Study On Deformation And Failure Of Deep Coal With Low Permeability By The Finite Deformation-Discontinuous Deformation Method

Gas is the main cause of coal mine disasters,and gas extraction is the fundamental way to prevent the gas disasters.More than 50% of coal seams in China have the characteristics of low permeability and strong adsorption,thus gas extraction is extremely difficult.Measures such as protective layer mining and dense coal drilling can effectively increase the gas permeability of coal seams and increase the gas drainage rate.However,there are problems,such as operation by experience and poor efficiency are common in engineering practice.And there is still lack of scientific guidance.In the process of permeability improvement of coal seams,the coal and rock masses have not only the large-scale nonlinear deformation,but also accompanied by discontinuous fracture propagation and fracture.Currently,there are no effective theoretical-description methods and analysis methods.Therefore,based on the finite deformation theory and the discontinuous deformation analysis method,combined with experiments and numerical simulations,the mechanical properties and deformation failure rules of deep low-permeability coal rocks are studied.The main developments are as follows:(1)The effects of temperatures,confining pressures,and gas pressures on the mechanical properties of coal and rock were studied by using the orthogonal test method.It is found that the confining pressure has the most significant influence on the mechanical parameters and the fractal dimension of failure surface.And the significance level is 95%.The significant values of the gas pressure and temperature on the mechanical parameters and the fractal dimension of failure surface are more than 90%.The linear model was used to describe the relationship between various factors and mechanical parameters and the fractal dimension of failure surface.(2)Based on the theory of finite deformation,the deformation and failure process of coal is analyzed with the mean rotation,and the large deformation constitutive equation of coal based on the mean rotation angle was established.The evolution law of the mean rotation angle with the deformation and failure of coal studied.The evolution law of the tangent modulus and energy density with the mean rotation angle was discussed.It is found that the mean rotation angle can reflect the various stages of coal deformation(compaction,elasticity,and yield stages).In this view,the large deformation constitutive equation of coal based on the mean rotation angle was established,and the accuracy of the model was verified by experimentaldata.(3)The creep characteristics of coal induced by different temperatures,confining pressures and gas pressures were discussed.It was found that the decaying stage of the coal creep can be described by the exponential function.The Burgers model was used to describe the steady-state creep of coal.Parameters of the creep model were identified,and the equations for the steady-state creep rate by considering the influence of temperature,confining pressure and gas pressure were established.(4)Combined with the finite deformation geometry field,the established coal and rock constitutive equation and creep equation were added to the DDA code,and the deformation and stress fields of the coal seam drilling-slotting model were analyzed.The so called " bottle cork effect",encountered in the site extraction project,is interpreted.The effect of the slot width,height and ground stress on the pressure relief discussed.Then the law of the shrinkage of the slot with time was studied.the results mentioned above can provided a theoretical basis for the gas extraction in engineering practice.