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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Excluding the pressure transducers TP7 and TP8, which exhibited an anomalous trend (an increase too much higher and rapid probably due to a not correct sealing of the head of the borehole), during the first flood peak, the total head variations measured by the other transducers (Fig.2a) arrived to a maximum of about 1.5m, enough for overcoming the ground level in the field (208 m a.s.l.). Fig. 2b shows a significant 1.5°C rise in river water temperature during the initial flood phase, followed by a gradual decrease to a final temperature 2°C below the initial reading. Recorded temperature fluctuations were minimal for the sensors located deeper than 10 m from the ground level. Sensor TT10, located close to the river detected an important temperature decrease. Also TT2, located 9m below the countryside berm, present a decrease of about 1°C at the peak of the flood and subsequently a continuous downward trend. This could suggest movements of colder water from the river to the embankment body, potentially contributing to observed springs at the levee toe. Further investigation is warranted to confirm this hypothesis and assess its implications. 3.1. Site A: DFOS installed in horizontal trench In Fig. 3, the temperature distributions obtained by the DTS measurement in site A during three different reading campaigns are presented: two campaigns were conducted quite at the flood peak in the river (the exact instances are indicated in Fig. 2) and one during low water conditions, specifically on 24 th November 2023. The temperature distributions reported in Fig. 3 are obtained by linear interpolation of the measured data at depths of 0.5 / 1.0 / 1.8 m along the surveyed stretch. The mesh grid used for the interpolation consists of 325 measurement points on the horizontal axis, corresponding to a sampling space of 1 m along the DTS. Vertically, the interpolation is obtained on 13 points with an interspace of 0.1 m, covering depths from 0.5 m to 1.8 m. During the flooding (Fig. 3a and 3b), a cooling effect of approximately 1°C is observed close to the ground surface, probably related to the rain infiltration, but also in the central portion of the monitored span. The cooling in the centre corresponds to the position as the gravel lenses identified in the geotechnical investigation, where the main water inflows occur and here the temperature decreases following a similar trend compared to the river temperature. On the contrary, in conditions of low water levels in the river, a consistent variation in temperature (Fig. 3c) is observed as expected, gradually changing with depth along the entire surveyed stretch.

c)

Fig. 3. Temperature measured at the base of the embankment: a) 31/10/2023 17:51; b) 03/11/2023 11:07; c) 24/11/2023 10:11