In most of the slope stability studies of earth dams, the pressure distribution below the free surface is either considered as hydrostatic or is derived from the flow nets. The assumption of hydrostatic pressure may not always hold good. The flow nets, apart from being subjective and unsuitable for computer aided design, do not hold for unsteady state and as such are not applicable for computing the factor of safety in respect of upstream slope under sudden draw down condition. In the present work, it is demonstrated that these problems can be overcome by numerically solving Richard’s equation for pore water pressure distribution. It has been found that the computed pore water pressures are significantly lower than the corresponding hydrostatic estimates especially at high degrees of hydraulic anisotropy (defined herein as the ratio of horizontal saturated hydraulic conductivity to the vertical saturated hydraulic conductivity). Subsequently, the factors of safety of the two slopes are computed using the computed pore water pressures in accordance with simplified Bishop’s method. It has been concluded that assuming the pore water pressure to be hydrostatic leads to an underestimation of the factors of safety. The extent of underestimation is quite pronounced at high degrees of hydraulic anisotropy.
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