A rock dilation angle model and its verification

Conventional Mohr-Coulomb model and strain-softening based Mohr-Coulomb model often take an assumption of constant dilation angle, which can not be supported by experimental data, and it is observed that the approach is not successful in characterizing the nonlinear deformation behaviors of rocks. Based on published data acquired from modified triaxial compressive tests with volumetric strain measurement performed on seven types of rocks, a mobilized dilation angle model which considers both confining stress and plastic shear strain is developed. Based on the model response and in combination with the grain size description and the uniaxial compressive strength of the seven types of rocks, the model is generalized for four rock types: coarse-grained hard rock, medium-grained hard rock, fine-medium-grained soft rock, and fine-grained soft rock. According to the principle of nonassociated flow rule in the strain-softening models in FLAC, the relationship between the plastic shear strain in the proposed dilation angle model and characteristic plastic parameter in strain-softening model is deduced; and the proposed dilation angle model is implemented in FLAC using FISH language. The dilation angle model is used to predict the volumetric-axial strain relationships of Moura coal; and the results are found to be in good agreement with experimental results. Finally, the importance of using the confinement and plastic shear strain dependent dilation angle model for rock engineering applications is discussed.