A smart inclinometer for monitoring the internal deformation of deep-seated landslides

Purpose: Very high landslide risks, related to complex phenomenologies involving both towns and strategic infrastructures, characterise numerous areas in the Southern Apennines in Italy. Understanding the landslide kinematics and monitoring its evolution is necessary to manage these risks, in order to keep people safe and to reduce the associated economic impacts. This is extremely important to reduce underinvestment that still affects these areas, which may – otherwise – be involved in Special Economic Zones able to attract both Domestic and Foreing Direct Investments in order to increase investments, employment, job creation, trade balance and, hence, many goals and targets of the 2030 Agenda for Sustainable Development. In presence of complex gravity-driven processes, inclinometer data remains one of the most useful information although it is characterized by low spatial resolution, and it’s a manual time-consuming activity. To overcome these limits, a New Smart Hybrid Transducer (NSHT), an innovative, distributed strain transducer, based on optical fiber sensing technology, has been developed (Minutolo et al., 2020) and its feasibility as smart inclinometer has been tested. Methods: Two smart inclinometers were installed in the study area of the San Nicola village, near the Cilento coast, to test the effectiveness of the new device to in-depth investigate the landslide mechanics. In the area, an active landslide system (Valiante et al., 2021) involves a superficial layer of landslide debris and a conglomeratic formation which extensively outcrops above the marl-clayey Mesozoic formation. The smart inclinometers were realized by disposing respectively two and four NSHTs along the outer surface of inclinometer tubes 40m long, so that traditional measurements can be performed in the meantime. The adopted sensing technique is based on the Brillouin scattering phenomena which allows detecting the changes of strain and temperature along the NSHT with a spatial resolution of 20cm and a sampling of 19 points/m. Results: Strain profiles retrieved by the smart inclinometers were consistent with inclinometer data revealing the main features of the slope movement. It also added information not always recognizable by conventional inclinometer monitoring as it reveals not only the horizontal component of the soil deformation, but also the vertical one. During winter 2021 a recorded constant strain profile indicated that the tube was subject to shortening and, hence, that it was suspended inside the landslide body. Indeed, the cumulative displacement profiles retrieved by conventional inclinometer, representative of an earthflow, did not given evidence of a sliding surface. During winter 2022 the smart inclinometer recorded increasing strain changes at three depths in the shallowest part of the conglomeratic formation, revealing the early formation of shallow sliding surfaces. Conclusions: The measures of more than one year show the feasibility of the NSHT in realizing a stand-alone smart inclinometer, demonstrating the advantage of near-distributed strain sensing over conventional displacement measuring techniques in accurate and long-term monitoring of subsurface deformation for complex landslides.