## 3 Distributed Optical Fibre Sensing Applied to Geo-Hydrological Applications
### Slopes and Landslides Monitoring
#### 3.2.1 Distributed Strain Sensing for Slopes and Landslides Monitoring
The application of BOTDR to landslide monitoring where fibres were installed in soil nails, inclinometer tubes (in loop configuration), and frame beams was extensively investigated by  
Figure 16: **(a)** The layout of the sensing cable used in the slope model of Ref. [204]: **(b)** Details of the sensing cable with anchor-like elements; **(c)** Strain distribution along the different layers of the sensing cable during the slope failure (from the top right graph in clockwise order: layer H1; layer H2; layer H3; and peak strain of the segment C (reproduced with permission from [204]). [PERSON] et al. [207,208]. In 2008, the Aggenalm landslide (Bavarian Alps) was successfully monitored using a BOTDR interrogator (along with Raman OTDR for temperature) with a sensing cable deployed both on the surface of the upper part of the landslide in a 15-20 cm-depth trench and inside a 23 m-depth inclinometer borehole in a loop configuration. The loop configuration allowed the arrangement of the back and forth fibre spans at 90\({}^{\circ}\) around the borehole section to allow a rough estimation of the sliding direction. Additional anchoring tools were applied at 1-3 m of spacing on the fibre buried at the surface (Figure 17). In the same year, in a field trial in the Three Gorges reservoir, a polarisation-sensitive OTDR scheme was applied to determine the local soil stress via the measurement of distributed polarisation mode coupling in polarisation-maintaining fibre encased in stress transducer soil nails. The soil nails were intended to be installed as inclinometers, to cross the interface at which the landslide was occurring [209]. Ten centimetres of spatial resolution over a distance of 500 m was achieved. The concept was further investigated by replacing OTDR detection with polarisation-sensitive OFDR, with half of the original spatial resolution (5 cm) and a great enhancement of theoretical spatial range (up to 10 km) [210]. In the years that followed, other authors proposed optical fibre inclinometers based on distributed strain sensing whose performance was very close to that of a traditional inclinometer. Figure 18 shows the common principle of a fibre-optic inclinometer. Among the many examples, in 2011 [PERSON] et al. [211] presented an inclinometer realised out of a plastic tube where three fibres were installed longwise at 120\({}^{\circ}\), 240\({}^{\circ}\) and 360\({}^{\circ}\), respectively, and embedded in a detachable membrane to allow the inclinometer to be assembled at the construction site. Please note that at least three fibres are needed to map the tube deformation. That inclinometer was interrogated using a commercial OFDR interrogator, but the authors confirmed the feasibility of Brillouin-based approaches. Then, in 2012, the same authors further developed the concept, proposing a P  
Figure 17: Example of optical fibre cable deployment for slope monitoring using a BOTDR interrogator. After [207]. [PERSON] et al. [213] proposed and tested in the laboratory and in situ a 7.5 m-long BOTDA-based inclinometer with four fibres cemented longwise to a PVC tube at 90\({}^{\circ}\), 180\({}^{\circ}\), 270\({}^{\circ}\), and 360\({}^{\circ}\), respectively (Figure 19a). The inclinometer measurements were in good agreement with those of a standard device (Figure 19b), and it was also tested in situ. Eventually, [PERSON] et al. [214] proposed a very similar inclinometer but much longer (up to approx 40 m) and BOTDR-based. The sensor was successfully tested in the Majiagou landslide at the Three Gorges reservoir site with excellent performance. As shown in Figure 20a the device was capable of detecting two shear zones at around 12 m and 35 m of depth, corresponding to two sliding surfaces. The agreement with measurements by standard inclinometer was indeed excellent (see Figure 20b). Figure 19: **(a)** Optical fibre inclinometer based on a BOTDA scheme; **(b)** Displacement curves measured by the optical fibre inclinometer (solid lines) vs. dial gauges (squares). Reproduced with permission from [213].