Parametric Study of Levee Saturation for Undrained Rapid Drawdown Analysis

Abstract

Rapid drawdown is a failure mode that may occur when the water level on a dam or levee suddenly drops, removing the stabilizing force that had been applied by the water.  The change in water level results in a change in hydraulic boundary conditions along the outer face of the structure, resulting in a complex, transient pore pressure state within the earth structure.  If the pore pressures that have developed are not able to dissipate quickly, the slope may experience a rapid drawdown failure.  Due to the complexity of the pore pressures, geotechnical engineers typically use multistage undrained analysis to evaluate the rapid drawdown stability of dams, assuming initial conditions that correspond to steady state seepage.  Using this method for levees, however, is more challenging.  Unlike dams, most levees only retain significant amounts of water during floods and may not reach a steady state seepage condition before water begins to recede.  Consequently, steady state seepage cannot be assumed as an initial condition for rapid drawdown analysis of levees.  In addition, most levees are much longer than dams and are usually variable in composition, making them difficult to characterize.  Therefore, it is not economically feasible to perform in-depth analyses on levees.  Considering the special challenges associated with levees, a simplified method of rapid drawdown analysis for levees would be useful to the geotechnical profession.  This research is developing a method to estimate the saturated zone following a flood based on the soil properties, flood hydrograph, and levee geometry.