Creep constitutive model of yellow sandstone under coupling action of unloading and wet-dry cyclic damage

The study focuses on the creep characteristics of significant yellow sandstone for water conservancy, hydropower, and other water-related slope excavation unloading rock-graded loading creep characteristics. It conducts a uniaxial graded loading creep test on yellow sandstone under different pre-peak unloading and wetting-drying cycles. The improved nonlinear Nishihara model was obtained by introducing a nonlinear viscous element with an accelerated creep threshold switch. The sensitivity characteristics of the parameters of the improved creep model were analyzed and a nonlinear creep constitutive model was established, considering the unloading-cyclic intrinsic damage induced by water intrusion. The research results show that: (1) With an increase in the unloading point, the porosity of the rock samples initially decreases and then increases. As the number of cyclic water intrusions rises, the porosity of the rock samples gradually increases, reaching a maximum of 9.58% at an unloading point of 70% uniaxial compression stress (0.7 R_c) after five cycles. (2) Total creep deformation increases with the number of cyclic water intrusions; however, with an increase in the unloading ratio, the original samples show an initial decrease, followed by an increase in creep deformation. With a higher unloading ratio and various instances of cyclic water intrusion, the total creep time of the rock samples, compared to the original samples, is reduced by 21.8% and 23.02%. The creep damage mode gradually changes from shear damage to tensile damage. (3) The sensitivity characteristics of the improved creep model parameters show that transient elasticity modulus E₁ is affected by the coupling of unloading and cyclic water intrusion. The viscoelastic modulus E₂ and viscous coefficient η₁ are mainly affected by unloading and cyclic water intrusion. (4) Based on the strain equivalence principle of damage mechanics, the damage treatment of the parameters in the original model is improved to construct a nonlinear creep constitutive model that considers unloading-cyclic water intrusion damage. A parameter inversion and comparison to the traditional Nishihara model reveal an average relative standard deviation of 0.271%, significantly less than 1%, indicating a more accurate nonlinear creep constitutive model. The research results are crucial for analyzing the long-term stability of water-related steep rocky slopes post-excavation and unloading and for preventing and controlling creep-type landslide disasters.