Research On Constitutive Models And Instability Rules Of Gas-saturated Coal

Our country is one of the most serious coal and gas outburst countries in the world. In the recent years, the intensity of coal and gas outburst and the fatalities caused by the outburst are growing with the increasing of mining depth, pressure of gas and complexity of mining condition, and the work of forecasting, preventing and controlling coal and gas outburst is very severe urgent. The comprehensive effect hypothesis shows that coal and gas outburst is caused by the comprehensive effect of ground stress, gas pressure and the physical and mechanical properties of coal. In the course of a coal and gas outburst, mechanical properties, creep characteristics and seepage characteristics of gas-saturated coal are the main factors which control the occurring and development of the outburst. Therefore, research on constitutive model and failure criterion of gas-saturated coal is very important for further revealing the mechanism of coal and gas outburst and preventing and controlling the outburst. Based on the research results of predecessors, applying experimental study, theory analysis and numerical simulation, the goal of this paper is establishing constitutive models and failure criterions of gas-saturated coal and validating them by using testing data.The main research results obtained in this paper are as follows:①By using of a testing system which is made up of a self-made triaxial gas-seepage device and a servo-controlled testing machine, mechanical properties, creep characteristics and seepage characteristics of gas-saturated coal were systematically studied in laboratory. In experimental study of mechanical properties of gas-saturated coal, the effects of both confining pressure and gas pressure on deformation characteristics and compressive strength were analyzed, and related rules were summarized. At the same time, the failure mode of gas-saturated coal under triaxial compression was also derived. In experimental study of creep characteristics of gas-saturated coal, the universal law of creep deformation of gas-saturated coal was summarized bases on testing results. In addition, the creep deformation and creep rate of attenuating and steady-state creep stages were analyzed in detail, the rules of effects of deviatoric stress, confining pressure and gas pressure on creep rate were obtained. Moreover, the start condition and creep rate characteristics of accelerating creep stage were also educed. In experimental study of seepage characteristics of gas-saturated coal, in according to the testing data, the effect rules of confining pressure and gas pressure on permeability of gas-saturated coal were worked out, as well as the Klinkenberg effect and the effect of complete stress-strain curve were analyzed simultaneously, and some valuable analysis results were found out.②On the basis of testing data, by means of model indentification method, Burgers body is considered as the optimum rheological model for describing the visco-elastic rheological properties of gas-saturated coal. In the meanwhile, a modified visco-elastic-plastic model was proposed for depicting the creep deformation of accelerating creep stage. The one-dimensional nonlinear visco-elastic-plastic rheological model for gas-saturated coal was built by connecting the modified model with a Burgers body in parallel, and then the three-dimensional nonlinear visco-elastic-plastic rheological model for gas-saturated coal was derived. The three-dimensional rheological model for gas-saturated coal was verified by using testing data finally. Besides these, the stability analysis of the three-dimensional rheological model of gas-saturated coal was carried out, and the rheological failure condition of gas-saturated coal was obtained.③In irreversible thermodynamics framework, a coupled elastoplastic damage constitutive model for gas-saturated coal was established according to the continuum damage mechanics method. The constitutive model can effective describe many physical phenomena and mechanical behavior of gas-saturated coal, such as elastoplastic deformation, gas adsorption effect, volume expansion, confining pressure sensitivity, degeneration of elastic modulus, anisotropic damage, strain hardening and softening, non-associated plastic flow, and so on. Furthermore, the constitutive model was verified in terms of testing results.④A solid-gas coupling dynamic model for gas-saturated coal was proposed by introducing the swell stress of gas adsorption into the effective stress principle of porous. The dynamic model can more truly and completely reflect the coupling effect between solid and gas in gas-saturated coal than previous similar models developed because not only the dynamic change property of porosity and permeability of gas-saturated coal but also the gas compressibility and the skeleton deformability of gas-saturated coal were taken into account in this model. A finite element model related to the solid-gas coupling dynamic model of gas-saturated coal was constructed by using COMSOL-Multiphyics software, and then the numerical results was yielded. In addition, the Klinkenberg effect was also discussed by means of numerical simulation method.⑤Based on the analysis system of testing machine and specimen, a cusp catastrophe instability model for creep failure of gas-saturated coal was built in terms of elementary catastrophe theory, and then the creep instability condition of gas-saturated coal under triaxial compression was obtained.⑥A strength criterion for gas-saturated coal was proposed in according to the summarization of strength criterions of rock-like material at home and abroad and experimental results. This strength criterion can describe not only the intermediate principle stress effect and the nonlinear strain hardening of gas-saturated coal under triaxial compression but also the tensile failure characteristics.