Study on creep characteristics and unsteady creep model of roadway surrounding rock

In order to study the creep behaviors of the surrounding rock under different confining pressure characteristics, the triaxial creep test of sandstone taken from Fuxin Hengda Coal Mine was carried out by MTS812.02 test system. Based on the Nishihara model, the parameters of the creep model were made unsteady by introducing the coefficients affecting the degree of damage, and the elastic creep, viscoelastic creep and viscoplastic creep were integrated respectively, and the improved Nishihara creep model was obtained by using the rheological model series theory. The model was substituted into the Origin fitting software, and the least square method was used to identify the parameters of the creep model, and the test data and the model curve were compared to verify the correctness of the creep model. The results show that the increase of confining pressure increases the creep bearing capacity of the rock, delays the acceleration creep start time, and prolongs the duration of rock failure. When the axial load is small, only the decay creep stage exists, and the creep tends to be stable. When the axial load is large, the creep curve is first convex and then transformed into a straight line, attenuating the creep phase and stabilizing the creep. The variable phases are more obvious. When the axial load is increased to the breaking load, the three stages of creep appear. When the confining pressure is 10 MPa and 15 MPa, the axial isochronous stress-strain curve of the rock begins to diverge obviously when the stress level is 60 MPa and 80 MPa respectively, indicating that the creep changes from linear to nonlinear. The established unsteady creep constitutive model can not only describe the creep three stages of sandstone well, but also make up for the disadvantage that it is difficult to describe the accelerated creep characteristics. The experimental data and the model curve have a good fitting degree, which indicates that the improvement of the unsteady parameter of the Nishihara creep model is correct and reasonable.