Investigation on the effect of intermediate principal stress on shear behaviour of unsaturated soils

Directional-dependent properties of the soil, like shear strength, stiffness and hydraulic conductivity, are known as anisotropy in soils. Shape and size of the soil particles and void distribution as microstructure characteristics and external factors such as stress history, environmental and geological conditions, and present stress condition can be the causes of the anisotropy in soils. In this paper, the behaviour of soil has been studied in stress-strain plain under monotonic anisotropic loading to investigate the effect of induced anisotropy on brittleness index of soil sample. The brittleness index of the soil is defined as the difference between the ultimate and peak shear strength divided by the peak shear strength of the soil. The two major parameters describing induced anisotropy or anisotropic loading are intermediate principal stress (b) and principal stress direction (alpha) which are representative of the difference between intermediate, maximum and minimum principal stresses and the rotation angle of the principal stresses' axis, respectively. This paper only takes the effect of intermediate principal stress with the values of 0.25, 0.5, 0.75. In addition, the soil is in the unsaturated state with the saturation degree of 80% using the constant water (C.W.) method.