Creep behavior of deep sandstones under stepwise incremental loading and unloading conditions

To study the creep deformation characteristics and visco⁃elastic⁃plastic deformation law of sandstone under deep high stress environment，the triaxial compression creep tests were conducted on sandstone specimens after the ini⁃ tial stress was restored by step increment loading and unloading method. According to the test results，the strain was decomposed into instantaneous elastic strain，instantaneous plastic strain，sticky glue elastic strain and plastic strain for further analysis. The steady⁃state creep rate method was used to estimate the long⁃term strength of deep sandstone，and verify the model based on fractional order to improve the applicability of the deep sandstone creep model. Contrast was conducted to analyze the fitting effect of the Nishihara model and the improved deep sandstone creep model on the test data. The results show that in the creep process of sandstone，in the instantaneous elastic strain stage the deformation is maximal，which accounts for 70% to 80% of the total deformation. With the loading level increasing，the transient elas⁃ tic strain and the transient plastic strain of the deep rock specimen increase gradually. However，the increment of the transient plastic strain decreases first and then increases，indicating that the defective microelements in the specimen are destroyed during loading. With the stress increasing，the unrecoverable viscous flow becomes stronger. Moreover， the viscoplastic strain increases rapidly and the plastic deformation accumulates. When reaching the high stress level， the creep deformation shows the coexistence of viscoelastic deformation and viscoplastic deformation. By analyzing the steady⁃state creep，the stress threshold when the steady⁃state creep rate is not zero was obtained. It was regarded as the basis to calculate the long⁃term strength of deep sandstones. It was estimated that the long⁃term strength of deep sand⁃ stone is 66 MPa，around 77% of conventional strength. The least square method was used to fit the creep model and the Nishihara model of deep sandstones and identify the parameters. It is concluded that the improved fractional model can better describe the constant velocity creep and accelerated creep of deep sandstones.