Anisotropic Behavior of Fiber-Reinforced Sands

The contribution of fibers to the strength of fiber-reinforced soils is dependent on multiaxial stress space. This study furthers knowledge on the dependency of steady states on anisotropy in unreinforced and reinforced (with 1.5% microsynthetic fibers) well-sorted sands with different shape and size properties. For this purpose, the direction of principal stress orientation varied from 15° to 60°, for an intermediate principal stress ratio of 0.5 and 1.0 and initial effective consolidation stress of 200 kPa. For this study, 24 undrained torsional shear tests were conducted using a hollow cylindrical torsional shear (HCTS) apparatus. When sands were randomly mixed with fibers, a dilative and strain-hardening behavior governed, and undrained strength generally improved. The samples’ anisotropy decreased, and a distortion of the deviatoric strength envelope was produced by the addition of fibers to the host sand. The results indicated that fiber reinforcement contribution is augmented with the growth of the major principal stress direction. This phenomenon is attributed to the sample loading combination changes from compression to torsion. The fiber strengthening contribution across all principal stress directions was higher in the sand with a larger median grain size (${\mathrm{D}}_{50}$) and angularity. Further, an increase in particle shape scale (${\mathrm{r}}_{\mathrm{s}}$ and ${\mathrm{r}}_{\mathrm{r}}$) ratios caused the stress-strain curves to approach another and reduced occurrence in specimen anisotropy. Further explanations on the effect of fiber addition are presented.