## 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
In those experiments, BOTDA was used to measure the strain exerted in a simple tight-buffered cable by a rainfall-induced landslide, with a spatial resolution of some tens of centimetres and measurement time of some minutes (up to 2 m and 15 min, respectively, in [195]). To avoid any relative slippage between soil and fibre, the system was then upgraded by cementing small pieces of plastic geogrids with a diameter of 2.5 cm to the fibre, located at regular intervals of 25 cm [200]. Please note that the distance among anchors was smaller than the spatial resolution of the interrogator, and therefore the resolution was not compromised. A similar solution was previously adopted by other authors [151]. Similar tests were carried out by [PERSON] et al. [201,202] and [PERSON] et al. [203], who experimented by embedding a 2 mm polyurethane, tight buffered cable opportunely manufactured for landslide monitoring into a small-scale physical model. Again, to further improve cable-soil coupling, 4 mm-thick shrinkable tubes were applied along the fibre at the regular interval of 20 cm (Figure 16b). The fibres were deployed in both horizontal and vertical layouts, and some portions of redundant fibres (2 m long) were used for temperature compensations and precise spatial localisation (Figure 16a). In the experiment discussed by [PERSON] et al. [204], an additional load was exerted utilising a hydraulic jack, and the slope was cut at the base to induce failure. A maximum strain value of 8000 \(\upmu\) was recorded during the test (Figure 16c), and the authors highlighted some discrepancy imputed to boundary conditions. Remarkably, physical model experiments are prone to be impaired by scale problems and boundary issues related to the setup that, despite the effort, are difficult to avoid. In this regard, high-resolution DOFSs--like OFDR-based ones--have more chances to highlight these issues and can therefore support mitigation actions or proper data interpretation. An experiment in a larger-scale physical model (\(6\times 2\times 3.5\) m) instrumented with 30 m of fibre cable driven to collapse by artificial rainfall was presented recently [205]. In the test, a commercial cable with corrugated outer sheath [206] was used to promote efficient coupling with the soil; a high-resolution OFDR interrogator allowed 10 mm spatial resolution at a measurement time of a few seconds. The fibre was installed in meanders at shallow depth, and overall resolution was 10 mm down the slope and 50 cm orthogonal to the slope direction. The landslide was triggered by a system of sprinklers, and temperature was compensated by measuring the Rayleigh shift on a strain-free portion  
Figure 15: **Above**: geometry and parameters of a numerical model aimed at investigating the effect of the fibre cable on soil strain field; **Below**: comparison of numerical solutions obtained with and without fibre–soil interaction according to two models (reproduced with permission from [190]). of the cable. For the first time, a very detailed map in space and time of the evolution of the landslide was produced with relevant insights about the failure process and the cable-soil coupling effectiveness. As the reader may have noticed, most of the examples presented so far regarding distributed fibre optic strain sensing for landslide monitoring in real sites or physical models deal with the installation of the fibre cables buried in shallow trenches or anchored above the slope surface. Therefore, only superficial relative ground motion can be directly observed. If a large portion of the slope surface slips rigidly due to a deep ground motion, these systems may observe significant strain only if the fibre crosses the superficial boundary of the landslide. Deeper ground motions of natural soil slope must be addressed by vertical fibre deployment, as for vertical shear zone measurements by inclinometers.