Abstract
It is difficult to detect and evaluate the structural damage in a shield tunnel during operation because many traditional techniques based on the observation of vibrations are limited in daily monitoring in tunnels. Thus, the curvature radius of a static longitudinal settlement curve is used to identify the residual health and safety of an in-service shield tunnel. However, there are still two problems. The curvature radius is suitable for a qualitative judgment rather than a quantitative evaluation for longitudinal damage detection. Moreover, the curvature radius, which is calculated from the measured settlements of three neighboring points, gives an average damage degree in a wide scope only and is difficult to use to identify the damage’s precise location. By means of the analysis of three kinds of longitudinal failure modes in a shield tunnel, this paper proposes: (1) a damage detection method based on the monitored increment of the neutral axis depth; and (2) an index to evaluate longitudinal damage. The index is composed of the residual ratios of the equivalent flexural stiffness (HFM1) and the equivalent shear stiffness (HFM3). The neutral axis position and the proposed damage index can be determined using long-gauge Fiber Bragg Grating sensors. Results from numerical simulations show that the deviation between the HFM1 and the true value residual ratio of the equivalent flexural stiffness is no more than 1.7%. The HFM3 is equal to its true value in the entire damage process. A loading experiment for a scaled-down model of a shield tunnel using long-gauge Fiber Bragg Grating sensors indicated that the errors in the HFM1 were no more than 5.0% in the case of early damage development (HFM1 ≥ 0.5). The maximum error did not exceed 9.0% even under severe damage conditions in the model. Meanwhile, the HFM3 also coincided with its true value in the entire testing process.
Highlights
Due to its safety, high efficiency in construction, and low impact on the urban environment, many metro tunnels have been built by the shield-driven method to satisfy the increasing demand for public transportation in many metropolises
This study proposes a new damaged detection and evaluation method based on neutral axis depth (NAD) monitoring in the longitudinal direction for in-service shield tunnels
Based on the analysis of two deformation modes, three kinds of failure modes are assumed, including the bending failure mode, the shearing failure mode due to dislocation, and the shearing failure mode of the equivalent shear stiffness reduction, which resulted from the bolts being subjected to an excessive bending moment
Summary
High efficiency in construction, and low impact on the urban environment, many metro tunnels have been built by the shield-driven method to satisfy the increasing demand for public transportation in many metropolises. Because a shield tunnel can be viewed as approximate to a beam-like structure, it is possible for the NAD to be a useful tool in damage detection and evaluation in shield tunnels. Another challenge is to obtain the strain distribution across the joint accurately. A method for the determination of the NAD based on LFBG sensors is proposed in Section 3 for longitudinal damage detection in shield tunnels. LFBG sensors for a scaled-down shield tunnel model in Section 5 are carried out to verify the accuracy of the proposed methods for damage detection and evaluation in shield tunnels
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