Levees are built to protect communities from floodwaters, minimizing the impact of floods on infrastructure and agriculture. The U.S.s Army Corps of Engineers estimates that levees have saved billions of dollars in flood damage. Despite their obvious benefits, the limited performance assessment of levees is driving a call for improvement. For a recent paper in the Journal of Geotechnical and Geoenvironmental Engineering, researchers focus on one area, rapid drawdown analysis. Rapid drawdown is as it sounds, a quick drop in the water level near a slope or levee that results in soil instability. When the stress from the water load is released quickly, the slope or levee can experience a pore pressure change, making it unstable.
The objective of this study from authors Prince Turkson and Daniel R. VandenBerge was twofold: 1) propose a framework for undrained RDD analysis and 2) determine the effects of different undrained shear strength methods. Starting with historical data from documented RDD events, the team evaluated four methods to characterize undrained shear strength. The framework outlined in their paper, “Rapid Drawdown Analysis of Levees,” defined the start-of-drawdown phreatic surface, modeled the undrained shear strength of the levee soil, and finally evaluated the levee embankment stability during RDD. Learn more about their test results for undrained strength and their method for estimating levee saturation at https://doi.org/10.1061/JGGEFK.GTENG-12754. The abstract is below.
Abstract
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 (๐ ๐ข) 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 ๐ ๐ข 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 ๐ ๐ข estimates undrained shear strength as a linear function with some initial value, and third, nonlinear increase in ๐ ๐ข 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, ๐ ๐ข as a constant value predicts the highest factor of safety followed by the comparison method and linear increase in ๐ ๐ข. Nonlinear increases in ๐ ๐ข 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.
Learn more about test results and how they could help strengthen levees you work on in the ASCE Library: https://doi.org/10.1061/JGGEFK.GTENG-12754.
