In the present study it is evident that the average pore water pressure ratio of the upstream and/or downstream slopes of an earthen dam is equal to that obtained using the Bishop & Morgenstern method. The average pore water pressure ratio is computed numerically by modelling the flow through an earthen dam to calculate the pore pressure distribution at the problem domain. Sensitivity analysis is performed to demonstrate the effect of soil properties and geometry on the average pore water pressure ratio. The soil properties considered in this study are hydraulic conductivity, porosity, specific weight, unsaturated zone parameters and hydraulic anisotropy, whereas the geometry is represented by the downstream and upstream slopes. The effect of unsaturated zone parameters is studied and conclude that these parameters have a significant effect in reducing the average pore water pressure ratio when compared with those results which consider the average pore water pressure in a saturated zone only. Moreover, it is evident that the specific weight and hydraulic anisotropy have a significant effect on the average pore water pressure ratio, while hydraulic conductivity and porosity have a negligible effect. Then, in order to provide further generalization, the results of average pore water pressure ratios for different non-dimensional soil properties and geometry combinations are tabulated for both steady-state and sudden draw-down conditions, which represent the critical cases of downstream and upstream slopes, respectively.
