Unsteady State Phreatic Surface in Earth Dams

A simple method is developed for determining the unsteady state phreatic surface in earth dams as a function of time. The method is useful for the stability analysis of temporary dams where the steady state seepage condition may not be reached during the life span of the dams. Although the method is based on the results of transient flownets by Cedergren and yields only approximate results, the uncertainty in determining the permeability and effective porosity of soils usually precludes the use of other more refined methods and thus makes this simple method particularly attractive. The development of the method by extending the results from a specific case to more general cases is described. It is shown that the factor of safety of a dam may increase significantly if it is used only temporarily because of the much lower phreatic surface as compared to permanent dams.