Insights into Rapid Drawdown Rate and Stability from Infinite Slope Analysis

Rapid drawdown (RDD) is an important consideration for the upstream slope of earth dams. Design against RDD failure typically specifies a maximum drawdown rate and/or requires a minimum factor of safety following rapid drawdown. While more in-depth analysis is warranted in many cases, infinite slope analysis can provide substantial insight into both the hydraulics and stability of an earth dam following RDD. Based on infinite slope geometry, a method for predicting pore pressure dissipation is developed to inform drawdown rate selection. Secondly, RDD stability is considered by applying infinite slope geometry to the multi-stage undrained method. The results are compared to known RDD failures and limit equilibrium analyses. This approach can be used as a preliminary design tool to select appropriate slope angles or a screening tool to assess suitability of compacted fill materials for the upstream slope of dams and levees.