# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 3 Development of FBG-Based Geohazard Monitoring Systems
### Installation and Protection Techniques  
One of the toughest tasks in utilizing FBG sensing system for field monitoring is how to install the sensors and protect them from physical damage. As FBG has inherent vulnerability, bare FBG sensors can hardly be applied to monitoring infrastructures without appropriate packaging methods [18]. Once a suitable protection measure is adopted, the FBG sensors should be calibrated, because any protection technique may affect the strain transfer efficiency from the fiber surface to the inner core [19; 20]. At present, until methods and standards for installing fiber optic sensors in the field are established and reliable, close collaboration of experienced geo-engineers and skilled optoelectronic engineers is necessary [21]. Existing packaging methods include gluing FBG to the target and embedding with adhesives [22; 23; 24; 25; 26], packaging FBG in pipes or on plates [27; 28; 29; 30; 25], or encapsulating FBG into composites or layered structures [20; 31; 32; 33; 34]. For field geohazard monitoring, the installation works can be extremely tricky since the optical fibers can hardly be adhered on geomaterials. Numerous studies have demonstrated the high adaptability of fiber-reinforced polymer (FRP) to FBG sensors. Therefore, it is wise to use FRP bars, tubes, of strips to make a sensor protection system in the field. Although the soil-embedded FBG sensors can capture the strain variations at different locations of geological structures, it is hard to directly bump them into in-situ soil masses. A common practice is to adhere the sensors on- or embed them in-soil reinforcements, such as geogrids, soil nails and anti-slide piles. As early as 1999, the use of glass fiber-reinforced polymer (GFRP) rock bolts with integrated FBG sensors for strain monitoring in a Switzerland tunnel was reported by [PERSON] et al. of the Swiss Federal Laboratories for Materials Testing and Research [31]. [PERSON] et al. successfully monitored strain and temperature fields of tunnels with FBG instrumented GFRP rock bolts [35]. [PERSON] et al. investigated the feasibility of smart geotechnical monitoring using FBG embedded in geotextiles [36]. Later on, the FBG sensors were substituted by fully distributed BOFDA fibers to make intelligent geosynthetics [24; 37]. Three optical fiber cables instrumented with an array of low reflectivity fiber gratings, with the same Bragg wavelength, were stitched onto a 1-m high geotextile by [PERSON] et al. [13]. The geotextile was not only capable of detecting minute deformation, but also provided a large resisting force against the landslide. [PERSON] et al. developed a smart geogrid embedded with FBG strain sensors [38; 39]. The sensor features were evaluated through finite element simulation and lab model tests. In the field study of [PERSON] et al. [25], painstaking precautions were taken to waterproof conventional strain gauges installed on GFRP soil nails to ensure their survivability. In comparison, there was no such need for FBG sensors but how to protect the FBG sensors from physical damage was difficult. In their field work, when the workers tried to insert the soil nails in the boreholes, some surface-adhered FBG sensors were damaged. Because of the complicated geological conditions and harsh installation processes, optical fiber cables and connectors used for transmitting monitoring data have to be well protected to prevent physical damage or signal loss. No-jacket-buffered optical fibers should be avoided for direct embedding into geomaterials. To reduce errors in obtained monitoring data, the coating of the cable should be correctly selected according to different site conditions and measurement requirements. Metals and plastics are two commonly used buffering materials. For geohazard monitoring, the cable diameter can vary from 0.25 to 10 mm. [PERSON] et al. declared that it is sufficient to adopt optical fiber cables with less than 2 mm diameter for an experienced geo-practitioner when performing manual integration [40]. In Canada, [PERSON] et al.