Rheological model of soft rock in Goupitan by in-situ plate-loading creep tests

Two in-situ plate-loading creep tests on fracture shale in Goupitan Hydropower Station are conducted, in which the pressure is at the same engineering stress level and is loaded under one cycle pattern and the test durations are 1686 and 2299 h, respectively. Some creep characteristics of the soft rock are revealed as follows: (1) Under the engineering stress level, after decelerating creep at the initial stage and the subsequent constant-rate creep continuing for a period of 682 and 910 h, the creep rate tends to zero instead of occurrence of plastic flow, showing that the creep is the attenuation creep in total. (2) After unloading, there's instantly rebound deformation and anelasticity and residual deformation, which is over 50% of the total deformation. A kind of one-way spring element (H_I), which will behave as elastic object under pressure but not rebound after unloading, is used to describe the residual deformation. By connecting the H_I element with the generalized Kelvin model, a changed generalized Kelvin model is established. It has simple components and can comprehensively describe the creep features of soft rock, that is, it is the attenuation creep while loading and there is the residual deformation after unloading. The constitutive equation of the model is derived, and the creep formula is also derived based on the corresponding principle. The model parameters are inversed by taking the creep formula to fit the trial curves.