A novel nonlinear fractional viscoelastic–viscoplastic damage creep model for rock-like geomaterials

In this study, a novel nonlinear fractional viscoelastic–viscoplastic damage creep model is developed for rock-like geomaterials in an attempt to describe the 3-stage creep behaviour. The proposed model consists of viscoelastic and viscoplastic parts. The former considers the viscoelastic response at different time scales, and incorporates the improved Kelvin and Maxwell components modified by the Abel dashpot. The two Abel dashpots represent the effects of the time dependence on the decaying and steady creep stages. The latter part is described by using a plastic slider parallel to a damaged Abel dashpot, and characterises the viscoplastic response adjusted by the overstress and non-orthogonal viscoplastic flow theory in the accelerated creep stage. The proposed model was written in finite difference form and transcribed into object-oriented C++ for implementation in FLAC3D. By calibrating the available experimental data, the numerical simulation with the proposed model shows a successful description of the full 3-stage behaviour. In addition, the proposed model adequately predicts the delayed deformation caused by the accelerated creep in the tunnel simulation, which provides a valid reference for the subsequent support.