Slope stability analysis is a geotechnical engineering problem characterized by many sources ofuncertainty. In slope stability computations, some of these sources are encountered, such as geologicaldetails missed in the exploration program, estimation of soil properties that are difficult to quantifyand many other relevant factors. Therefore, accurate reproduction of the spatial variability in the fieldcould be essential to decrease uncertainty. The Sabzkuh-Choghakhor water conveyance tunnel iscurrently under construction using the NATM and TBM tunnelling methods in the Zagros Mountainsin south-western Iran. In the slope stability assessment of the Sabzkuh tunnel portal, despite adequategeotechnical investigations, field studies have not been performed with appropriate accuracy. Alandslide and collapse has occurred in a part of the portal due to tunnel excavation. In this paper, theimportance of having a precise and predetermined schedule for selecting site location, monitoring,complexities of the geological model, uncertainty and its effects on the stability of the trench wereinvestigated and the necessity of comprehensive slope management was emphasized.