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Health monitoring of ground movement via highly sensitive fiber optic sensors allows operators to detect early potential or ongoing failures in critical geotechnical structures. Particularly, the fiber optic sensors can be embedded in geosynthetics which are nowadays widely used in many geotechnical applications including earth dikes, railway embankments, landfill liners, quarries and mines. Thereby, such smart geosynthetics can be used for reinforcement, layer separation, filtration or drainage while the embedded fiber optic sensors provide information about the condition of the geotechnical structures in real time. The paper highlights the results achieved in this innovative field in the framework of several German and European projects. The presented measurement methods for long-term monitoring are based on Brillouin scattering in silica glass optical fibers (GOFs) and optical time domain reflectometry (OTDR) in polymer optical fibers (POFs).
Responding to a growing need for intelligent monitoring of critical geotechnical and civil infrastructures, new multifunctional geotextiles and geogrids with integrated optical fibers were developed and investigated within several research projects. Such smart two-dimensional geosynthetics intercorporating optical fibers as distributed sensors provide solutions both for soil reinforcement, erosion control, drainage or environmental protection and for cost-effective monitoring of critical mechanical deformations and temperature distribution in geotechnical and masonry structures at the same time. The integrated fiber optic sensors provide online information about the condition of the monitored structure and about the occurrence and localization of any damage or degradation for the purpose of preventing a total collapse. The optical fibers have a serious advantage over other kinds of sensors due to their fibrous nature, so they can be ideally processed like standard textile yarns.
The monitoring of extended geotechnical structures like dikes, dams, railways, embankments or slopes requires sensor technologies with measurement lengths of at least some hundred meters which can be realized by the use of silica fibers. However, the integration of silica fibers into geosynthetics during the manufacturing process experienced problems of sensor brittleness, low strain range not exceeding 2 % and bending-related attenuation increase optimized to the value of 1.5 dB/km. Unlike silica fibers, the integration of polymer optical fibers (POFs) into various geosynthetics was easily achieved free of bending losses enabling distributed high strain measurements up to 40 % using PMMA POFs. The first industrial product GEDISE based on a geogrid with integrated PMMA POFs is on the market.
Due to the five times lower attenuation of low-loss perfluorinated graded-index POFs (PFGI-POFs), compared to PMMA POFs, the measurement range can be extended to 500 m. The low-loss PFGI-POFs offer the possibility of implementation of Brillouin optical fiber frequency domain analysis (BOFDA) which will significantly improve the measurement accuracy and spatial resolution in comparison with existing POF-based distributed measurement methods like POF OTDR. The development of a distributed Brillouin system based on PFGI-POFs is in progress.
The topic of the presentation are distributed fiber optic measurement techniques that can be used for long-term monitoring of strain, temperature and radiation distribution in the radiation environment. The fiber optic sensors applied to the surface of waste packages provide detection and localization of cracks.