This paper presents laboratory experiments on the effect of matric suction on the compressibility of unsaturated gypsum sand soil under various types of loading. The soil samples were obtained from Al-Najaf city in Iraq with gypsum contents of 14 %, 22%, and 29 %. Two series of laboratory tests were conducted in a modified triaxial cell on these samples. The first one is loading-path likes when a building is constructed on these soils in a specific degree of saturation (specific matric suction). Four matric suctions were adopted; 100, 60, 30, and zero percentages of initial matric suction under two different confining stresses; 100 kPa and 200 kPa. Also, two saturated tests (CD) were performed under the aforementioned confining stresses. The second method is wetting-path, likes when a structure has been constructed and the degree of saturation of a foundation soil increases (decreasing in matric suction). The changes in the degree of saturation may be due to rainfall, water table rising and/or leaking of sewage and water pipes. The results from both paths revealed that the volumetric strains were increased by decreasing the matric suction and increasing the gypsum content. This type of soil shows significant volume changes during the wetting process which can lead to a large amount of deformation of the soil following damage to the substructures.

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