Experimental Research on Unsaturated Soils Shear Strength Theory

Unsaturated soil is a 3-phase medium, so the mechanical properties of vapor, liquid and solid phase, and energy balance are much more complicated than saturated soils. The macroscopic parameter matric suction and suction stress are widely involved in the research of unsaturated soils shear strength. On the basis of the previous tests results presented by scholars - choosing the unconsolidated red-clay soils as the research object - this paper analyzes the effect on shear strength that resulted from matric suction, suction stress, and moisture content. In the study, we did the soil water characteristic curve tests for unsaturated soil and the direct shear tests by applying the soil pore pressure dewatering and controlling clay content to make soil samples of differing moisture content and gradations. We found that the shear strength of unsaturated soil depended largely on the degree that soil matric suction changed into the effective cohesion c', or the conversion rate that the matric suction transformed into suction stress. There must be a certain moisture content that makes the soil shear strength a maximum regarding that the matric suction and moisture content was in keeping with the soil water characteristic curve. In addition, clay content and gradation of grain has an enormous impact on the transformed rate. It should be paid enough attention to in results analysis.