Abstract This study investigates the dependency of steady states on anisotropy in unreinforced uniform angular silica sands and those reinforced with micro-synthetic fibres. In doing so, the principal stress orientation varies from 15° to 60° for an intermediate principal stress ratio of 0.5 and 1.0 and the initial effective consolidation stress of 200 kPa. Twenty-four undrained torsional shear tests are conducted using a hollow cylindrical torsional shear apparatus. On randomly mixing sands with fibres, dilative and strain-hardening behaviour attains prominence, and the undrained strength generally improves. Anisotropy in the samples decreases and distortion of the deviatoric strength envelope is produced by the addition of fibres to the host sand. The results show that fibre reinforcement contribution growth by the increase of the major principal stress direction. This phenomenon is attributed to the change in the loading combination of the sample from compression to torsion. The contribution of the fibre to the strength in all principal stress directions is higher in the sand with smaller median grain size (D50). Furthermore, increasing the median grain size caused the stress-strain curves to be adjacent to one another and increased specimen anisotropy. The flow potential decreases and begins to follow an inverse relationship with the principal stress direction.