PSI - Issue 64

Lorenzo Brezzi et al. / Procedia Structural Integrity 64 (2024) 1589–1596 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3. Composite Anchors: a) project configuration; b) Anchors n.1 (A1) monitored with DFOS after installation; c) Side overview at the end of second phase. 3. Monitoring campaign and preliminary results The installation of the 42 anchors required some months due to the frequent rainfalls that slowed down the operations in site. Consequently, anchor A1 was realized on 5/10/2023, while anchors A2 and A3 were installed only on 27 /10/2023. No DFOS readings could be conducted during the second day of installation due to the detection of damage to the fibers' joints. The fibers were measured on the date 5/10/2023 (A1); 14/12/2023 (A1, A2); 18/01/2024 (A1, A2, A3); 7/02/2024 (A1, A2, A3); 21/03/2024 (A1, A2, A3) and, as of today, they are still operative. During these measurement campaigns, damages to the optical fibers were discovered over time. For this reason, no temperature compensation was applied, due to absence of some temperature measurements. Outcomes have been calculated from the Brillouin Spectral Shift (BSF) measured in the cables and employing the temperature and strain coefficient provided by the cables’ manufacturer. As mentioned above, the temperature is exclusively measured by the cable integrated into the anchor A1, but is not reported here for lack of space. Before commenting on the deformation monitoring results obtained with DFOS, it is appropriate to evaluate the landslide activity during the period. In this regard, it was deemed appropriate to present the measurements obtained from three automatic inclinometers chain that were installed close to the A1, A2 and A3 locations, named S2, S4 and S5, respectively (Fig. 3). S2 monitoring unit is composed by 3 sensors, positioned at different depth. Similarly, S4 and S5 are furnished with 5 measuring units, each. The displacements were obtained assuming that, for each available angle measurement, a segment of 1.0 meter would rotate by that angle. The resulting displacement was thus calculated considering this rotation in both directions (N-S and E-W), allowing for a single continuous measurement for each sensor over time (Fig. 4). Between 10/2023 and 3/2024, S2 exhibited minimal displacements in the deeper layer, whereas the shallower layer indicated some instability. S4 displayed overall movement during the initial 2 months (October and November 2023), followed by a consistent trend thereafter. Lastly, S5 indicated relevant displacements concentrated in October-November 2023 and February-March 2024, particularly at depths of 8.0 m and 10.0 m. Overall, the data indicate that there were no significant instabilities in the landslide during the examined period, only some localized ground movements. Therefore, a full activation of the passive anchors is not expected yet. It should be noted that in October, during the installation phases of the anchors near inclinometer S4, the bar struck the inclinometer casing, undoubtedly altering its inclination. It cannot be ruled out that the same incident also occurred near S5. Fig. 5 and 6 show strains measured in the monitored anchors during the measurement sessions. An initial measurement was performed on the 5/10/2023 only for anchor A1, while the cementitious mortar was still setting. Although this reference measurement may be subject to the effects of mortar hardening, resulting in significant thermal variations, it was decided to include this measurement precisely to emphasize how the choice of the reference