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Abstract: In this study, the influence of cyclic wetting and drying on swelling behavior of polymer-stabilized expansive clays has been investigated. Such a study is required to assess the long-term behavior of these soils in geotechnical applications. Three expansive soils with different plasticity indexes were obtained from a region around Tehran (capital of Iran). These soils were stabilized by different amounts of two polymers, Urea Formaldehyde and Melamine Formaldehyde. The swelling tests have shown that these polymers decreased the swelling potential of the expansive soils. Cyclic swelling and shrinkage potential and cyclic swelling pressure tests were performed on both treated and un-treated expansive clays. In addition, to simulate partial shrinkage, specimens were dried to their initial water contents in each cycle. Variations in grain size distributions and Atterberg limits of both treated and un-treated expansive soils due to cycles of wetting and drying were assessed. The microfabric changes were also studied by scanning electron microscopy (SEM). The results showed that the suppression of swelling in polymer-stabilized specimens was not lost after the first cycle and the swelling potential and pressure of treated specimens were less than those of un-treated specimens during the cyclic wetting and drying; nevertheless, in cyclic swelling and shrinkage, the swelling potential and pressure of all specimens started decreasing after the first cycle. All of the specimens reached equilibrium after the fourth cycle. [ABSTRACT FROM AUTHOR]