Transient Seepage Analyses: Applications for Embankment Dams

Steady-state seepage conditions are typically used to analyze and design earth dams and size internal drains. The maximum seepage through the dam occurs under steady-state conditions. Steady-state conditions also represent the critical condition for evaluating the stability of the downstream slope due to the maximum location of the phreatic surface. In certain cases, a transient seepage analysis, which accounts for changes in the seepage behavior during the passage of time, may be applicable. For example, transient seepage analysis may be utilized when steady-state conditions do not develop due to fluctuating water levels, as might be the case for dams at dry detention basins or levees that experience rapid filling and drawdown. An internal drainage system for a dry detention basin or levee designed without considering transient seepage conditions can result in an overly conservative design by over-sizing the internal drains. Transient seepage modeling, used in conjunction with traditional steady-state seepage modeling, can more accurately predict drainage behavior during planned filling and drawdown cycles, thus effectively meeting dam safety standards while achieving a more economical design. Another potential application for transient seepage analyses is the evaluation of the stability of the upstream slope of an earth dam under drawdown conditions. Specifically, a transient analysis may be used to determine appropriate operational drawdown rates to reduce the likelihood of sloughing of unprotected upstream soil slopes in earth dams. While not typically a dam safety issue, surficial sloughing may necessitate additional maintenance such as re-grading following drawdown events. 8 pp., 5 figures, 1 table, 11 references.