# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 3 Development of FBG-Based Geohazard Monitoring Systems
### Development of FBG Displacement Sensors  
Monitoring of large strain and displacement is a crucial part for geohazard monitoring. Although the FBG sensors developed for displacement monitoring generally have satisfactory accuracy owing to the intrinsic characteristics of the sensing principle, the measuring range is a bottleneck problem. The ultimate strain of silica optical fibers is merely around 5% [45], which significantly limits the wide application of this technology. The polymer optical fiber (POF) can overcome this problem, since its ultimate strain is about 40% [24]. However, FBG can only be fabricated on a silica fiber. Recently, some FBG sensors are developed for displacement monitoring in boreholes. [PERSON] and [PERSON] designed an FBG extensometer to monitor rock deformations in tunnels, which was embedded in GFR rock bolts [46]. [PERSON] and [PERSON] introduced the application of FBG settlement tubes to measure the vertical profile of settlement in boreholes under a mat foundation [12]. In this sensor system, a pre-tensioned FBG is packaged in a plastic tube, which is connected to a stainless steel spring. The measured strain \(\epsilon\) is thus proportional to the tube compression \(S\), i.e., [12]:  
\[S=\left(\frac{\pi dtE}{k}+l\right)\epsilon=\left(\frac{\pi dtE}{k}+l\right) \frac{\left(\Delta\lambda_{B}-\Delta\lambda_{B}{}^{T}\right)}{\epsilon_{e} \lambda_{B0}} \tag{7}\]  
where \(E\), \(d\), \(l\) and \(t\) are the Young's modulus, outer diameter, length and thickness of the tube, respectively; \(k\) is the spring coefficient; \(\Delta\lambda_{B}\) and \(\Delta\lambda_{B}{}^{T}\) are the shifts in Bragg wavelength of the FBG strain sensor and an FBG temperature compensation sensor. The conventional inclinometer has been widely utilized to monitor slope displacements and ground movements[PERSON] However, long-term use of inclinometers is impractical due to the disadvantages of low accuracy, poor durability, complex operation, and high costs[PERSON] In recent years, several studies of in-place slope displacement monitoring using FBG-based techniques have been reported [16; 47; 48; 49; 50; 15]. Figure 4 shows the typical structures of FBG in-place inclinometers. The combination of conventional inclinometers and the FBG technology effectively improves the measurement range of displacement. In Japan, [PERSON] et al. developed the first generation of FBG-based borehole inclinometer for slope deformation monitoring following the principle of the conventional deflectometer [47] (Figure 4a). This type of inclinometer is composed of a series of rigid pipes connected to each other with elastic joints. Bending of the joints is measured by FBGs [PERSON] on the elastic joints. The inclinometer was installed in boreholes for deformation monitoring of an artificial slope under construction. Late on, a borehole multiple deformation sensing system was developed by [PERSON] et al. using internal FBG sensor units [2]. When ground displacement occurs, the FBG-instrumented plate element will bend accordingly and the FBG readings can be self-temperature compensated. This sensing system was successfully used for ground deformation monitoring of a tunnel in Japan. Figure 4: _Cont._Another FBG segmented deflectometer was invented by [PERSON] et al. [16]. The deflectometer is designed to be inserted into the conventional inclinometer casing installed in vertical boreholes to measure the relative deflection between the segments of the inclinometer casing (Figure 4b). Laboratory  
Figure 4: Schematic illustration of typical structures of FBG-based inclinometers. (**a**) FBG borehole inclinometer [47]; (**b**) FBG segmented deflectometer [16]; (**c**) FBG in-place inclinometer [15]; (**d**) New-generation FBG in-place inclinometer [45]. and field experiments were conducted to verify the effectiveness of the FBG system. The test results were consistent with those of a conventional inclinometer probe, indicating that the system could be used for ground movement monitoring.