This research presents an in-depth analysis of the volumetric and mechanical behavior of expansive soils surrounding the Khangiran gas well in Sarakhs, emphasizing the effects of matric suction and confining pressures on soil mechanical properties. The study employs both laboratory and numerical approaches, utilizing an unsaturated triaxial apparatus and GeoStudio software, to assess (1) the influence of matric suction and confining pressure on volumetric deformation and shear strength, (2) the impact of annual precipitation on soil swelling, and (3) the tensile stresses exerted on the well casing due to soil expansion. Laboratory results reveal that shear strength increases from 195 kPa to 235 kPa as confining stress rises from 100 to 200 kPa, while cohesion climbs from 68 kPa in saturation to 95 kPa under 100 kPa of matric suction, signifying enhanced resistance in drier soil conditions. Numerical modeling indicates that annual precipitation induces a maximum tensile force of 163 kN at a depth of 13 m, with the expansive zone extending approximately 15 m from the well. The thickness of the steel used for tensile strength resistance against soil swelling is sufficient, and if extensive corrosion of the steel casing is not a concern, tensile strength failure will not occur. These findings offer critical insights into soil-structure interaction in expansive soils and provide practical guidance for the design of resilient gas well casings in similar geotechnical settings.

A fully-automated unsaturated triaxial device is developed at the Advanced Soil Mechanic Laboratory of the Sharif University of Technology (SUT) to investigate the hydromechanical behavior of unsaturated soils under any complicated path. The main improvements of the developed device are (1) the ability to continuously measure accurately different stress/ strain variables during long duration of unsaturated tests. This is of great importance in capturing the sudden deformations of the specimen, under wetting (2) providing a user-friendly controller software enabling the definition of any desired stress/strain variable and stress/strain paths under either stress-controlled or strain-controlled conditions. Finally, to evaluate the performance of the developed apparatus, two wetting tests were carried out to study the effect of initial shear stress on the hydromechanical behavior of unsaturated reconstituted specimens of Gorgan loess. In these wetting tests, the specimens were wetted under isotropic and anisotropic stress states by stepwise reduction of suction to approach to the saturated condition. The obtained results showed the excellent performance of the developed device to accurately follow the defined stress path along with the continuous measurement of stress/strain variables. The tested collapsible soil specimen that was wetted under the effect of initial shear stress exhibited a larger volume decrease accompanied by a larger degree of saturation compared to those of the specimen wetted under the isotropic stress state with the same initial mean net stress.