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Traditional in-place inclinometers are widely used for monitoring displacements inside a slope. However, these traditional inclinometers have several disadvantages such as electromagnetic interference, signal loss for long distance transmission, gravity dependence and poor durability. Scholars have proposed fibre Bragg grating (FBG)-based in-place inclinometers which can overcome these limitations. However, all such in-place inclinometers are based on the relative displacement between two tips of the rigid segment, but the deformation of the segment is not considered in the calculation, which is not consistent with the actual condition. The error of measurement is due to linear superposition. According to classical indeterminate beam theory, the authors developed a new type of FBG-based in-place inclinometers where the deformation of the segment is coordinated with the casings installed in boreholes. This type of FBG-based in-place inclinometer has been fabricated and calibrated in the laboratory. The measured displacement data are in good agreement with theoretical results obtained using equations derived by the authors based on classical beam theory. After the laboratory calibration, a series of FBG-based in-place inclinometers have been installed in one vertical borehole in a slope in China. Monitoring data of all FBG wavelengths have been collected in rainy and dry seasons, and have been analysed in this paper. It has been found that the slope moves quickly in the rainy season and remains stable in the dry season.

In recent years, natural and anthropogenic geohazards have occured frequently all over the world, and field monitoring is becoming an increasingly important task to mitigate these risks. However, conventional geotechnical instrumentations for monitoring geohazards have a number of weaknesses, such as low accuracy, poor durability, and high sensitivity to environmental interferences. In this aspect, fiber Bragg grating (FBG), as a popular fiber optic sensing technology, has gained an explosive amount of attention. Based on this technology, quasi-distributed sensing systems have been established to perform real-time monitoring and early warning of landslides, debris flows, land subsidence, earth fissures and so on. In this paper, the recent research and development activities of applying FBG systems to monitor different types of geohazards, especially those triggered by human activities, are critically reviewed. The working principles of newly developed FBG sensors are briefly introduced, and their features are summarized. This is followed by a discussion of recent case studies and lessons learned, and some critical problems associated with field implementation of FBG-based monitoring systems. Finally the challenges and future trends in this research area are presented.

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Recently the distributed fibre optic strain sensing (DFOSS) technique has been applied to monitor deformations of various earth structures. However, the reliability of soil deformation measurements remains unclear. Here we present an integrated DFOSS- and photogrammetry-based test study on the deformation behaviour of a soil foundation model to highlight the role of strain sensing fibre–soil interface in DFOSS-based geotechnical monitoring. Then we investigate how the fibre–soil interfacial behaviour is influenced by environmental changes, and how the strain distribution along the fibre evolves during progressive interface failure. We observe that the fibre–soil interfacial bond is tightened and the measurement range of the fibre is extended under high densities or low water contents of soil. The plastic zone gradually occupies the whole fibre length when the soil deformation accumulates. Consequently, we derive a theoretical model to simulate the fibre–soil interfacial behaviour throughout the progressive failure process, which accords well with the experimental results. On this basis, we further propose that the reliability of measured strain can be determined by estimating the stress state of the fibre–soil interface. These findings may have important implications for interpreting and evaluating fibre optic strain measurements, and implementing reliable DFOSS-based geotechnical instrumentation.

In the past few years, fiber optic sensing technologies have played an increasingly important role in the health monitoring of civil infrastructures. These innovative sensing technologies have recently been successfully applied to the performance monitoring of a series of geotechnical structures. Fiber optic sensors have shown many unique advantages in comparison with conventional sensors, including immunity to electrical noise, higher precision and improved durability and embedding capabilities; fiber optic sensors are also smaller in size and lighter in weight. In order to explore the mechanism of seepage-induced slope instability, a small-scale 1 g model test of the soil slope has been performed in the laboratory. During the model’s construction, specially fabricated sensing fibers containing nine fiber Bragg grating (FBG) strain sensors connected in a series were horizontally and vertically embedded into the soil mass. The surcharge load was applied on the slope crest, and the groundwater level inside of the slope was subsequently varied using two water chambers installed besides the slope model. The fiber optic sensing data of the vertical and horizontal strains within the slope model were automatically recorded by an FBG interrogator and a computer during the test. The test results are presented and interpreted in detail. It is found that the gradually accumulated deformation of the slope model subjected to seepage can be accurately captured by the quasi-distributed FBG strain sensors. The test results also demonstrate that the slope stability is significantly affected by ground water seepage, which fits well with the results that were calculated using finite element and limit equilibrium methods. The relationship between the strain measurements and the safety factors is further analyzed, together with a discussion on the residual strains. The performance evaluation of a soil slope using fiber optic strain sensors is proved to be a potentially effective approach.

Recently fiber-optic sensing technologies have been applied for performance monitoring of geotechnical structures such as slopes, foundations, and retaining walls. However, the validity of measured data from soil-embedded optical fibers is strongly influenced by the properties of the interface between the sensing fiber and the soil mass. This paper presents a study of the interfacial properties of an optical fiber embedded in soil with an emphasis on the effect of overburden pressure. Laboratory pullout tests were conducted to investigate the load-deformation characteristics of a 0.9 mm tight-buffered optical fiber embedded in soil. Based on a tri-linear interfacial shear stress-displacement relationship, an analytical model was derived to describe the progressive pullout behavior of an optical fiber from soil matrix. A comparison between the experimental and predicted results verified the effectiveness of the proposed pullout model. The test results are further interpreted and discussed. It is found that the interfacial bond between an optical fiber and soil is prominently enhanced under high overburden pressures. The apparent coefficients of friction of the optical fiber/soil interface decrease as the overburden pressure increases, due to the restrained soil dilation around the optical fiber. Furthermore, to facilitate the analysis of strain measurement, three working states of a soil-embedded sensing fiber were defined in terms of two characteristic displacements.