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The initial and induced fabric anisotropy of granular materials

A granular material is most likely to possess an initial anisotropic structure due to the depositional process or the previous stress history. There is also induced fabric anisotropy during shearing. Concerning the significant influence of fabric anisotropy on the stress-strain behaviour of a granular material, in-depth investigation on the two kinds of fabric anisotropy is essential. Initial fabric anisotropy is studied by generating anisotropic specimens using the deposition method, and induced anisotropy is investigated by applying deviatoric shearing. Observations show that prominent fabric anisotropy is developed during deposition, while deviatoric shearing can also change the internal structure significantly. When subjected to continuous shearing, unique fabric anisotropy is developed at large strain levels. This paper also explores the possibility of measuring the initial fabric anisotropy through conventional laboratory experiments. Numerical simulation shows that it is possible to compensate the initial fabric anisotropy by external anisotropic consolidation. A standard procedure is proposed. A different stress ratio leads to a different amount of induced anisotropy, hence residual stress ratio after unloading. The stress ratio, which brings the specimen back to the isotropic state, is used to serve as an index to quantify the initial fabric anisotropy.