Due to high energy demand, recently dams are one of the most encouraged projects by the state in Turkey. Dam locations are determined in order to obtain maximum energy output and minimum cost to attract the private investors. On streams alluvial and permeable areas possess special problems for controlling groundwater seepage under the dam. For this purpose cut-off walls are well known relatively impermeable groundwater barriers that have been used for decades in various sizes and types. This paper examines a cut-off wall case study beneath Seyrantepe Dam Project in Elazig, Turkey. Seyrantepe Dam is planned to be constructed on alluvial deposits of Peri Stream which discharges into Munzur River. Maximum 30 meters deep alluvial and slope debris layers exist under the Seyrantepe Dam site. A slope failure has occured during the initial stage of the construction that added great difficulty and complexity in construction of seepage barrier. To avoid the groundwater seepage in the alluvial and slope debris layers, a cut-off wall with 800 mm thickness was planned to be constructed penetrating two meters into relatively impermeable marl base rock. Mechanical grab and hydrofreze cutter diaphragm wall machine was used to construct the cut-off wall considering very unfavorable subsoil conditions given by slope debris containing irregular large size basalt boulders. In addition systematic grouting of limestone formations was planned in order to control permeability within of the dam settlement area. The plastic concrete which used in the cut-off wall was determined based on the hydraulic conductivity tested in the laboratory. Special permeability tests are also implemented after construction in order to verify the impermeability of the cut-off. Construction method and encountered difficulties with special emphasis are discussed in the scope of this case study.