PSI - Issue 62

Luca Comegna et al. / Procedia Structural Integrity 62 (2024) 484–491 Luca Comegna/ Structural Integrity Procedia 00 (2019) 000 – 000

488

5

1 = 1 − 1

(1) where CWB 1 represents the monthly value of the Cumulative Water Balance (Freeze and Cheery, 1979), that is the difference between P 1 and the monthly value of the potential evapotranspiration PET 1 calculated using the formula proposed by Hamon (1961) 1 = 13.97 ∙ ∙ 2 ∙ (2) where d is equal to the number of days in a month, D is the average number of daylight hours in the month, and W t is the density of saturated water vapor expressed through the equation = 4.95 ∙ 0.016020∙ (3) As shown in Figure 3b, the CWB 1 values are featured by a fluctuation typical of the Mediterranean area: the balance is usually positive from October to April, showing peaks up to about +400 mm, while it results essentially negative during Summer, with minimum up to about -100 mm. 3. Monitoring results Information about the pore-water pressure regime have been provided by four hydraulic piezometers all installed within the parent formation: three open-tube piezometers, named P1, P2, and P3 in Figure 2b, and one Casagrande type piezometer, named P4 in Figure 2b. Piezometers P1 e P2 worked for about one year, from 2002 to 2003. Piezometer P3 was operational during about four years, 2015 to 2019. Piezometer P4 provided information from 2019 to 2020. Monitoring of movements began in 1998 after the installation of inclinometer G1 (Fig. 1b) that provided data until 2003 (Fig. 2c). Additional inclinometers were added at different following stages giving information from 2002 to 2003 (G2 in Figure 2c), and from 2015 to 2020 (G5, G6, G7, and G8 in Figure 2d). Field monitoring has been therefore affected by an overall lack of data for about 11 years (from 2004 to 2014) as a consequence of unexpected technical problems and a change in the local management of the area. Due to the large number of natural water springs recognised in the study area (Chiarini et al., 2008), the average piezometric levels were relatively high. In particular, as highlighted by Figure 2b, the minimum position of the water table corresponds to the original ground surface of the natural slope until the uppermost location of the embankment, and then it is essentially parallel to the top of the parent formation. The highest water table level was detected within the embankment at a distance of 2.0 m above the embankment/parent formation contact, and within the debris at a depth of 5.1 m above the debris/parent formation contact. Piezometers P3 and P4 measured the same pressure heads. As shown by Figure 4, the analysis of the longest working piezometer P3 highlighted that the seasonal oscillation of the water pressure heads, h w,P3 , was well correlated to the fluctuation of the potential hydrological balance over a five months time interval, as determined through the corresponding cumulative value CWB 5 . In particular, the most effective empirical relationship that overlaps with the monitored h w,P3 values can be expressed as ℎ , 3 [ ] = 3.5 + 0.0025 ∙ 5 [ ] (4) Figure 4a shows the h w,P3 variation calculated using equation (4) for the period spanning from 1991 to 2020. Based on this estimation, h w,P3 exhibited an average fluctuation of 2.5 meters, with recorded minimum values of approximately 3.0 meters and maximum values reaching around 5.5 meters. Concerning movements, the measurements allowed the identification of two continuous sliding surfaces: the shallowest one, about 80 m long, crossed the embankment and the debris, while the deepest one, about 120 m long, developed within the parent formation along a 20° inclined plane (Fig. 2a). All the movements were featured by a main NW-SE direction consistent with that of the natural slope. Having no direct evidence of exit points on the ground surface for both the surfaces, it is likely that the observed phenomena is the result of a local failure mechanism rather than of a global one (Comegna et al., 2023). The measured rates of displacement exhibit a variability, ranging between