The long-term creep deformation of soft and hard rock strata may be very uncoordinated

which poses a great threat to the safety and stability of excavation engineering. A series of creep tests and numerical simulations were carried out on two types of rocks (i.e.

sandy mudstones and argillaceous sandstones) with large differences of hardness in the same excavation section of diversion tunnel of a hydropower station under unloading conditions. Results show that under the condition of unloading confining pressure

the lateral creep deformation of the specimen develops more rapidly than the axial deformation and a significant lateral expansion occurs. The specimen under a higher initial confining pressure exhibited a greater lateral expansion effect in the unloading creep failure. The deformation rate of argillaceous sandstone in the steady creep stage was slower than that of the sandy mudstone. Meanwhile

the strain values of argillaceous sandstone in the creep failure were smaller than those of the sandy mudstone with weaker signs. Based on the Burgers model

damage variables were introduced to establish a creep damage constitutive model. The model curve well described the linear creep characteristics of rocks before the failure deviatoric stress level and the nonlinear accelerated creep characteristics under the failure deviatoric stress level. The numerical simulations of creep deformation of soft and hard rock strata show that at the early stage

the deformation difference between soft and hard rock contact points was not large. With the increase of time

interlayer dislocation occurs at the interface

which is very unfavorable to the overall stability of surrounding rock and should deserve sufficient attention in the engineering practice.