The nonlinear creep behavior and creep damage constitutive model of coal rock

The creep behavior of coal rock is one of the most important factors which impacts on the stability of coal mine. In this study, the raw coal blocks mined from Baode coal mine in Shanxi were collected and prepared into briquette coal specimens. And the creep behavior of coal rock was accurately analyzed. Results show that the stress levels have a significant effect on coal rock creep processes; the complete three stage of creep process only occur at relatively high stress level. And the ratio of creep deformation to the total strain shows a trend of decreasing firstly and then increasing, which indicated the damage effect caused and accumulated by creep. Moreover, the failure angle decreases with the increase of the confining pressure, indicating the deformation increasement of specimens, which is consistent with the larger increase of radial deformation in the test results. An elastic–plastic damage body was proposed based on the tests and the damage mechanics theory and then replace the viscous element in Nishihara model to construct a new nonlinear creep damage constitutive model. Comparative analysis of experimental data and fitting results found that the model can better reflect the three stage of creep process (decelerating creep stage, stable creep stage, accelerating creep stage) and the nonlinear creep damage behavior of coal rock. The new model can provide theoretical basis for further revealing the creep damage behavior of gas well coal rock.