Rapid Drawdown Analysis of Levees

Rapid drawdown (RDD) is an important stability design condition for the waterside slopes of levees. A framework for levee RDD analysis that evaluates both seepage and undrained shear strength (su) has been assessed in this study. Methods for determining the start-of-drawdown phreatic surface considering different unsaturated soil models have been proposed. Three different methods of su evaluation are also explored in this study for RDD and are compared with a method based on the current state of practice to assign shear strength to embankment soil for RDD analysis. First, undrained strength as a constant value assigns undrained shear strength as a constant value to the embankment soil mass. Second, linear increase in su estimates undrained shear strength as a linear function with some initial value, and third, nonlinear increase in su considers undrained shear strength as a power function. These four undrained strength methods are used in RDD stability analyses based on data from three case histories, an existing levee, and a hypothetical levee model. For the same slip surface, su as a constant value predicts the highest factor of safety followed by the comparison method and linear increase in su. Nonlinear increases in su predict the lowest factor of safety because this method results in lowest undrained shear strength values assigned to a slip surface. A probabilistic analysis illustrates the potential of using this RDD framework along with relatively simpler and cheaper methods for determining undrained shear strength as an alternative to the multistage undrained method for routine RDD analysis of the thousands of kilometers of levee embankments in the United States.