Assessment of shear strength characteristics of the unsaturated gypseous soil at various saturation degrees

Abstract The purpose of this study is to determine whether or not unsaturated gypseous soil can function well as a substrate for the foundations of carrying loads. A comprehensive program of testing was carried out with the objective of validating the geotechnical parameters and behavior of the unsaturated gypseous soils. The testing program included specific gravity, moisture content, classification tests, Proctor’s compaction, relative density, and the triaxial test. Additionally, chemical analysis was performed on the samples as well. This approach was employed in a granular soil suction process to eliminate gaps of air in the soil until the soil grains held together. The sample was prepared by using a pump of vacuum with a suction process (approximately −20.0 kPa), and this method was used in the granular soil suction process. As a consequence of this, the suction prevents a specimen from collapsing when it is removed from the apparatus. The next step consisted of conducting a consolidated-undrained triaxial test on the soil. Experiments were performed on materials with a relative density of 35% and several degrees of saturation, such as normal saturation (6%), unsaturated (30, 60, 80%), and 100% saturated. It was shown that there is a reduction in the internal friction angle for the effective and total stresses is caused by an increase in the water content of the soil at any saturation degree. This occurs in both the unsaturated and saturated states of the soil. The angle of friction decreased by 80% of the natural value for both stresses, effective and total. As gypseous soil moisture increases up to the saturation degree of 60%, the soil cohesion for the total and effective stresses rises, where it increased by (220% and 125%) of the natural value for both the effective stress and the total stress, respectively, leading to an increase in the soil’s shear strength (ϕ and c). After then, there was a steady weakening of the force when it reached saturation degrees of 80% and 100%, where it decreased by (44% and 47%) of the maximum value at 60% saturation degree for both the effective stress and the total stress, respectively.