subway tunnel.

The concrete floating slab, as a widely-used measure to reduce the subway-induced vibration and noise, can deteriorate with the accumulation of trainload. To monitor the dynamic performance of the floating slab during operation, this study develops a lightweight sensing unit based on microelectromechanical systems (MEMS). This sensing unit, which can be easily mounted on the concrete slab, enables synchronous monitoring of vibration and noise at different sampling frequencies with a very low power consumption. With the advantage of easy installation, the sensing units are mounted on various concrete slabs and tunnel walls to collect massive vibration and noise data. With the monitoring data, the dynamic performance and vibration reduction capability of the floating slab can be assessed. A case study is conducted on two floating slab sections: one section has a floating slab with a potential defect on its shear hinge and the other has a normal floating slab. The monitoring results show that: (1) the train condition has a significant effect on the amplitude of the track-side vibration; (2) the train-induced noise data last longer than vibration data during the passage of trains; (3) for the floating slab with shear hinge defect, the vibration levels at edge points are much higher than that in the middle. Through the monitoring test, the performance of the MEMS-based sensing unit and its adaptability in subway circumstances are verified.

The surface defects of shield subway tunnel can significantly affect the serviceability of the tunnel structure and may compromise the operation safety. To effectively detect the multiple surface defects, this research employs a tunnel inspection trolley (TIT) based on the mobile laser scanning technique. By conducting the inspection of the shield tunnel on a metro line section, various surface defects are identified by the TIT, including water leakage defect, dislocation, spalling, cross-section deformation, etc. To explore the root causes of the surface defects, the association rules between different defects are calculated via an improved Apriori algorithm. Results show that: i) there are significant differences in different association rules of various surface defects of the shield tunnel; ii) the average confidence of the association rule “dislocation & spalling → water leakage” is as high as 57.78%, indicating that most of the water leakage defects are caused by dislocation and spalling of the shield tunnel in the sections being inspected; iii) the weakest rule appears at “water leakage → spalling”, with the average confidence of 13%. The association analysis can be used in predicting the critical defects influencing the structural reliability and operation safety, such as water leakage, and optimizing the construction and maintenance work for the shield subway tunnel.