Case History: Rapid Drawdown Analysis of Village Creek Plant Levee: Fid-Bau Portal

Case History: Rapid Drawdown Analysis of Village Creek Plant Levee

VandenBerge, Daniel R. / Gregory, Garry H. / Turkson, Prince

Rapid drawdown (RDD) controls the design of the upstream face of many earth dams and can also be important for the waterside slopes of levees. However, unlike dams, levees retain high water for relatively short flood durations and may not reach steady state seepage. Little guidance is available for selecting an appropriate start-of-drawdown phreatic surface that can be used with multi-stage undrained analysis procedures. Rapid drawdown analysis of the Village Creek wastewater treatment plant levee near Fort Worth, TX, is explored in this case study. The levee consists of a blend of clayey sand to fat clay, underlain by layers of mixed alluvium. Three methods are used to select an approximate start-of-drawdown phreatic surface and are compared to a single piezometer measurement from a past flood. Method 1 uses steady state results for water level at about 80% of the peak flood stage. Method 2 uses results directly from transient seepage analyses. Method 3 utilizes a recently developed response surface. The phreatic surfaces predicted by each method are used as the starting point for undrained rapid drawdown analyses. Methods 1 and 3 give similar factors of safety, despite differences in the start-of-drawdown seepage patterns. Method 2 results in a higher factor of safety because it predicts significantly less seepage development during the flood. The results illustrate the importance of the final piezometric surface used in the third stage of the rapid drawdown analysis.