The failure criterion represented by matric suction has been proposed on unsaturated soils. Although there is plenty of unsaturated ground under low confining pressure(lower than 100kPa) near the surface of the earth, the application of the failure criterion under low confining pressure has not been confirmed.

To achieve this objective, strength and deformation characteristics of an unsaturated cohesive soil were studied, with close attention paid to tensile stress using a hollow cylinder torsional shear apparatus. Experiment was carried out under the conditions of constant suction, where it is easy to estimate the effect of matric suction. The difference between the failure mechanism under compressive stress and under tensile stress is investigated in this study. Then, the application range of the existing failure criterion is discussed with a focus on low confining stress range. Finally, new failure criterion on unsaturated soil considering tensile stress is proposed on the basis of the experimental results.