Creep behavior and damage constitutive model of sandstone: an experimental study on seepage-load coupling

Triaxial creep tests of sandstone under different seepage pressures were carried out to research the effect of seepage on the rheological laws of sandstone. The effect of seepage pressure on the rheological properties of rocks is investigated by analyzing the creep deformation, creep rate and permeability of sandstones. The creep rate curve is related to the seepage pressure and the axial load level. At a constant load level, the change in the creep curve mainly shows a trend of rapid decline, followed by stability for a long time, and finally a rapid increase under the next load level, which is linked to the variables of axial strain, radial strain, and volumetric strain of the sandstone. Permeability, which can reflect the hydration effect of rocks, exhibits a typical three-phase characteristic under seepage pressure: decreasing phase, a steady phase, and an increasing phase. For the damage creep model, firstly, the traditional Nishihara model is modified based on the fractional order theory, and the coupling model reflecting the whole creep process of sandstone is obtained by connecting the acceleration elements describing the accelerating phase of the rock in series, and finally, it is shown through the validation that the model can describe the whole creep process of sandstone under the seepage pressure. This study can provide theoretical support for the stability analysis of slope engineering under seepage conditions.