Slope stability is the resistance of inclined surface to failure both in sliding or collapsing forms. The main purpose of a slope stability analysis is to find endangered areas, investigation of potential failure mechanisms, determination of sensitivity of slope to different triggering mechanisms, designing of optimal slopes with regard to safety , reliability and economics . Successful design of a slope requires enough geological information and site and laboratory characterizations. Measurements of soil / rock properties, beside establishing slope geometry , considering groundwater conditions and also mapping discontinuity features and mechanisms are necessary. In this study, the effect of pore water pressure on the slope stability in the Gol-e-Gohar open pit mine is analyzed using a numerical finite difference method based codes; FLAC3D and FLAC2D considering dry and saturated conditions. The numerical slope stability analyses was performed on 9 two dimensional sections and also 2 three-dimensional models. The results showed that in all sections, the difference betweethe safety factor in completely dry and saturated modelis approximately 30 percent. The maximum instability, were found in the southern and western walls of the mine. The difference between the results in two and three dimensions was not significant. Two methods including reducing slope inclination and also excavating drainage tunnel were examined in those endanger slopes. The sensitivity analysis was also performed in order to determine the exact location and size of the drainage tunnel. The results showed that the drainage tunnel must be excavated in the lowest height and closest site to surface of bench. When a slope was considered without any benches, the overall safety factor was increased by 12 percent compared with the real condition while the failure mode did not changed remarkably.
