Effect of Matric Suction on Deformation of Gypseous Sand Using Modified Oedometer

Gypsum is a soluble material, and it is one of the problematic components in the soil in the west of Iraq. Al-Najaf is one of the governorates in Iraq which suffers from the gypsum content in different levels. The soil of the city is mainly sand-sized particles bonded by different percentages of gypsum. The main problem of this component is the dissolution upon the wetting process. This paper investigates the effect of decreasing the matric suction (wetting) on soil deformation under a specific stress level. The investigation includes three matric suctions of (50, 20, and 0 kPa) under three net normal stresses of 221 kPa, 442 kPa and 885 kPa respectively. The soil specimens are remolded to 95% of the maximum dry density from the proctor test. A modified Oedometer setup is used to perform the tests, including the application of air and water pressures up to achieve the specific matric suctions. The results revealed that the highest value of collapse potential (CP) is under the stress of 221kPa, and the greatest part of the CP is achieved prior to the saturation of the soil. This issue must be considered in the analysis and design of the foundation in unsaturated gypseous sandy soils.