An Improved Creep Model for Granite Based on the Deviatoric Stress-to-Peak Strength Ratio

Time-dependent behaviors of rocks can be attributed to long-term applied stress, which occurs in most underground engineering projects. The creep behavior of granite from the Shuangjiangkou underground powerhouse in western Sichuan Province, China, was investigated via creep tests. It is shown that granite has an obvious creep behavior under different confining pressures and deviatoric stresses. The new dimensionless parameter DPR, which is defined as the ratio of the deviatoric stress to the peak strength, is proposed for experimental analysis. The relationships between the instantaneous strain and final visco-elastic creep strain of granite with DPR are established under different confining pressures. It is shown that when DPR is equal to 0.63, the final visco-elastic creep strain of rocks will reach a minimum value. Based on DPR and the secondary creep strain rate, a unified method for defining the long-term strength of rocks is proposed, and the granite will enter in secondary creep with a nonzero strain rate when DPR > 0.9501. The Nishihara model is improved, and the relationships between the parameters of the improved model with confining pressures and DPR are obtained. The improved creep model can satisfactorily represent the primary creep strain of granite and secondary creep strain of granite with time under a given confining pressure and deviatoric stress.