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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).
The distributed measurement of relative humidity is a sought-after capability for a wide range of applications in civil engineering and structural health monitoring. We show that polymethyl methacrylate (PMMA) optical fi-bers can be employed as a sensor medium to conduct distributed humidity measurement by analyzing Rayleigh backscattering traces obtained by OTDR. We make use of the effect that water penetrates the fiber core and directly influences the local fiber attenuation and Rayleigh backscatter coefficient. We conducted distributed backscattering analysis for two different pulse wavelengths: 500 nm and 650 nm. The 650 nm results are susceptible to both, attenuation changes and backscatter changes, whereas backscatter results at 500 nm are not affected by humidity-induced attenuation and only exhibit a change of Rayleigh backscattered power as a function of humidity. The combined measurement and analysis of both parameters at these two wavelengths has the advantage that cross-sensitivities on backscatter change and attenuation, such as strain and tempera-ture changes, could be separated from the humidity response of the fiber. We present laboratory results for a humidity range between 30% and 90% for both pulse wavelengths: including step responses, humidity cycles and hysteresis analysis. In addition to the attenuation and backscatter coefficient dependence, we also analyze optical runtime changes as a function of humidity. POFs have the advantage that they can be directly embed-ded into materials such as concrete or soil to measure water content or localize water ingress. Standard step-index PMMA POFs can be used as a distributed relative humidity sensor up to 200 m distance.