Verification of rationality of two stress state variables of unsaturated soil from deformation, moisture change and strength

In order to verify the rationality of two stress state variables of unsaturated soil, the conventional triaxial apparatus is improved: three pressure-volume controlles are employed to respectively control the pore water pressure, inner and outer chamber pressures so as to improve the measurement precision of volume change and water content; and the axial load by loading derice can conveniently control the deviator stress. Based on the previous verified results of isotropic tests by the authors, the improved unsaturated soil triaxial apparatus and remolded Q3 loess are used to conduct two types of experiments: one is two groups of consolidation undrained shear tests under the net confining pressure, deviator stress and suction, and the other is three groups of consolidation drained shear tests under the net confining pressure and suction. In the first type of tests, with the equivalent change (increase or decrease) of the confining pressure, pore water pressure and pore air pressure, and without the change of the net confining pressure, suction and deviator stress, it is found that the volume change and moisture change are very small and can be neglected. In the second type of tests, each group contains two consolidation drained shear tests with the same net confining pressure and suction. Among the tests, one is that after the completion of the consolidation, the direct drained shear tests are performed; the other is that after the completion of the consolidation, with the equivalent increase in the total confining pressure, pore water pressure and pore pressure and without the change of the net confining pressure and suction, the drained shear tests are performed. It is found that their shear strengths, volume changes in the shearing process, generalized shear strains and drainages are very close and can be considered to be equal. The use of two stress state variables to describe the stress state of unsaturated soil is reasonable in terms of deformation, moisture change and strength, which consolidates the mechanics theory of unsaturated soil and provides a reliable experimental basis for the theory of two stress state variables of unsaturated soil.