Fiber grating-based deformation monitoring technology for island-tunnel structure of Hong Kong-Zhuhai-Macao Bridge

Objective This study investigates the effective structural deformation monitoring for island-tunnel connection of Hong Kong-Zhuhai-Macao Bridge immersed tunnel in long-term complex conditions, i.e., waves, settlement, and traffic loads. A high-precision, stable, and automated monitoring method is needed to ensure operational safety. Method This study developed three specialized sensors based on fiber Bragg grating (FBG) sensing technology, and constructed a distributed monitoring system. The system consists of FBG displacement sensor for 3D joint movement; FBG deformation sensing rod for tunnel vault settlement monitoring based on modified conjugate beam method; and FBG inclinometer for measuring the inclination of wave wall. The sensor design principle, temperature compensation scheme, and field installation were described. The monitoring network was deployed at key sections (E1 and E33) and on the east and west artificial islands wave walls. The system enabled automated data collection and remote transmission. Result FBG 3D displacement sensor has a sensitivity of 51.707 pm/mm within the range of ±25 mm; and the measurement accuracy reaches 0.01 mm. It operates stably for long term and successfully captures the strong correlation between axial displacement of pipe section joints and temperature change, i.e., range from 0 to 5 mm. While the transverse and vertical deformations are small, i.e., within ±0.5 mm. The 10-meter FBG deformation sensing rod reaches 0.01 mm measurement resolution and 0.63 mm laboratory calibration accuracy. The sensitivity of FBG inclinometer is 146 pm/°with the range of ±15°, and the accuracy is 0.01°. The field data, from March 11 to April 22, 2023, show the minimal wave wall deformation. The maximum settlement is only 0.63 mm, indicating structural stability. Conclusion FBG-based monitoring system effectively achieves long-term, synchronous, high-precision, multi-dimensional deformation monitoring. It targets critical locations of immersed tunnels, and provides reliable technical support for the structural safety and maintenance of Hong Kong-Zhuhai-Macao Bridge.