Specifying the mechanical characteristics of soil is as one of the major steps in designing the foundation for civil projects, particularly hydraulic structures. This study examines the water salinity effect on engineering characteristics of fine-grained soil in clayey core of Gotvand dam which it is located on the Karun River in Khuzestan province, Iran. For this purpose, three types of salt; sodium chloride, magnesium chloride and calcium sulfate is considered that the last one has the highest concentration in the water of reservoir behind the dam, and then their effect on the mechanical characteristics of clay including density, Atterberg limits, angel of internal friction, adhesion and open inflation have examined. Therefore, treatments required for testing were provided by combining the mentioned salts considering the weight percentage of salt in the water behind the dam and designing a test using response surface methodology (RSM). The results were statistically analyzed by RSM. Results showed that type and amount of soil salinity has no significant impact on the compaction characteristics of soil namely optimum moisture content and the maximum dry density. Results of changes in Atterberg limits showed that presence of salt ions reduces the plasticity of the soil and it was seen a certain decrease in the liquid limit and subsequently a dramatic drop in soil plasticity by increasing the percentage of soil; while the plastic limit remained almost unchanged. Eventually, results of open inflation showed that whatever the salt concentration be higher, the rate of sample's open inflation will be lower
