PSI - Issue 44

Andrea Ciancimino et al. / Procedia Structural Integrity 44 (2023) 323–330 Andrea Ciancimino et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction

When subjected to dynamic loading, earth dams tend to accumulate permanent crest settlement and displacements due to soil densification and earthquake-induced sliding instabilities. Such permanent settlement may in turn lead to the reduction of the freeboard, compromising also the static stability of the embankment in the aftermath of seismic events. The evaluation of the seismic vulnerability of earth dams is therefore a major issue in high seismicity regions. Consequently, several studies have been devoted in the past years to the assessment of the seismic response of earth dams (e.g. Makdisi and Seed, 1978, Seed et al., 1979). The Italian National Code (Ministero delle Infrastrutture e dei Trasporti, 2014) suggests, as a good practice, employing different methods for the seismic vulnerability assessment of earth dams in compliance with the principle of gradualness. These approaches, ranging from very simplified to highly sophisticated methods, include pseudo-static numerical simulations, simplified empirical relationships, displacement-based approaches derived from Newmark’s rigid block method (Newmark, 1965), and advanced dynamic numerical simulations. As reported by Seed et al. (1979) based on some case histories, reliable seismic vulnerability analyses of earth dams require a deep knowledge of the structure (e.g., geometry, mechanical properties of the embankment and foundation, etc.). Therefore, advanced dynamic simulations can be performed only for a few specific situations. This is, for instance, the case of large dams (i.e. dams with a height >15 m or/and a reservoir volume >10 6 m 3 ) for which comprehensive geophysical and geotechnical characterizations are usually available. Conversely, in absence of a detailed geotechnical model, only simplified methods are suitable to assess the seismic vulnerability of the structure. Such condition is very common when small earth dams are considered. Such structures, although characterized by reduced height and reservoir volume, can be associated with a high seismic risk due to their proximity to highly populated areas and usually lack proper geotechnical information. As a consequence, a first-level screening is usually carried out by using empirical relationships to identify the small dams for which more refined analyses are required. The main goal of the “ReSba” (Resilience of Dams) project, funded by the European fund for regional development (Interreg-ALCOTRA), was to improve the knowledge of natural risks associated with dams located along the French Italian Alps area. A simplified methodology, briefly summarized in the following, was first developed within the project to identify critical embankments associated with high seismic risk (Cosentini et al., 2020). The developed approach was then applied to select some small earth dams classified from medium to high seismic risk, that have been subject to extensive geophysical and geotechnical characterization (Cosentini et al., 2021). Finally, advanced dynamic analyses were performed to analyze the seismic performance of two of them, namely the Arignano and Briaglia dams. The paper presents the results of the fully coupled analyses in terms of expected crest settlement concerning the associated seismic hazard. The results of the simulations are compared to predictions from simplified empirical relationships for testing their reliability in predicting the seismic response of small earth dams. 2. Selection of the case studies The last survey promoted by ISPRA – “Institute for Environmental Protection and Environmental Research” highlighted that there are 533 large dams and 10ú642 small dams in Italy (ISPRA , 2019). More than 900 small dams are located in the Piedmont region; approximately 100 of them are located close to populated areas of the French Italian Alps area (Fig. 1). Despite their limited size, the risk associated with the potential failure of small dams is therefore significant. Nevertheless, these structures often lack proper characterization of the dam and foundation materials. A prioritization criterion is therefore needed to identify the critical structures for which further analyses are required. The simplified approach developed within the ReSba project is based on the compilation of data sheets that lead to a preliminary classification of the earth dams (Cosentini et al., 2020). The seismic vulnerability of the reservoir is defined through an index V structure associated with the ultimate limit state of the dam. Such index is correlated to different vulnerability levels as shown in Table 1.