Experimental investigation on two stress variables proposed by Fredlund from deformation, strength and moisture change under triaxial state of stress

<p>In describing the stress state of unsaturated soil, the two independent stress state variables advocated by Fredlund et al. (1977) is approved by many scholars. However, the experimental verification is not sufficient at present, which is mainly embodied in two aspects. One is that only isotropic compression tests are conducted, and the complex stress condition is not considered. The other is that the verification only involves volumetric strain and moisture change of specimens and does not involve strength and shear strain. In order to further verify the rationality of the two independent stress state variables for unsaturated soil, a triaxial apparatus for unsaturated soil was improved to control pore water pressure, confining stress and deviatoric stress. Two types of tests were conducted with remolded Q3 loess. The first type is consolidated undrained shear tests in which confining pressure, pore water pressure and pore air pressure were equivalent increased or decreased, and net confining pressure, suction and deviatoric stress were kept constant. It is found that volumetric strain and moisture change of specimens are very small and can be ignored. The second type is three groups of consolidated drained shear tests. Each group contains two tests with controlled same confining stress and same suction. One specimen directly sheared after consolidation. The other equivalent increased confining pressure, pore water pressure and pore air pressure after consolidation and kept net confining pressure, suction and deviatoric stress constant, and then sheared. It is found that the shear strength, volumetric strain, moisture change and generalized shear strain of the two specimens are very close. The results of this paper further illustrate that the two independent stress state variables for unsaturated soil are reasonable from the aspects of deformation (including volumetric strain and shear strain), strength and moisture change.</p>