Abstract: To study yield characteristics of unsaturated loess in the effective stress space, a series of undrained isotropic net stress compression consolidation and shear tests with different intermediate principal stress parameters b is conducted on unsaturated intact loess by using true triaxial apparatus. The effective stress yielding behaviors of unsaturated loess under true triaxial compression conditions are studied. The results show that the effective stress ratio decreases with an increase in intermediate principal stress or net confining pressure, and the effect of intermediate principal stress on effective spherical stress is greater than that of generalized shear stress. The yield curves determined by the effective stress ratio-volume strain curves have good regularity in the effective stress space, the effective yield stress at yield point increases with an increase in intermediate principal stress and initial suction. The effective stress yield strength surfaces and the strength failure surfaces on the π plane are in good agreement with the SMP strength criterion. As the effective spherical stress and initial suction are larger, the yield strength surface and strength failure surface are more larger. The equations of elastic shear strain and plastic shear strain under true triaxial conditions are proposed. By analyzing the relationship between effective stress and plastic strain, it is concluded that the plastic potential surfaces on different meridian planes in the effective stress space are elliptical, and the elliptical yield surface increases with an increase in intermediate principal stress.

Key words: unsaturated intact loess, effective stress, yield, plastic potential surface