Abstract: To simulate the failure of loess under undrained condition in the actual engineering, a series of isotropic consolidation and shear tests with different intermediate principal stress ratios b under constant water content is performed on intact loess with various initial suctions by using unsaturated soil true tri-axial apparatus. The mechanical characteristics of unsaturated intact loess are studied. The results show that the isotropic compression yield stress increases with initial suction, suction decreases with the increase of net mean stress, the extent of suction impact of net mean stress increases with increasing of the initial suction. The damage state lines of generalized shear and net average stress are parallel to the linear relationship and the test points of generalized shear and effective net mean spherical stress can be normalized to drainage shear failure state line of saturated soil under different the ratios b. The suction of shear failure increases linearly with the ratio b; void ratio is approximately linear parallel to logarithm of net average stress critical state lines, whose slopes are larger than those of saturated loess and larger than those of isotropic compression before yielding; both void ratio and logarithm of effective net mean spherical stress critical state lines of unsaturated soil are approximately linear parallel to the critical state line of saturated soil. The test points that ratios of damage state void ratios between unsaturated loess and saturated loess and gas saturations are distributed in a linear zone of 1-1.2 under different net confining pressures, but those points can be normalized to a nonlinear curve at the same net effective stress.

Key words: unsaturated intact loess, unsaturated soil true triaxial apparatus, suction, void ratio