Isotropic damage constitutive model for time-dependent behaviour of tunnels in squeezing ground

The conventional constitutive models employed to describe rock mass creep behaviour associated with the squeezing mechanism are widely adopted in research and design practice. However, these models fail to explicitly replicate the 3-phase creep behaviour especially the accelerated creep phase; only the transient and steady-state creep phase are described explicitly. Hence, this paper presents a viscoelastic viscoplastic with isotropic damage (EVPD) constitutive model that describes the 3-phase behaviour explicitly. This is achieved by the Newtonian dashpot replacement with the fractal-order spring-pot that incorporates isotropic damage effect. Derivation of the fractal-order derivative-based creep constitutive equations is conducted using scaling transformations. The model is then calibrated using experimental data and its derived constitutive equation is implemented in FLAC(3D) using object-oriented C + +. Furthermore, its numerical implementation is employed to simulate delayed response of a tunnel excavated in squeezing ground. The average radii of yield zone around the tunnel estimated by the CVISC and EVPD constitutive models are approximately 8.7 m and 14.7 m, respectively. Whereas the deformation is approximately 29% and accounts for 8% more than the CVISC estimation, the disparity attributed to time-dependent behaviour. It is observed that the EVPD constitutive model describes the creep mechanism and mean deformation in squeezing ground reasonably well.