The Ataturk Dam is the fourth largest clay cored rock-fill dam in the world. Located in southeastern Turkey, it is the centrepiece of a project called the Southeastern Anatolian Project (GAP). The construction of the dam was finished in the short time of 3 years and 8 months in August 1990. As the reservoir level started to rise, settlement problems started to occur along the crest reaching considerable levels by May 1992 and the weathered vesicular basalt used in the rock-fill section of the dam started to slake seriously. The elevation of the crest has fallen down to such an extent that now the upper part of the dam is being reconstructed to the original height of 549 m in order to maintain the 7 m freeboard and keep the dam operational. The settlement and slaking problems of the dam were investigated both in the field and the laboratory using soil and rock mechanical techniques. Standard proctor tests were run to determine tbe optimum moisture content at which the highest dry density (best compaction) is obtained during compaction of the impervious clay core. Consolidation tests were performed on undisturbed compacted clay core samples to determine if the field compaction of the clay core was done to the standards determined in the laboratory. Unconfined compression tests were run on the two different (sound aphanitic and weathered vesicular) basalts used in the rock-fill section of the dam to determine their unconfined compressive strengths. Slake durability tests were run on the same basalts to determine their durability under wetting and drying cycles. Petrographic and X-ray analysis were done so as to study the mineralogical contents of the two basalts and in turn, to determine the cause of rapid slaking of the vesicular textured weathered basalt. The consolidation test results show that, the field compaction of the impervious clay core of the dam was not done to the standards determined through the proctor tests in the laboratory. The unconfined compressive strength of the sound aphanitic basalt is more than ten times greater than the weathered vesicular basalt, and while the former has extremely high durability the latter only has medium durability. Because of nontronite (iron montmorillonite) and secondary calcite amygdules filling the vesicles, the weathered vesicular basalt used in the rock-fill section is slaking badly causing additional settlement and landslides both in the clay core which is already settling and the rock-fill section of the dam. With the continuing slaking, the present reconstruction of the crest up to the theoretically intended height of 549 m to maintain the 7 m freeboard and keep the dam operational seems only to be a temporary solution. (C) 2000 Elsevier Science B.V. All rights reserved.