Coupled creep-damage and plastic softening/hardening model: Development and engineering application in soft-hard interlayered rock mass

Existing creep constitutive models rarely incorporate studies on the coupling mechanism between creep damage and rock strain softening/ hardening. This study analyzed the strain softening and hardening behaviors of argillaceous sandstone and sandy mudstone during load-induced failure based on the plastic increment theory. These behaviors were then coupled with an improved Burgers creep model to establish a coupled creep-damage and plastic softening/hardening model. Finally, the validity and engineering applicability of the proposed model were verified through FLAC^3D numerical simulations. The numerical simulation results of standard cylindrical specimens show that the established coupling model can effectively reflect the unloading creep deformation law and failure characteristics of argillaceous sandstone and sandy mudstone. Taking the diversion tunnel of a hydropower station in Northwest China as an example for engineering application, the coupled creep-damage and plastic softening/hardening model is introduced into FLAC^3D to carry out numerical simulation calculation of the tunnel under excavation and unsupported creep conditions. The results show that the uncoordinated deformation of the upper and lower walls of the surrounding rock of the tunnel is more prominent. When the buried depth of the tunnel increases to 80 m, the monitoring point C in the sandy mudstone area of the upper wall shows nonlinear accelerated deformation under unsupported creep conditions, and the maximum displacement in the horizontal direction reaches 44.5 mm, and the maximum displacement in the vertical direction reaches 53.5 mm. The coupled creep-damage and plastic softening/hardening model established in the research results can well describe the whole process of uncoordinated deformation and failure in the unloading creep process of soft-hard interbedded rock mass.