Fine gypsum soils, when saturated and loaded, because of their high porosity, have large settlements. Compaction of the soil and removal of the load causes the soil to swell in saturated mode. These properties of gypsum soils made them both collapsible and expansive soils. Shrink-swell gypsum soils cause widespread damages to the buildings annually in many cities, among them Mashhad city, which is the second largest city in Iran. Unsymmetrical settlement, uneven ground floor and cracks in façade are some of the problems. Likewise in road engineering, swelling of this type of soil located under the asphalt after absorbing water is one of it most important problem. Identifying the behavior of gypsum soils and effort in finding solutions for decreasing the amount of settlement and swelling are the main objectives of this study. The soil samples were from the City of Mashhad. These soils naturally contain high amount of gypsum and high porosity. In this study, samples with different amount of gypsum were gathered and classified to seven sample groups. Preliminary tests done showed that the increase in the amount gypsum causes the soil plasticity index and unit weight to decrease, which in turn increases settlement. The results show that with increasing unit weight and amount of gypsum and water in samples, the soil swelling potential increases. In order to improve the shrink-swell potential of fine gypsum soils, two chemical and physical methods applied to reduce the amount of swelling and settlement. These suggested methods are adding lime and loading and unloading the saturated soil samples.

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