A rock creep constitutive model is the core content of rock rheological mechanics theory and is of great significance for studying the long-term stability of engineering. Most of the creep models constructed in previous studies have complex types and many parameters. Based on fractional calculus theory, this paper explores the creep curve characteristics of the creep elements with the fractional order change, constructs a nonlinear viscoelastic-plastic creep model of rock based on fractional calculus, and deduces the creep constitutive equation. By using a user-defined function fitting tool of the Origin software and the Levenberg–Marquardt optimization algorithm, the creep test data are fitted and compared. The fitting curve is in good agreement with the experimental data, which shows the rationality and applicability of the proposed nonlinear viscoelastic-plastic creep model. Through sensitivity analysis of the fractional order β2 and viscoelastic coefficient ξ2, the influence of these creep parameters on rock creep is clarified. The research results show that the nonlinear viscoelastic-plastic creep model of rock based on fractional calculus constructed in this paper can well describe the creep characteristics of rock, and this model has certain theoretical significance and engineering application value for long-term engineering stability research.

The creep mechanical properties of rock under dry-wet cycles are of great significance for studying the long-term aging stability of engineering rock and soil. In the past, there were few studies in this area, and most of the dry-wet cycle tests on rock samples did not conform to the actual stress state of the rock. In view of the shortcomings of these studies, this paper innovatively carried out the dry-wet cycle test of the rock under the continuous state of the stress field, and studied its mechanical properties. The specific method is to take carbonaceous shale as the research object, and use the soft rock shear rheological test system independently developed by our research group to carry out the shear creep test of carbonaceous shale under the action of dry-wet cycle. The test results show that the creep full-time curves of carbonaceous shale under different dry-wet cycles show a step-shaped curve shape. The dry-wet cycle has a significant effect on the deformation characteristics of carbonaceous shale. With the increase of the number of dry-wet cycles, the instantaneous strain of the rock gradually increases, the instantaneous shear modulus decreases from 596.650 MPa at 0 times to 365.199 MPa at 12 times, and the attenuation rate reaches 38.79 %. The creep strain and cumulative creep strain become larger, the stress required for accelerated creep decreases from 3.29 MPa to 2.75 MPa, and the accelerated creep time in the third stage increases from 11.892 h to 5.316 h, and the creep effect is more significant. The long-term strength of carbonaceous shale decreases from 3.05 MPa to 2.49 MPa, and the decrease increases with the increase of dry-wet cycles. The more the number of dry-wet cycles, the smaller the undulation of the shear failure section of the carbonaceous shale, and the smoother the surface. The research results have important guiding significance for the long-term aging stability analysis of engineering rock and soil mass subjected to repeated dry-wet cycles.

For a long time, one of the important safety problems in open-pit mines is the stability of a large number of high slopes with gently inclined soft interlayer. Rock masses formed after long geological processes generally have some initial damage. Mining works also cause varying degrees of disturbance and damage to rock masses in the mining area during the mining process. This phenomenon means that accurate characterization of the time-dependent creep damage for rock masses under shear load is necessary. The damage variable D is defined based on the spatial and temporal evolution laws of shear modulus and initial level of damage for the rock mass. In addition, a coupling damage equation between the initial damage of the rock mass and shear creep damage is established based on Lemaitre’s strain equivalence assumption. Kachanov’s damage theory is also incorporated to describe the entire process of time-dependent creep damage evolution for rock masses. A creep damage constitutive model that can reasonably reflect the actual mechanical properties of rock masses under multi-stage shear creep loading conditions is established. This takes into account multi-stage shear creep loading conditions, instantaneous creep damage during the shear load phase, staged creep damage and factors influencing the initial damage of rock masses. The reasonableness, reliability and applicability of this model are verified by comparing the results of the multi-stage shear creep test with calculated values from the proposed model. As opposed to the traditional creep damage model, the shear creep model established in this present study takes into account the initial damage of rock masses and can describe the multi-stage shear creep damage characteristics of rock masses more convincingly.

Under field conditions, the moisture content of rock changes with the weather during prolonged creep. In order to investigate the effect of moisture content change on long-term shear strength and deformation behavior, a shear apparatus for intact rock was developed. Since three prefabricated holes are drilled in the upper part of the rock sample, the water injection device and the gas injection device can be used to inject water and gas into the rock sample alternately during the test to adjust the moisture content without removing the normal load and shear load. By using silicone gasket and seals in the shear box, fluid injection at a pressure of 5 MPa was achieved without leakage. Shear creep tests of argillaceous shale were conducted under both constant and dynamic moisture conditions, and the results were described by the Nishihara model. The experimental results revealed that there are significant differences in the long-term shear strength and deformation of argillaceous shale under different moisture content conditions. The proposed rock shear apparatus can advance the quantitative study of the shear creep properties of rock samples during moisture content changes and has certain practical application value for the prediction of engineering rock mass stability during rainfall.

The moisture content is closely related to the shear creep deformation behavior of soft rock, and the linear creep deformation behavior of soft rock can be described by the classical Nishihara model. However, the classical Nishihara model cannot describe the deformation characteristics of the whole process of shear creep including nonlinear deformation of rocks under the influence of moisture content. In this study, we presented an improved Nishihara model that connected a strain-triggered nonlinear dashpot in series on the classical Nishihara model to describe the whole process of rock creep, and a damage factor was proposed to reflect the effect of moisture content on the rock creep characteristics. The damage factors and related model parameters were determined from results of the shear creep tests, which were performed under four moisture conditions (0%, 0.46%, 0.87%, and 1.24%). The comparisons between model predictions and experimental results show that the improved creep constitutive model proposed here can not only describe the whole creep process well, but also reveal the influences of the moisture content on the creep behavior of rock, which demonstrate its accuracy and usefulness.