Investigation of strength anisotropy properties of tufanbeyli lignite basin units

In this study, the shear strength anisotropy of the main lignite unit in the Tufanbeyli Basin was investigated by direct shear (shear box) tests performed on four block samples. From each lignite block, oriented specimens were prepared with the shear plane parallel (β = 0°), perpendicular (β = 90°) and inclined at 30°, 45° and 60° to the bedding planes, and changes in shear strength parameters were quantified. The relationships between shear stress (τ) and normal stress (σₙ) obtained from the tests were evaluated on the basis of the Mohr–Coulomb failure criterion, and peak (c, φ) and residual (cᵣ, φᵣ) shear strength parameters were derived under undrained (total stress) conditions. Peak cohesion values (c ≈ 86–219 kPa) generally reach their minimum at β = 0° and 30°, whereas the highest values occur at β = 90°. The internal friction angle (φ ≈ 33–57°) varies with orientation angle, but does not exhibit as clear a pattern between blocks as the cohesion. For each block, individual anisotropic strength functions were defined, and representative bench slope stability analyses showed that the factors of safety obtained with the anisotropic strength definition are significantly higher than those computed using an isotropic c–φ pair. The results demonstrate that the shear strength behaviour of lignite, as a bedded rock, is sensitive to orientation, and that incorporating anisotropy into geotechnical design is essential for a realistic representation of the engineering performance of such units.