EFFECT OF NON PLASTIC SILT ON THE ANISOTROPIC BEHAVIOR OF SAND

The inherent anisotropic behaviour of sand and the effect of adding different amounts of non plastic silt are studied in this paper. A series of undrained torsional shear tests with constant alpha and b-values were conducted by hollow cylindrical torsional shear (HCTS) apparatus on Firoozkuh sand containing a wide range of silt percentage. The dry deposition method was selected in the tests. The principal stress direction or inclination angle with respect to the depositional (vertical) direction has a considerable effect on the response of sand that is related to inherent anisotropy in sand fabric during sedimentation. In addition, adding different amount of silt to the host sand causes dramatic strain softening response in the behaviour even at high relative densities, whereby silty sands present higher tendency to flow compared to pure sands. The results show that adding silt to sand skeleton decreases the anisotropy effect up to a certain silt content and then it starts to increase. This phenomenon is attributed to the mixture's unstable structure. Sand-silt-sand contacts cause the meta-stable and unstable fabric. Flow potential increases by increasing the silt content till thirty percent and it has a direct relation with inclination angle alpha. Also, brittleness index is influenced much more by silt content at thirty percent compared to inclination angle.