Application of suction stress to shear strength in direct shear test for unsaturated soil under low confining pressure

The effects of matric suction on the shear strength of unsaturated soil have not been fully understood for low stress level. In this study, the relationships among the matric suction, the shear strength and the deformation under low confining pressure were examined by using the modified opening type direct shear apparatus for compacted specimen of weathered granite. The Suction stress-SWCC Method (SSM), which determines the matric suction value for the direct shear test, was proposed to analyze the shear strength behavior for unsaturated state. It was found that the peak shear strength for unsaturated state are arranged uniquely on the same failure line for saturated state when the suction stress is took account as a component of confining pressure. We also found that the shear strengths for unsaturated soil can be estimated by using the shear strength equation applying SSM.