The geotechnical behavior of residual soil differs essentially from that of sedimentary soil because of the weathering pedogenesis of the former, thereby posing significant difficulties in predicting soil response. In this study, the shear strength and stiffness of natural granite residual soil are evaluated through systematic monotonic and cyclic simple shear tests performed using a hollow-cylinder apparatus. Simple shear testing provides critical information about soil behavior under plane-strain conditions and involves principal stress rotation, which is beyond the scope of triaxial shear tests. The mechanical properties of granite residual soil measured in monotonic simple shear are found to be different from those obtained through other routine laboratory tests such as triaxial shear and resonant column tests. Whereas the conventional triaxial compression test gives unconservatively high soil strength parameters, those from simple shear testing appear more reasonable than the triaxial results. The cyclic behavior of residual soil in simple shear is dominated by the cyclic stress ratio, a higher value of which results in more significant deformation and pore water pressure build-up as well as more rapid stiffness degradation. This is particularly the case when the cyclic stress ratio exceeds a critical value in the range of 0.125-0.1875. No consistent pattern can be established for how the loading frequency influences soil responses within the range of 0.01-1.0 Hz. This study enriches the techniques for characterizing residual soil and provides new data sets about its mechanical behavior.

Although extensive investigations have been performed on soil anisotropy, little information is available regarding its quantification. The quantification of anisotropy reflects the extent to which the inherent anisotropy of soil controls its mechanical behavior and so it is crucial for connecting comprehensive experimental research with soil constitutive models and engineering practices. The present study evaluates the level of shear strength anisotropy in various types of soil. A database is compiled of the variations of shear strength with the direction of major principal stress (alpha) as established via hollow-cylinder torsional shear tests, covering more than 20 types of soil. This study reviews critically the existing methods for strength anisotropy, finding none of them quantifies anisotropy satisfactorily. The present study proposes using the bracketed area by the S(alpha)/S(0)-alpha curve and the line S(alpha)/S(0) = 1 to measure the level of strength anisotropy, where S(alpha) is the soil strength as a function of alpha. The proposed method in this study can measure the strength anisotropy levels of sands, silts, clays, residual soil, calcareous sand, mudstone, and glacial till, among others. Additionally, the anisotropy degree measured using the proposed method appears to be consistent with the results of microstructural anisotropy evaluations. This study enhances the understanding of soil anisotropy by providing a comprehensive database of soil strength anisotropy and proposing a general method for its quantification.