Stability of embankment dams following reservoir drawdown

The observed behavior of embankment dams was evaluated in terms of appraising the existing methods of stability analysis used for drawdown conditions, assessing the shear strength and pore water pressure characteristics of compacted soils, and considering the performance of actual embankment dams. Seven embankment dams that failed due to reservoir drawdown and four embankment dams that remained stable upon drawdown condition were evaluated utilizing both the Effective Stress Stability Analysis (ESSA) and the Undrained Strength Stability Analysis (USSA). The existing procedures of expressing the undrained shear strength were evaluated utilizing the results of the laboratory-testing program of the present study, which was conducted on four compacted clay soils (namely, Champaign Till, Boston Blue Clay, Chicago Blue Clay, and Yacycreta Clay). These four soils were selected with a wide range of plasticity (i.e., liquid limit ranges from 19 to 67%, and plasticity index ranges from 6 to 41%) in order to cover the range of properties and behavior of the compacted soils that were used in actual embankment dams.; All the four compacted clayey soils that were tested in the present study exhibited "prestressing" pressure analogous to the preconsolidation pressure in natural clay deposits.; In the present study, the shear strength was defined at the yield state, eliminating the influence of dilation. The undrained shear strength defined at yield condition was found, for all practical purposes, to be directly related to the consolidation major principal stress independent of consolidation principal stress ratio. This relationship is linear in the compression range and it is nonlinear in the recompression range.; A new USSA method is proposed for conducting the stability analysis for embankment dams upon reservoir drawdown. The proposed USSA method takes into account the effect of prestressing pressure, utilizes the actual history of embankment material and reservoir level in evaluating the consolidation stresses, and eliminates the need for determining or estimating the ACU strength.; Empirical relationships for the undrained and effective stress shear strength parameters were established in the present study based on soil plasticity through the results of the laboratory-testing program and stability analyses for 11 embankment dams.