In order to prevent leakage in the basement of dams, there are cutoff  walls or membranes below the dam foundation is required. When digging cutoff wall in the pre-excavation section, in order to avoid the fallout local soil and its consequences such as bonding chamber inside the trench excavation, bentonite slurry is used. In order to determine the elements in a slurry that can provide temporary stability is essential. In this study, due to conditions in the dam area Nohob Takestan, and assuming continuum finite difference method is used for numerical simulation and analysis of the pre-excavation. Excavation area is divided into six sections according to the digger bucket capacity and each stage is analyzed separately. After determining the critical modes using empirical relationships most hydrostatic pressure applied to the wall is determined and ultimately determined the desired slurry elements. The results of numerical modeling with the rules of procedures of the International Commission on Large Dams corresponded. And it can be concluded that continuum in the region is not far from reality excavated Nohob dam