Long-Term Monitoring of a Tunnel in a Landslide Prone Area by Brillouin-Based Distributed Optical Fiber Sensors

Abstract

This paper shows the results of the monitoring of the deformations of a tunnel, carried out using a distributed optical fiber strain sensor based on stimulated Brillouin scattering. The artificial tunnel of the national railway crosses the accumulation zone of an active landslide, the Varco d’Izzo earthflow, in the southern Italian Apennines. Severely damaged by the landslide movements, the tunnel was demolished and rebuilt in 1992 as a reinforced concrete box flanked by two deep sheet pile walls. In order to detect the onset of potentially dangerous strains of the tunnel structure and follow their time trend, the internal deformations of the tunnel are also monitored by a distributed fiber-optic strain sensor since 2016. The results of the monitoring activity show that the deformation profiles are characterized by strain peaks in correspondence of the structural joints. Furthermore, the elongation of the fiber strands crossing the joints is consistent with the data derived by other measurement systems. Experiments revealed an increase in the time rate of the fiber deformation in the first and last part of the monitoring period when the inclinometers of the area also recorded an acceleration in the landslide movements.

Innovative Photonic Sensors for Safety and Security, Part III: Environment, Agriculture and Soil Monitoring

“…The second example is a long-term monitoring campaign carried out in a railway tunnel located in a landslide prone area [ 66 ]. For these tests, a portable Brillouin Optical Time-Domain Analysis (BOTDA) sensor unit was used, featuring a spatial resolution of 1 m. The sensor was employed to detect the strain distribution along the two sidewalls of a 200 m long railway tunnel (tunnel “Calabrese”), consisting of eight contiguous sectors separated by joints with irregular spacing.…”

“… ( a ) Crack on the tunnel wall; ( b ) parget swelling; ( c ) salt precipitate accumulation (from Ref. [ 66 ]). …”

“…The symbol “?” indicates that measurements could not be taken due to fiber failure, while the symbol “x” denotes fiber failure (from Ref. [ 66 ]).…”

Differential Deformation Identification of High-Speed Railway Substructures Based on Dynamic Inspection of Longitudinal Level

“…Traditional methods, on the basis of settlement monitoring pile, sedimentation plate, sedimentation cup, and leveling measurement [ 11 , 12 , 13 , 14 ], are widely used, but also blamed for the high cost, low efficiency and large manual workload. For the purpose of automatic monitoring, optical fiber sensors [ 15 ], such as FBG (fiber Bragg grating) sensor and BOTDR (Brillouin optical time domain reflectometry), are also adopted, but mainly for the settlement monitoring of the deep soil layer [ 11 ], and they are usually impractical for covering the whole range of a HSR line. In recent years, wide-area monitoring methods based on GNSS (global navigation satellite system) [ 16 , 17 ] and InSAR (interferometric synthetic aperture radar) [ 18 , 19 , 20 ] have attracted much attention from researchers and engineers.…”