,Gypseous soils are considered problematic when used as the foundation in civil engineering structures such as roads, buildings and dams, due to their solubility. These soils are resistant and have good engineering properties in their dry state. However, when saturated by rainwater or a rising groundwater table, the soluble minerals are washed out, resulting in the subsidence of the structures built on them. In the recent decades, buildings constructed in the Southern Mashhad Metropolitan Area, Iran, have been widely faced with this problem. Since the changes in chemical composition and engineering properties of these soils are based on the amount of dissolved gypsum, the focus of this study is to characterize the soluble soils of this area and their changes throughout the leaching process. Thirty-eight samples were taken from different locations in the area. Chemical tests were conducted on the samples and the gypsum and sulfate concentration maps were produced based on these results, combined with the previously available data from 511 boreholes drilled in the area. Seven soil samples with different gypsum concentrations were selected for further analysis in four major groups of tests, including hydraulic tests (permeability and solubility), chemical tests (chemical analysis of soils samples and total dissolved solids, calcium hardness and chlorine of the leachate samples), physical tests (grain size analysis, Atterberg limits and specific gravity) and mechanical tests (consolidation and direct shear). Changes in the mentioned parameters were investigated through a 5-day leaching process. The results indicate that extensive dissolution of gypsum and removal of gypsum bonding between soil particles change soil chemical composition and decrease the soil compressibility and strength parameters. Therefore, the structures built in this area are in high risk of subsidence and foundation failure; proper measures should be taken to improve the soil quality before construction.

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