Evaluating the Effect of the Reservoir Water Level on Seismic Behavior of Embankment Dams

Level of the water table on reservoir is one of the parameters that effects on the seismic behavior of embankment dams. Water table in embankment dams changes every month of the year. Also changing of water table causes to the change of stress and strain in the body of dam. In this situation, earthquake can always happen. So to evaluate the effect of this parameter, Masjed Soleiman dam as a case study was selected. The Masjed Soleiman dam is a rock-fill type with clay core and a maximum height of 177m located on the Karoon River in the Southwest Iran. This embankment dam is located in Khuzestan province at the distance of 25.5 km to Masjed Soleiman town. Finite element model of Masjed Soleiman dam was constructed. The Mohr-coulomb elasticperfectly plastic constitutive model was taken into account to reflect the soil stress-strain relation. First, layer analysis was carried out considering 12 layers at the end of construction stage. Then, the analysis was continued considering the water table and weight of the dam reservoir in steady seepage condition. The 4 earthquake records in the far field condition were applied horizontally to the bedrock as the input for dynamic analysis. Then, by considering the result of dynamic analysis, changing of the maximum acceleration and maximum displacement in time domain was evaluated. In this study, to perform stability analysis and calculate the factor of safety, critical sliding surface on upstream and downstream, as reported by the consultant engineers, were considered. The semi- empirical Newmark method used for estimating permanent earthquake-related deformation of the slopes is based on the sliding block framework. This conceptual framework approximates the potential sliding mass as a rigid body resting on a rigid sloping base. Using the Newmark method, the equal dynamic factor of safety, minimum factor of safety and applied deformation were evaluated. For studying the water table effect on the behavior of embankment dams, 20 models were used with different water tables in the reservoir. In this study, dynamic analyses were done for 4 earthquake records and for 5 elevations of water table that were considered. Then the seismic response of embankment dam was investigated. The results showed that the level of the water table has low effect on occurred maximum acceleration and maximum displacement. Minimum factor of safety in the downstream critical slip surface was has increased when the water table elevation was increased. But this parameter in the upstream critical slip surface was decreased when the water table elevation was increased. Equal dynamic factor of safety in the downstream critical slip surface was increased when the water table elevation was increased. But the upstream critical slip surface was decreased when the water table elevation was increased to 0.6 height of the dam, and then the equal dynamic factor of safety was decreased. So water table elevation in the upstream wedge was in critical condition when the water table elevation was equal to 0.6 height of the dam. Permanent deformation occurred after the earthquake in the downstream critical slip surface was decreased when the water table elevation was increased but the permanent deformation occurred in the upstream wedge was increased when the water table elevation was increased.