Utility Tunnel Settlement Monitoring Using Distributed Fiber Optic and Ground Penetrating Radar Technologies

Settlement and deformation of multi-purpose utility tunnels (MUTs) are critical factors affecting their structural integrity and service life; however, effective identification methods remain limited. This study proposes a comprehensive approach integrating Brillouin frequency domain analysis (BOFDA), fiber Bragg grating (FBG), and ground penetrating radar (GPR) technologies, which was successfully applied to an MUT comprising three tanks in Baiyin City, Gansu Province, China. BOFDA enables precise localization of settlement points, FBG-based dislocation meters facilitate posture recognition of the MUT, and GPR is employed for detailed analysis of settlement causes. The results indicate that MUT deformation primarily manifests as displacement at joint locations, supplemented by deformation of the tunnel structure itself. Rotation, even settlement, and uneven settlement were identified through three FBG-based dislocation meters installed on the top and side walls. The primary causes of MUT settlement include mudstone compression and collapse of loess.