Little is known about the application of the failure criterion under low confining pressure. However, it is a very important problem because there are lots of problems of unsaturated soils near the surface, which is under low confining stress level. Therefore, hollow cylinder torsional shear tests were conducted in order to examine the mechanism of failure in low confining stress level, taking into account of tensile stress on an unsaturated cohesive soil. α=45o and b =0.5 under a constant pnet (=25 to 300kPa) and matric suction, s (=100 to 400kPa), were chosen as the shear conditions in this study. The difference between the failure mechanism under compressive stress and under tensile stress was discussed in detail. Finally the application range of the existing failure criterion and a new failure criterion was proposed using the experimental results. The main results of the study are summarized as follows:
1. When the matric suction, s, is larger, the initial stiffness becomes steeper and a more dilative response is observed during shearing.
2. When the net stress is larger, a more contractive response appears and the failure occurs at a larger shear strain level during shearing.
3. The failure lines regarding matric suction on q - pnet plane tend to converge in one line through the strain softening behaviour after failure.
4. The angle of failure plane becomes steeper in lower net stress. It can be interpreted that tensile failure might be induced in low net stress.
5. It was confirmed from the experiment that the failure occurred promptly when the (σ3-ua ) became negative. It means that the existing failure criterion cannot apply to this failure mode.
6. It was shown that the assumption, in which the failure occurs at (σ3-ua )=0, was practically effective. Finally, the general failure criterion on unsaturated soils was achieved by addition of the new failure criterion, which is (σ3-ua )=0, to the existing failure criterion.