Comprehensive Dilation Angle Model for Strain-Softening and Brittle Rocks

Triaxial compression tests are conducted to determine the dilation behavior of five different rocks, namely, shaly sandstone, fine-grained sandstone (weak), coarse-grained sandstone (weak), fine-grained sandstone (strong), and granite. After yielding, volumetric and shear plastic strains and dilation angles are determined for each sample at every 5-s interval. The dilation angle increases sharply with the initial development of plastic shear strain for strain-softening rocks. On the other hand, for brittle rocks, the dilation angle reaches its peak gradually with plastic shear strain. Based on the experimental data, a three-parameter dilation angle model is developed as a function of plastic shear strain. These parameters depend on the rock types and the ratio of σ3/σc, where σc is the uniaxial compressive strength of rock. In terms of the coefficient of determination (R2), the proposed dilation model fits the experimental data better than the existing dilation model proposed in a previous study . The model also predicts the peak dilation angle more accurately if the angle increases sharply at the initial stages of the plastic shear strain increment. Results show that the prediction error of the proposed model is within 10.27% compared to 15.06% by the existing model. The results also show that the peak dilation angle decreases hyperbolically with the increase of confining stress (σ3).