Modeling the Stress-Dilatancy Relationship of Unsaturated Silica Sand in Triaxial Compression Tests

It is well known that partial saturation increases the shear strength and dilatancy of unsaturated sand. However, little research has been carried out on the actual stress-dilatancy relationship. This paper shows that the increase in peak shear strength caused by partial saturation is consistent with an increase in dilatancy, and that conventional stress-dilatancy theories are still valid for unsaturated sand. The use of state indexes as a proxy for dilatancy were investigated and extended to unsaturated sands. Additionally, these indexes can be used to establish a critical state line that is based on material properties only. The validity of the stress-dilatancy theories and the use of state indexes offer simplicity in modeling the shear behavior of unsaturated sand. This will be demonstrated in this paper with the Nor-Sand model, with which the wetting collapse can be explained as a consequence of a loss of dilatancy characteristics.