Gypsum soils are considered to be a class of problematic soils which cause severe damage to highways and many other engineering infrastructures. They have an acceptable strength and stiffness in their natural state, but often have low strength and large settlement when soaked. This article presents the results from an experimental investigation into the mechanical behaviour of two artificial clay-gypsum soils. Experimental tests including Atterberg limits, compaction, settlement, unconfined compressive strength and California bearing ratio were carried out on samples of clay–gypsum soil with different percentages of gypsum. The results indicated that the maximum dry unit weight decreased and optimum water content increased by increasing the gypsum content. Furthermore, settlement, strength and California bearing ratio values were considerably decreased by increasing the gypsum content before and after soaking.