## 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  
Several distributed fibre-optic sensors have been proposed in the last 20 years for the measurement of strain and displacement in landslide monitoring, including many single-point sensors mimicking traditional devices [166] and integral interferometric sensors [167; 168]. In 2001, at the Public Works Research Institute (PWRI) in Japan, the BOTDR technique was applied to the monitoring of the Okimi Landslide (Niigata Prefecture) [169; 170]. In the same years, some proof-of-concept papers appeared about the application of OTDR schemes to landslide monitoring [171; 172]. A few years later, in 2007, other researchers from PWRI proposed an OTDR-based scheme to detect soil displacement at the surface of the Takisaka Landslide (Japan) [173; 174], yet with very limited spatial accuracy. The proposed setup was very basic but quite effective, as the fibre cables were simply anchored to the ground through stakes. In that way, opportune anchorage points can be chosen, also in the light of expected landslide dynamics (Figure 13). One of the main drawbacks of this kind of installation is that the cable is exposed to harsh conditions and rodents. Curiously, five years before, [PERSON] [175] had proposed a similar idea as a proof-of-concept (i.e., to monitor a landslide by attaching optical fibres to telephone poles) [176]. A BOTDA interrogator was used to monitor the St. Moritz landslide in Switzerland, employing a fibre cable buried in a road crossing the landslide, with a spatial resolution of a few metres. The fibre-equipped road acted like a long distributed strain meter, with sufficient spatial resolution to identify the landslide boundaries [177; 178]. The same site was then used to test a soil-embedded \"micro-anchor\" cable system. The system consisted of a compact metal-free cable and tridimensional \"micro-anchors\" mechanically clamped to the cable at regular intervals. The anchors provided bearing capacity in three perpendicular planes so as to prevent the relative slippage between the strain sensing cable and the surrounding soil. Furthermore, the anchors allowed the cable to be pre-strained during the installation [179]. Regarding the use of anchoring systems, it is important to highlight that DOFSs' performance may be affected by the use of such systems. In particular, the resolution may be reduced to a value corresponding to the distance between the anchors, as the cable may be simply pulled by the soil movement at the anchor points. From the same research group, [PERSON] et al. [180] proposed a field experiment using an 80 m-long tight buffered fibre measured over three years with a BOTDA-based system. The authors pointed out the importance of considering temperature and water-related strain cross-sensitivity. The measurements were in fact slightly affected by the variation of temperature and the degree of saturation that changed over time and space due to the soil heterogeneity. In addition to the intrinsic temperature cross-sensitivity of the optical technique used, the authors also highlighted the importance of cable materials, because they may induce additional strain due to thermal expansion and swelling. However, they concluded that these effects might be partially mitigated by the friction between the soil and the cable that somehow tightly confines the cable. Furthermore, it was suggested that long-term monitoring campaigns may be useful to get rid of these spurious environmental effects, whose magnitudes were in any case quite small. A BOTDR interrogator was also used to monitor the slow-moving Ripley landslide in Ashcroft, British Columbia, Canada [181] with the sensing cable anchored to the lock-block retaining wall. Curiously, the system worked for three months before being damaged by a black bear and the long exposures to sub-zero temperatures caused the cement (epoxy resin and caulking product) to fail so that it was no longer attached to the retaining wall. Following the approach of using nearby structures, [PERSON] et al.