La risposta di grandi dighe in terra durante eventi sismici intensi

Instability phenomena of large existing earth dams are one of the major sources of seismic vulnerability in Italy, as most of them were constructed in the absence of specific seismic regulations. Therefore, it is necessary to investigate the response of such earth structures when subjected to severe earthquake loading, estimating the safety conditions with respect to deformation patterns which can impair the water retention capability. However, earth dams are complex structures and their seismic behavior is strongly affected by the inertial forces that develop during the ground motion and change with time and space. Moreover, positive excess pore water pressure may build up in the saturated portion of the dam during cyclic loading, leading to a reduction of the shear strength of the earth dam and the foundation soil. This paper focuses on the seismic performance of large earth dams. Specifically, a plane strain numerical model of an existing homogeneous earth dam was calibrated using the monitoring data taken during the construction and the impoundment phases. The seismic performance of the dam was evaluated through a series of nonlinear dynamic analyses carried out in the time-domain using four real seismic records applied at the base of the model. The selected input motions are characterised by the same energy content and by a very high return period. Both the horizontal and the vertical component of the seismic action were applied at the base of the grid. In calculations, cyclic excess pore water pressure build-up was accounted for using the Finn-Byrne model. The seismic response of the dam is discussed showing the effects of the pore water pressure distribution within the embankment on the deformation patterns of the dam. Two hypothesis were adopted for the pore pressure distribution in the portion of the dam located above the “free surface”, defined by the condition u = 0: in the first, negative values of pore water pressure were allowed to develop, while in the second, the condition u = 0 was maintained above the “free surface”. The results show that the differences in the deformation patters observed for the two schemes can be ascribed mainly to the different distribution of the available shear strength of the soil within the embankment. Anyway, despite the high return period of input motions considered in the analyses, this corresponding to a very severe seismic scenario, the maximum permanent settlement computed at the crest of the dam was always lower than the available freeboard, this indicating a satisfactory performance of the dam to intense earthquake loading.

In questa nota vengono discussi alcuni aspetti del comportamento sismico di grandi dighe di terra omogenee. Nello specifico, vengono mostrati i risultati di analisi dinamiche eseguite su un modello numerico alle differenze finite, in condizioni di deformazione piana, di una diga esistente e ben documentata in letteratura: la diga sulla Marana Capacciotti. Le analisi dinamiche non lineari nel dominio del tempo sono state condotte applicando alla base del reticolo di calcolo le componenti orizzontale e verticale di quattro registrazioni reali. Gli input sismici selezionati sono caratterizzati dallo stesso contenuto energetico e da un periodo di ritorno molto elevato. Nelle analisi si è tenuto conto del possibile sviluppo di sovrappressioni interstiziali positive mediante il modello di Finn-Byrne. Nella nota viene discusso l’effetto della presenza di pressioni interstiziali negative nella zona satura del corpo diga sui meccanismi deformativi e sui cedimenti permanenti della cresta, che possono essere considerati un indice di prestazione sismica della diga.