## 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
[182] exploited a 100 km-long buried pipeline to detect horizontal and vertical landslide, yet simulated, employing another BOTDR-based system. Most of the examples discussed so far qualitatively detect landslide/soil movements rather than quantitatively assessing them. Despite the efforts made in more than one decade of research, the correlation of landslides dynamics to strain measured by optical fibre sensors is still not clear [183]. It is important to note that (assuming no slippage between cable and soil) due to the different mechanical behaviour of the fibre-optic cables and characteristic of the surrounding soil, the strain measured in the fibre-optic cable is not that of the soil [184]. Recent work by [PERSON] et al. [185]  
Figure 13: Installation scheme of a distributed strain sensing system for landslide monitoring proposed by [PERSON] et al. [173]. suggested that overburden pressure, density, and water content of the soil strongly affect the coupling between soil and fibre cable. The experiment was very basic and consisted of measuring the stress exerted on a 900 \(\mu\epsilon\) tight buffered fibre, 1 m-long, progressively pulled out from a small soil tank (\(150\times 20\times 20\) cm). The cable-soil coupling showed a \"brittle\" behaviour, as witnessed by the pull-out force vs. pull-out displacement curve. The curve initially followed a linear trend up to the point at which the fibre slipped in the sand due to the failure of shear friction among cable surface and soil. The pull-out force then dropped dramatically, and after the friction was failed, the pull-out displacement continued to increase at almost constant pull-out force. Additional tests carried out by the same authors [186] using a 2 mm-diameter cable showed a different response, with the soil-cable coupling behaving like a \"ductile\" bond. The curve pull-out force vs. displacement had the following trend: up to a peak, the pull-out force increased with the displacement, following a highly nonlinear behaviour. After the peak, the pull-out force remained almost constant with no drop of its value, despite the continuous increase of the displacement. The strains--measured using a BOTDA interrogator--initially emerged at the loading point for small pull-out displacement, and then propagated towards the far end of the fibre as the displacement increased. The authors concluded that failure of the fibre-soil interface was highly progressive during the deformation process of soil (Figure 14). Please note that the different behaviour reported in the works of [PERSON] et al. [185; 186] is likely due to the particular soil conditions and to the specific friction between cables and soil. The response of optical fibre cables to shear was also investigated many years before by installing the fibre transversally to the direction of slippage [169]. Again, the forces resulted to be redistributed: in practice, while the soil immediately at the shear interface slipped away, the stresses exerted on the cable were distributed over some distance that depends on the friction still present. Furthermore, each cable shows a specific cable mechanical transfer function [179; 187] that relates the strain in the host material to the strain exerted on the cable. It is indeed important to know or measure this function  
Figure 14: Results of pull-out experiments carried out by [PERSON] et al. [186]: (**a**) pull-out force vs. pull-out displacement; (**b**) pull-out force measured by fibre vs. load cell; (**c**) strain along the fibre vs. pull-out displacement; (**d**) pull-out force vs. percentage of the fibre detached from the soil. for proper data analysis and interpretation. For example, mechanical transfer functions of cables that redistribute the strain over large distances are broad functions. For these cables, both the spatial resolution of the employed optical technique and the extension of the mechanical transfer function determine the effective spatial resolution.