GNGTS 2015 - Atti del 34° Convegno Nazionale

The Frassine River, to which the reconstructed embankment belongs, is one of the six segments into which the Guà River is divided. It flows between Borgo Frassine (Montagnana, PD) and Brancaglia (Este, PD), with a length of 13.42 km, and is part of the Brenta-Bacchiglione catchment. From a hydrological point of view, this river has a mixed regime that implies higher flow rates during spring and autumn and lower flow rates on the other seasons (Bondesan, 2001). The flooding event that caused the levee rupture took place in the early afternoon of November 1, 2010, when the water level of the Frassine river reached its historical peak (4.54 m) after a three-day heavy precipitation event that resulted in a cumulative rainfall amount of 300 mm (ARPAV, 2010). The collapse occurred on the right levee and had spread up to a total length of 100 m when a preliminary barrier with boulders of different diameters was built. Then, the positioning of several reinforced concrete bars on the levee outer side increased the structure stability, while the installation of sheet piles on the inner side allowed the stopping of the water flow. Finally, three days after the flood event, the whole preliminary structure was covered with aggregates (tout-venant), thus ending the first step of the embankment restoration. In order to provide a long-term solution, new reinforcement works from the embankment crest took place in 2012 (second step). They can be divided into three phases: i) concrete injection along the whole preliminary structure (110.0 m length, 9.5 m depth), ii) jet-grouting diaphragm construction, extended also to the undamaged levee (170.0 m total length, 22.0 m total depth) and iii) Tube a Manchette grouting to further waterproof the whole structure. Despite these actions to improve the embankment’s functionality, several water infiltration phenomena occurred during the following months, thus highlighting a lack of homogeneity within the new structure. More in detail, a more diffusive seepage process in the embankment toe marked the upstream sector, while water infiltration in the lower part of the outer face characterized the downstream portion. Therefore, nine geotechnical soundings were carried out in 2013: six of them analysed the reconstructed part, showing a rather complex domain, where the different reinforcing interventions overlap. Since they provide only punctual information, these soundings Fig. 1 – Map of all profiles on the Frassine site. The low-outer face SP profiles are not represented. 44 GNGTS 2015 S essione 3.2