Application of Distributed Optical Fiber Strain Measurement into Geotechnical Engineering Monitoring

The characteristics of geotechnical engineering related structure such as tunnels, foundations and retaining walls are as follows: (1) The structure is normally buried underground and visual inspection of the health condition is not possible in most cases. (2) Due to the complicated geological and hydrological conditions, a number of uncertainties are involved in the design and construction of geotechnical structures. (3) Some of the geotechnical structures are super long and large. Therefore, the requirements of real-time and long-distance monitoring of these geo-structures are of great importance. Some key geo-structures such as large-span tunnel, underground facilities and foundations require the measurement accuracy in the order of micron or millimeter. Hence the conventional monitoring techniques cannot meet the demands for geo-structure health monitoring. Brilliouin Optical Time Domain Reflectometry/Analysis (BOTDR/A) is a fully distributed measurement technique, which utilizes Brilliouin spectroscopy and Optical Time Domain Reflectometer/Analyzer to measure strains generated in optical fibers as distributed in the longitudinal direction. Fiber Bragg Grating (FBG) is a qusai-distributed monitoring technique, which is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. The reflected wavelength is proportional to the applied strain and temperature and the FBG sensor can therefore be used as a strain or temperature sensor. Because of the distinctive characteristics of the distributed optical fiber sensing technologies, they have been paid more and more attention and began to be used to construct monitoring systems of various geo-structures such as tunnels, embankments and soil nailed slopes. In this paper, the operation principles and merits of BOTDR/A and FBG are introduced; their application feasibility on the geo-structure health monitoring are analyzed based on the authors' research achievements and application cases; the related topics about application of BOTDR/A and FBG are discussed in detail, which include the installation methods of optical fiber, the impact of distance, temperature and humidity resolution on measurement results, the fatigue effect of optical fiber, the conversion from strain to deformation and the establishment of intelligent monitoring system. The research results show that the distributed optical fiber sensing technologies are quite applicable to the geo-structure health monitoring.