PSI - Issue 64

Nicola Fabbian et al. / Procedia Structural Integrity 64 (2024) 1649–1656 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1990s. Despite this intervention, moderate piping issues persisted, prompting further investigation into the effectiveness of the seepage cutoff wall. The geotechnical investigation involved a comprehensive examination of the river body and the underlying foundation soil, revealing crucial insights into the composition and characteristics of the embankment. Moreover, taking advantage of some interventions carried out in the meanwhile, an optical fiber cable was installed in a trench 1.8m-deep excavated at the toe of the embankment on the landside. Samples collected in the trench were subjected to laboratory analysis, with a primary focus on assessing grain-size variability beneath the embankment's landside toe. Although the upper half-meter of soil displayed relatively homogeneous characteristics, deeper layers exhibited the presence of gravel lenses concentrated in the middle of the levee segment. These permeable laminations may serve as preferential pathways for groundwater flow, potentially leading to landside sand boils and backward erosion piping. For the sake of brevity, the full geotechnical characterization is omitted: a detailed description was already reported in Cola et al. (2021). A combination of conventional and DTS monitoring systems was implemented. The conventional system comprised two cross sections instrumented with pressure (TP) and temperature (TT) transducers in order to furnish data essential for evaluating the barrier efficacy and examining seepage patterns in terms of groundwater pressure and temperature variations. At the same time a DTS was employed. Within a 350m-long trench situated at the levee toe in rural terrain, the DTS was installed at three different levels - approximately at a depth of 1.8m (Level 1), 1.0m (Level 2), and 0.5m (Level 3) - spanning an overall length of 1100m.

Fig. 1. Test site locations: maps from national to local scale.

In the flood event occurred in 2018 (Cola et al., 2021), the temperature variations recorded with temperature transducers and DTS were minimal and thus very difficult to be definitively interpreted. It was hypothesized that the shallow position of DTS was not optimal, as they were overly influenced by external seasonal and daily thermal variations. Furthermore, despite the efforts done to minimize disturbances by keeping the fibers in close contact with undisturbed soil, the installation process itself caused some soil disturbance that can reduce the reliability of measurements. To overcome these limitations, a new field test (test site B) was conducted in 2021. It comprises five boreholes positioned at the corners and the centre of a 20m-side square area. The central borehole, denoted as S3 and aligned with the central axis of the embankment, reaches a depth of approximately 30 m. In contrast, boreholes S1 and S2, located on the landside embankment berm, and S4 and S5, positioned on the waterside, reach depths of about 25 m to achieve a roughly equivalent elevation. The geotechnical characterization of site B involved a detailed laboratory investigation conducted also on undisturbed samples of granular soils, collected using the Gel-Push type TR technique, as described by Fabbian et al. (2023). Like for Site A, in Site B the embankment body is primarily constituted by fill material, consisting of poorly graded gravel with silt and sand or poorly graded sand with silt (GP GM or SP-SM). The foundation soil exhibits alternations of fine and coarse layers. No cut-off wall is present in Site