Measurement of Shear Strength of Unsaturated Gypseous Soil with Different Degrees of Saturation by Triaxial Test

Abstract

AbstractThe purpose of the present work is to assess the suitability of unsaturated gypseous soil as subgrade layer for carrying machines foundations. An extensive laboratory testing program was carried out to study the geotechnical properties and the behavior of unsaturated gypseous soil. Gypseous soil samples were brought from a region in Tikrit city in Salah Al-Dean governorate, north of Baghdad. The tests include physical tests such as moisture content, specific gravity, classification tests, relative density, compaction's Proctor, and consolidated undrained triaxial test. Chemical tests were also carried out. The specimen was prepared by using vacuum pump and suction operation (the value of the suction operation ranges -20 kPa approximately), where this method using to the granular soil suction operation such as sandy gypseous soil to pull void air spaces out of the soil until the soil grains of holds together. Therefore, once the sample former is removed the suction will stop the sample from collapsing. Then, triaxial test was carried out on the gypseous soil. The tests were performed for samples at 35% relative of density for the natural, unsaturated, and fully saturated gypseous soil at degrees of saturation (30%, 60%, and 80%). It was found that increasing the moisture of the gypseous soil at all degrees of saturation causes a decrease in the angle of internal friction for both the total and effective stresses (ϕ, and ϕ'), where the decrease ranged between (25.5° - 5.0°) in relation to the total stresses, and (27.5° - 5.5°) in relation to the effective stresses. While the soil cohesion for both of the total and effective stresses (c, and c') increased with the increase of gypseous soil moisture up to the saturation degree (60%), this leads to an increase in shear strength of the soils. Then a gradual decrease was observed at the saturation degrees (80%, and 100%), respectively. Thus, this leads to a decrease in shear strength of the soil, which ranged from (12.5 - 9.5) kPa for the total stresses, and between (11.5 - 9.0) kPa for the effective stresses, respectively.