Diffuse optical fiber sensing: geotechnical application in real case studies for structural and landslide monitoring

This thesis work is part of the research field of geotechnical monitoring by means of fibre-optic reflectometry, an innovative measurement system that exploits the behaviour of light as the structure of the medium it passes through changes in mechanical stress or temperature. Compared to the systems traditionally used in this field, fibre-optic measurement has several advantages, including the possibility of obtaining spatially dense data, short measurement times, high resolution over long distances covered and high application flexibility. However, it still needs further field trials in order to be validated. The proposed research focused on the analysis of three case studies, all of them located in Italy, the main objective of which is to validate the measurements obtained through fibre-optic systems. The first consists of the Ponte Cola dam in Santa Maria Valvestino, in the province of Brescia. This structure is equipped with both traditional monitoring systems designed to measure its seasonal structural deformations and a fibre-optic measurement system located along the crest. The fibre-optic data were previously processed to obtain measurements in the form of deformation and temperature; subsequently, the displacements of fixed points of the crowning were calculated by means of these data and the measurements obtained from a topographic system. At the same time, measurements obtained through a series of dilatometers placed in the joints between the segments of the dam body were also taken into account, and similar displacements were calculated from the optical fibre signal included in the positions of these joints. The validation of the fibre-optic monitoring system was then developed by comparing the results achieved through its measurements with those obtained from the topographic system and the manual dilatometers. The second and third case studies consist of two landslides, the first located in Recoaro Mille, in the province of Vicenza, also known as the Fantoni landslide, and the second located in Borgata Paradisi, from which it takes its name, in the province of Belluno. Both sites were equipped with a series of mitigation and containment measures to reduce the sliding force, including floating anchors. In the case of the Fantoni landslide, six anchors were installed, all instrumented with fibre optics positioned along the rods, while in the case of the Paradisi landslide, 56 anchors were installed, of which only three were instrumented. For both sites, an analysis and processing algorithm was developed to align the traces measured on different dates, to eliminate superfluous portions and to convert them into mechanical deformation and temperature data. Finally, these data were further processed to obtain strain profiles in millimetres. The validation of the data was then carried out through the analysis of the results and their interpretation in relation to the expected behaviour of the physical phenomenon. The second objective of this work concerns the last two case studies, which represent phenomena that are still in progress. From here, the need for further measurements to be taken in the near future emerges, so an algorithm was developed that allows for faster processing and the addition of new measurements to the previous ones. This is also being carried out with a view to creating an algorithm that can be easily adapted for other case studies, since the promising results reported in the scientific literature and the ongoing investments point to an expansion of the application of fibre-optic monitoring.