An elasto-plastic and viscoplastic damage constitutive model for dilatancy and fracturing behavior of soft rock squeezing deformation

Soft rock squeezing deformation mainly consists of pre-peak damage-dilatancy and post-peak fracture-bulking at the excavation unloading instant, and creep-dilatancy caused by time-dependent damage and fracturing. Based on the classic elasto-plastic and Perzyna over-stress viscoplastic theories, as well as triaxial unloading confining pressure test and triaxial unloading creep test results, an elasto-plastic and viscoplastic damage constitutive model is established for the short- and long-term dilatancy and fracturing behavior of soft rock squeezing deformation. Firstly, the criteria for each deformation and failure stage are expressed as a linear function of confining pressure. Secondly, the total damage evolution equation considering time-dependent damage is proposed, including the initial damage produced at the excavation instant, in which the damage variable increases exponentially with the lateral strain, and creep damage. Thirdly, a transient five-stages elasto-plastic constitutive equation for the short-term deformation after excavation that comprised of elasticity, pre-peak damage-dilatancy, post-peak brittle-drop, linear strain-softening, and residual perfectly-plastic regimes is developed based on incremental elasto-plastic theory and the nonassociated flow rule. Fourthly, regarding the time-dependent properties of soft rock, based on the Perzyna viscoplastic over-stress theory, a viscoplastic damage model is set up to capture creep damage and dilatancy behavior. Viscoplastic strain is produced when the stress exceeds the initial static yield surface fs; the distance between the static yield surface fs and the dynamic yield surface fd determines the viscoplastic strain rate. Finally, the established constitutive model is numerically implemented and field applied to the -848 m belt conveyer haulage roadway of Huainan Panyidong Coal Mine. Laboratory test results and in-situ monitoring results validate the rationality of the established constitutive model. The presented model takes both the transient and time-dependent damage and fracturing into consideration.