Triaxial mechanical properties and constitutive model of transversely isotropic slate

The slate is a typical transversely isotropic material, and its physical and mechanical properties have obvious directivity. In this paper, a series of triaxial compression tests were carried out on the slate specimens with five groups of bedding angles under two confining pressure levels (5 MPa and 10 MPa). The testing results indicated that the peak and residual strength values increase with increasing confining pressure, but the confining pressure has less effect on the radial strain of the specimens under the current confining pressure range studied. The axial strain and radial strain values increase by 17% and 6% during the elastic stage, respectively. The triaxial strength of the slate has nonlinear relationship with its bedding angle. The maximum peak strength occurs at the horizontal bedding angle of the specimens, and the minimum peak strength occurs at the bedding angle of 45° and the minimum residual strength occurs at the bedding angle of 60°. The angles between minimum principal stress of slate and bedding plane of rock mass change due to stress redistribution after excavation. Based on the relationship between mechanical properties of slate and bedding angle, the elastoplastic constitutive model of slate was established. In the global coordinate system, the elastic part of the stress-strain relationship was described by the generalized Hooke’s law, and the elastic stiffness matrix was calculated. While considering the effect of bedding angle on rock strength, the plastic part was obtained based on Mohr-Coulomb criterion, non-associated flow rule and strain hardening criterion. The parameters (e. g., elastic and plastic parameters) in the elastoplastic constitutive model can be obtained from the conventional triaxial tests. Through the secondary development of UMAT subroutine by FORTRAN language and implementation of the elastoplastic constitutive model of slate, the constitutive model of slate was numerically established in ABAQUS. The results of theoretical model were compared with those of triaxial tests, and it was found that the elastoplastic constitutive model of transversely isotropic slate was reasonable. © 2023, Central South University Press. All rights reserved.