خاکریز آسیب دیده در جریان زلزله
Abstract
1. Introduction
2. The case study
3. Reconstruction of the reference input motion
4. Basic geotechnical model and preliminary seepage and stability analyses
5. Model and procedure for the dynamic analyses
6. Results of the dynamic analyses
7. Conclusions and perspectives
References
Abstract
The May 20th, 2012 Emilia earthquake triggered significant fractures, deformations and liquefaction occurrences along a number of riverbanks located close to the epicentre area. One of the most severely damaged earth structures was a levee of an irrigation channel, where large, longitudinally-oriented ground cracks were observed along a 3 km stretch. The ground fissuring was apparently associated to a lateral spreading mechanism causing structural damage to the buildings settled on the bank crown. An extensive study, including in-situ and laboratory investigations permitted a detailed definition of the geotechnical model and to back-figure the reference input motion at the deep bedrock. On such a basis, a dynamic effective stress analysis was carried out on a representative cross-section of the dyke showing that liquefaction occurred within the soil constituting the foundation of the levee. The results of the analysis allowed also for computing the permanent displacement along the critical sliding surface, which turned out to be compatible with the observed damage.
Introduction
Liquefaction phenomena observed during strong-motion earthquakes very often affected natural or artificial embankments. During the Hyogoken-Nambu earthquake (M = 7.2) which struck Kobe city (Japan) on January 17, 1995, loose riverbank sand deposits liquefied causing damages to about 9 km of levees stretches [1]. At most sites, sand boils were observed on the ground surface near the dykes [2]. The 1993 Kushiro-oki earthquake (M = 7.8), which struck the Hokkaido island in the North of Japan, induced significant dyke failures along the Kushiro river for a length of more than 10 km [3]. River dykes suffered extensive damage consisting of cracking, settlement, lateral spreading and slumping, all induced by liquefaction occurred in the body of the embankment (Fig. 1). Due to the huge economic loss produced by the damage observed in the above mentioned cases, the analysis of seismic performance of levees was approached with increasing interest in Japan, as well as in United States. A systematic study of the dynamic response of characteristic levee cross sections for Central California was conducted by Miller and Roycroft [5], as well as by Athanasopoulos-Zekkos and coworkers ([6,7]), who proposed a simplified procedure for the evaluation of the seismic vulnerability of earthen levees. More recently Kwak et al. [8] proposed a simplified procedure to evaluate the fragility curves of levee systems, starting from the observed performance of those along the Shinano river, in Japan, heavily damaged by the 2004 M 6.6 Niigata-ken Chuetsu and 2007 M 6.6 Niigata-ken Chuetsu-oki earthquakes. The results of [8] were used for the fine-tuning of a more general guideline for multi-hazard analysis of river levees proposed by Zimmaro et al. [9].