Some aspects of the seismic behaviour of a large homogeneous earth dam

Instability phenomena of large existing earth dams are one of the major source 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 phenomena which can com-promise the water retention capability. This paper discusses some results of a study on the seismic behaviour of a large homogeneous earth dam. Specifically, monitoring data taken during construction and impounding of the dam were used to calibrate a plane-strain numerical model. Iterative pseudo-static analy-ses were first conducted by uniformly accelerating the finite-difference model to investigate the plastic mechanisms forming under critical conditions. The seismic performance of the dam was then evaluated through a series of dynamic analyses in which a real seismic record was used as input motion. In the analyses, the effects of the bedrock deformability and of the presence of the vertical component of the seismic action were investigated. The results of the dynamic analyses evidenced that a significant reduc-tion of the seismic energy is obtained if the deformability of the bed-rock is accounted for, resulting in much lower permanent displace-ments of the embankment. Conversely, the vertical component of the seismic action induces a sensible increase of the seismic displace-ments, so that analyses conducted neglecting the vertical component of the ground motion can lead to an unsafe evaluation of the water re-tention capability.