## 3 Fibre-Optic-Based Smart Geosynthetics
### Production Processes and Applications of Fibre-Optic-Based Geosynthetics  
A suitable production or encapsulation method is one of the most important prerequisites for protecting OF sensors and generating essential coupling between geotechnical structures and sensors [64; 65; 66]. However, packaging or encapsulation may induce intricate mechanisms for transferring strain and stress from host materials to the OF core [66]. Indeed, comparatively investigating common production and encapsulation techniques elucidates the primary challenges of using OF-based geosynthetics to monitor geotechnical health and the basic perspectives to develop other smart geosynthetics. Figure 12 illustrates a cross-sectional schematic of a sequence of optical fibres packed into a geotextile wherein OF sensors are incorporated into and adhered to geotextiles using epoxy resin and then coated with another geotextile layer to form a classic \"sandwich\" structure. This form of packaging can be performed swiftly in the field [67]. Figure 11: DOFS structures applied to slope monitoring: (**a**) rippled metal-sheathed, (**b**) metal-reinforced single core, and (**c**) aluminium-packaged fibre-reinforced cables. Figure 10: Cross-sections of OF-based sensors: (**a**) tight-buffered strain sensor, (**b**) loose-tube LSOH (low smoke zero halogen) cable for temperature detection, (**c**) 900 \(\upmu\)m fibre, and (**d**) connecting cable. A schematic of an OF-sensor-based geosynthetic fabric at a settlement test trench is shown in Figure 13. In this method, a system consisting of multiple OF cables was connected to the sensing fibre chains at both ends of the measurement sensing unit. A large box was separated into two portions to simulate soil settling such that one part could be quickly set up and withdrawn following the test requirements [68; 69]. The fabricated OF-sensor-based geosynthetic was implanted between 3 and 20 cm thick sand and gravel-sand composite layers, respectively. This monitoring system is like those used to monitor railway embankments with sensor-equipped geosynthetics [70] and reinforce slopes under surcharge loads with OF-based geogrids [71]. In the latter monitoring investigations, 200 m tight-buffer-covered and 900 m tight-buffer-jacketed OF sensors were encapsulated and fastened to the geosynthetic using epoxy resin. OF-sensor-based geosynthetics effectively sense strain distributed over long distances and can, thus, be integrated into geomaterials to predict landslides around dams or dikes [70]. A schematic of a typical OF-based geosynthetic monitoring system placed at the landslide foot of a dike is shown in Figure 14. To monitor geost textile undimensional strain fields, distributed OF sensors were utilised, and this method outperformed current landslide monitoring methods. In this system, OF sensors were integrated into a geostatile by a series of warp-knitted fixation positions between the fabric and OF sensors. While the OF sensors are installed into the geostatile, a minimum bending diameter of 10 mm should be avoided to protect the optical fibre sensor from damage in the field. In addition, suitable >>3 mm protective coating materials were used for this purpose. OF sensors can also be incorporated into various fabric elements such as braided bars, fibre-reinforced polymers (FRPs), and glass-fibre-reinforced polymer (GFRP) bars for field applications [72; 73; 74]. Figure 15a,b show schematics of the OF-sensor-based FRP anchor and GFRP bars, respectively. Although this production and encapsulation method is relatively structurally simple, it includes a complex fabrication process [75; 76]. The FRP fibre and OF sensor were first braided together and then fed through pre-drilled holes in a dividing plate. Figure 12: Cross-sectional schematic of interior geotextile OF packing. Figure 13: Schematic of OF-sensor-based geosynthetic fabric for monitoring soil settlement.