Rapid drawdown in slopes and embankments

The rapid drawdown condition arises when submerged slopes experience a rapid reduction of the external water level. Classical procedures developed to determine the flow regime within the slope and the resulting stability conditions are reviewed in the paper. They are grouped in two classes: the stress-based'' undrained approach, recommended for impervious materials, and the flow approach, which is specified for rigid pervious materials (typically a granular soil). Field conditions often depart significantly from these simplified cases and involve materials of different permeability and compressibility arranged in a complex geometry. The drawdown problem is presented in the paper as a fully coupled flow-deformation problem for saturated/unsaturated conditions. Some fundamental concepts are first discussed in a qualitative manner and, later, explored in more detail in synthetic examples, solved under different hypotheses, including the classical approaches. Some design rules, which include a few fundamental parameters for the drawdown problem, have also been solved in a rigorous manner to illustrate the limitations of simplified procedures. A significant portion of the paper is devoted to the discussion of a comprehensive case history. In Shira, earth dam pore pressures were recorded at different points inside the embankment during a controlled drawdown. Predictions of four calculation procedures (instantaneous drawdown, pure flow, coupled flow-elastic, and coupled flow-elastoplastic, all of them for saturated/unsaturated conditions) are compared with measured pressure records. Only the coupled analysis provides a consistent and reasonable solution. The role of the different soil properties in explaining the phenomena taking place during drawdown is finally discussed.