# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 4 Current Status of Applying FBG Systems for Geohazard Monitoring
### Slope Stability and Landslide Monitoring  
#### 4.1.1 Performance Monitoring of Slope Reinforcements  
A number of slope stabilization methods have been used in engineering practices, such as geosynthetics, anchors, soil nails, side-resistant piles, and retaining walls. As previously mentioned, the bare FBG sensors are fragile in strength. Due to the magnitude of potential geohazards and chaotic nature of construction sites, the sensors are easily damaged. If they are installed on the slope reinforcing members, adequate protection of the sensors can be realized. More importantly, the FBG measurements can help evaluate the slope stability condition and even forecast potential disasters. Comparing with electrical resistance and vibrating-wire strain gauges, FBG sensors can be quasi-distributed along the slope reinforcements, so that the distribution of axial force, shear force and bending moment can be obtained.  
[PERSON] et al. [66] and [PERSON] et al. [67] introduced the use of FBG sensors for mine monitoring in Germany. For an anchor bolt, the strain sensing fiber was glued in a groove and covered by special epoxy resin, with a fiber for temperature compensation loosely laid on the other side of the anchor bolt. They suggested to install the sensing fiber on the reinforcing steel bars inside anti-sliding piles, shaped in the form of the letter U. Monitoring of slope displacements could then be made. Another case was reported by [PERSON] and [PERSON] [68], who employed 50 FBG-instrumented anchor bolts to monitor the Xiaolongtan slope in Yunnan, China.  
[PERSON] et al. successfully monitored strain distributions along soil nails and soldier piles using FBG sensing systems in Hong Kong [15,25]. They evaluated the performance of two GFR soil nails during field pullout tests based on the monitoring results of quasi-distributed FBG sensor arrays [25]. The surface-glued and tube-packaged FBG sensors were installed along the soil nails for axial strain monitoring, together with several strain gauges. The monitoring results show that, compared with strain gauges, the FBG sensors can perform accurate strain measurement on multiple locations, which  
Figure 6: Schematic illustration of geohazards and infrastructure monitoring using FOS technologies.  
clearly depicted an approximately linear strain distribution with respect to the nail length during the pullout tests. As the observed maximum strain was over 7000 microstrain, the GFRP soil nail was found to be a highly extensible soil reinforcement, which requires additional displacement-based criteria to limit the allowable pullout resistance.  
On the Luk Keng roadside slope when upgrading works were being implemented, a 14 m-long soil nail was instrumented with 10 FBG strain sensors and 10 corresponding FBG tube packaged temperature sensors [15]. The soil nail was then inserted into the drillhole and grouted with cement grout. An H-shaped steel column instrumented with 18 FBG strain sensors and nine temperature sensors was grouted in a 16 m-long borehole to form a soldier pile. During installation, PVC trunkings were used to protect the optical fiber cables. The strain variations in the soil nail and the soldier pile with monthly rainfall were compared to each other (see Figure 7). It was seen that the soil nail strains were in general consistent with monthly rainfall, but such a relationship was not found for the soldier pile, which was not been clearly documented in previous works.  
[PERSON] et al. installed an integrated fiber optic sensing system to monitor the performance of a lock-block retaining wall on the Ripley Landslide in Canada [41; 42]. This is a retrogressive translational landslide that have been active since 1951. Two FBG sensors were fixed on the retaining wall, together with surface-glued BOTDR cables. Three months of monitoring data show there was no obvious displacement, indicating the retaining wall was stable.