Dams are one of the most important engineering structures that need detailed geological and geotechnical investigation since several types of engineering problems are encountered during construction and postconstruction phases of dams. In May 2010, during the spillway construction of an earthfill dam from the northwest Turkey (Akcin Dam), toe buttress supporting the spillway was disturbed, and a mass movement occurred at the right bank. Furthermore, the spillway started to move in the dip direction of the slope, and the mass movement direction at the right bank. Thus, shear diagonal cracks on the walls of the spillway constituted. This study aims to describe the slope failure mechanism and the damages on the spillway of the Akcin Dam and to suggest effective remedial measures. In the first stage of the study, the mechanism of the failure was investigated, and it was found that claystone-tuff contact acting as a weakness plane comprised the failure surface. Then, the seismicity of the study area was assessed, and the peak ground acceleration was determined as 163 gal. Finally, as the necessary remedial measures, drainage control, grouting, reinforcement of buttresses, and concrete piles front of the tuffite mass were suggested to provide the dam safety.