## 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
For most of the applications and interrogators presented here, the optical spatial resolution is fairly larger than the extension of the mechanical transfer functions of the cables employed for sensing; therefore, the effective spatial resolution of the system is most of the time that of the interrogator. The only exception is represented by OFDR-based interrogators, for which this may not always be true. About the capability of cables to follow the soil deformations, [PERSON] et al. [188] investigated the behaviour of cables and geotextiles and showed that the strain measured by single optical fibre cables with a BOTDR system is directly correlated to soil mass movement, with better correlation than that obtained by optical fibres integrated into geotextiles and geogrids. Nonetheless, as demonstrated in structural health monitoring, geotextiles may still provide sufficient information for warning and safety assessment purposes [189]. A further confirmation to the above argument was provided by [PERSON] et al., according to whom fibre-optic cables exhibit high flexibility in comparison to common soil [190]. Please note that the term flexibility was used there instead of a more specific terminology that should refer to stiffness and Young's modulus. Common values of stiffness for cables used in the monitoring of soil movements--either in situ or in laboratories--range from 0.9 kN for 900 um tight buffered cable to several tens of kN for armoured cable (with steel tube and outer sheath) [191]. The corresponding Young's modulus can be up to several GPa for very stiff cables, while typical values of longitudinal stiffness for a uniform soil range from 7 to 320 MPa [192]. This paper represented a significant contribution to the field because it tried to answer a key question; that is, whether the sensor does affect the system to be monitored or not, and if so, how much. [PERSON] et al. developed an analytical model and showed that soil displacement is minimally affected by any cable whose stiffness is in the range above, concluding that \"a horizontally laid fibre will follow the soil, regardless of its type\" (Figure 15). Remarkably, the model was successfully supported by BOTDR measurements. Although that work pertained to tunnels excavation and the soil was not very loose (from a geotechnical point of view), we may observe that the model still holds for more loose uniform soils, at least up to early ruptures. [PERSON] also highlighted that OFDR systems with high spatial resolution and fast sampling are better candidates if compared to BOTDA schemes; however, both technologies were more sensitive than laser-based displacement systems [193]. Regarding the general effectiveness of distributed fibre optic strain sensing in timely alerting of slope failure, the debate in the community is still open. According to [PERSON] et al. [194], a common situation in which optical fibres may be effective is that of steep slopes covered by unsaturated granular soils, whose collapse due to water infiltration is rainfall-driven. From a geotechnical point of view, the failure is caused by a reduction in the cohesion due to a decrease of suction. For loose soil attaining full saturation, liquefaction occurs due to volumetric collapse. In that case, the slope fails quite rapidly and a real-time monitoring even with the fibres buried at shallow depths in the same direction of the slope or parallel trenches normal to the slope profile can be effective in detecting precursory signals of failure. The same principle does not apply to dense soil that does not liquefy and for which pre-failure deformation is very modest. The issue was thoroughly tested in a series of experiments carried out by the same group of researchers in the last years [195; 196; 197; 198; 199]. The setup consisted of a small-scale physical model (1.35 \(\times\) 0.5 \(\times\) 0.1 m of volcanic ash) whose collapse was driven by artificial precipitation.