Investigating the Influence of Principal Stress Directions and Intermediate Principal Stress on the Shear Strength of Unsaturated Silty Soil Using the Hollow Cylinder Apparatus

The present study investigates the simultaneous influence of two parameters: the principal stress direction (α) and the intermediate principal stress parameter (b = (σ2 - σ3) / (σ1 - σ3)) on the shear strength of unsaturated soil. Experiments were conducted using an unsaturated hollow cylinder apparatus, and the soil mixture consisted of Firouzkouh silt and bentonite. The samples were prepared using the static compaction method at optimal moisture content and were dried to reach 30% saturation. It is observed that the sensitivity of the soil to changes in b depends on α. Specifically, for intermediate principal stresses of 0.25 and 0.75, an increase in the α angle results in a decrease in shear strength up to approximately 60 to 80 degrees, after which the shear strength increases again. However, when the intermediate principal stress is 0.5, the maximum shear strength occurs at an angle of approximately 30 to 40 degrees, and the behavior of α with respect to b is symmetric regarding the intermediate stress of 0.5.