Embankment Drainage After Instantaneous Drawdown

A method is presented for use in determining the position of the saturation line and the per cent drainage in the upstream portion of an earth dam following complete or partial drawdown of the reservoir based upon: (1) A dimensionless time factor which is a function of elapsed time, permeability, effective porosity, and boundary conditions, and (2) a drawdown ratio which expresses the drop in reservoir elevation as a function of the original height of water above the slope. Since an evaluation of slope stability under drawdown conditions is dependent upon pore pressures related to the position of the saturation line, a quick method of predicting its location is desirable. The drainage theory used in this method is similar to the approach proposed by Casagrande for evaluating base course drainage of airfield pavements and later adapted by Shannon for use in dams. Expressions are derived for predicting the effect of partial drawdown. Model studies indicate that hyperbolas related to a coordinate system, which is a function of the time factor and the drawdown ratio, can be used to define the saturation line.