Influence of structural properties on strength and yielding characteristics of unsaturated Q(3) loess

To investigate the influence of structural properties on strength parameters, yielding stress and yielding suction of unsaturated Q(3) loess, a series of unsaturated soil tests, including direct shear test, hydrostatic triaxial compression test and triaxial shrinkage test, is carried out on Lanzhou undisturbed and remolded Q(3) loess, using four-alliance direct shear apparatus and improved triaxial shear apparatus. The results show that the cohesions and internal friction angles of undisturbed and remolded loess increase linearly with matric suction. The greater the matric suction, the greater the shear strength. Because the undisturbed Q(3) loess has well-developed structures, it possesses a higher strength. Compared to the undisturbed loess, the bonds between the remolded loess particles are weaker, so that the remolded loess shows lower shear strength than undisturbed loess, especially at low matric suction and high water content. The cohesion structural parameter c M-phi and internal friction angle structural parameter M-phi are defined, and the variations of the coupling values of both cohesion and internal friction angle with suction are determined, providing a new approach to selecting shear strength parameters of practical engineering projects. It is shown that both yielding stress and yielding suction of undisturbed loess are higher than that of remolded loess, and elastic area of undisturbed loess is larger than that of remolded loess in p-s plane. The difference between the yielding stresses of undisturbed and remolded loess increases linearly with the increase of the suction, and yielding suction approaches a constant, which is almost unaffected by net mean stress. Before yielding, due to intensive structural characteristics, the deformation of undisturbed loess is less significant than that of remolded loess; but after yielding, the deformations of both are similar. The test results provide the experimental foundation for the development of structural model and some guidelines for the engineering construction in loess area.