Study On Creep Test And Constitutive Relation Of Surrounding Rock In Bojianghaizi Mine

The Bojianghaizi coal mine is located in the town of Borzihaizi in the Inner Mongolia Autonomous Region.Because of the long service life of this large coal mine,it is necessary to conduct theoretical research and experimental research on the rheological properties of its surrounding rock in order to ensure the safety and long-term stability of its structure.The rock samples tested were taken from the shallow Cretaceous sandstones and deep Jurassic sandstone mudstones in the Bojianghaizi coal mine.The conventional uniaxial compression tests and conventional triaxial compression tests were first performed on these two rock samples.The results obtained not only the basic mechanical parameters and deformation parameters of the two rock samples,but also their compressive strength provides a reference for stress grading in the subsequent compression creep tests.At the same time,for the conventional triaxial compression test of rock,the full stress-strain curve is divided into five stages: initial compaction,approximate linear elasticity,plastic hardening,plastic softening,and residual flow.Then,a four-line model is used to characterize the test curve.The results show that the model is consistent with the description of the full stress-strain curve.Among them,the sandstone mudstone has a better fitting effect in the plastic hardening and plastic softening than the medium sandstone.Second,the energy conversion principle is used to analyze the energy conversion during the triaxial compression test.The results show that the kinetic energy density and dissipated energy density increase with the increase of the axial strain,while the elastic strain energy density increases with the axial strain.When it increases,the elastic strain energy density reaches a maximum value when the peak strain is reached,and then decreases as the strain increases.Creep test is the most common and effective way to study the creep properties of surrounding rock.Therefore,it is necessary to carry out uniaxial compression creep tests and triaxial compression creep tests on medium sandstones and sandy mudstones.Subsequently,based on the uniaxial creep experimental data of sandy mudstone and the triaxial creep data of medium sandstone,the non-stationary Nishihara creep model and the damage model based on the Nishihara damage creep model were established.For the unsteady Nishihara creep model,the relationship between the four model parameters of Nishihara model and time or stress is established.Then the four parameters are substituted for the steady parameters of the traditional Nishihara model,and the unsteady Nishihara creep model is proposed.The experimental data identify the model and the results show that the model is reasonable and feasible.For the Nishihara damage creep model,the creep equation of the Nishihara model under the three-dimensional stress state is deduced firstly,and then the evolution equation of the damage variable with the different creep stages of the rock is deduced based on the damage theory.Finally,the damage variable is introduced into the Nishihara model.Based on the comparison and analysis of the triaxial creep test curves of the medium sandstone with the traditional Nishihara model,the results show that the Nishihara damage model is called the traditional model to describe the acceleration stage better.