Research On Mechanical Properties And Seepage Law Of Coal And Rock Under The Condition Of Fluid Structure Interaction

Due to the engineering disturbance,stress of coal and rock is redistributed in the process of coal mining.It is subjected to the effect of axial stress loading and confining pressure unloading.While because of the flow of fluid,the coal and rock is coupled by the stress field and seepage field.Therefore,this paper takes the coal and sandstone as the research object,and then the study on the mechanical properties and seepage law of coal and sandstone under different stress conditions were carried out by using self-developed “THM coupled with triaxial servo-controlled seepage apparatus for gas Infiltrated coal and rock”.The effects of different loading and unloading rates on the mechanical properties and seepage of coal and sandstone are discussed.The influence of gas pressure on the failure criterion of sandstone was analyzed,and the modified H-B strength criterion was put forward.The main results are as follows:(1)Strength characteristics and evolution rule of permeability of coal under loading-unloading conditions were conducted.The Results show that: The mechanical properties of coal are closely related to the rate of loading and unloading.The peak stress,axial strain and circumferential strain increase linearly with the increase of the rate of loading and unloading,but the change of volumetric strain is opposite.The variation law of deformation modulus of coal under different loading unloading rate ratio is approximately the same.The deformation modulus of coal increases with the decrease of loading and unloading rate,and the same applies to the change of permeability of the rule.(2)Experimental study on the mechanical properties of sandstone under different initial confining pressure and different loading and unloading rates were conducted.The results show that: The peak stress of sandstone is linearly related to the axial loading rate V?1.The relationship between the peak stress and confining pressure can be expressed by exponential function.the variation rate of axial strain from the yield stage to the peak stress was defined as ??1 under different confining conditions.According to the research data,??1 increased with the increase of loading rate.That is to say the axial strain increases rapidly at the yield stage,and the rate of that increases with the growth of loading rate.(3)Experiment was conducted to study the effect of gas pressure on the mechanical properties of sandstone.The results show that: Gas pressure had a weakening effect on the strength of sandstone.Under the effect of gas pressure,the peak stress of sandstone increases nonlinearly with the increase of confining pressure.So the parabolic Mohr criterion and Hoek-Brown strength criterion were employed to analysis the nonlinear strength characteristics.Then based on the results of research,the modified Hoek-Brown strength criteria which considered the effect of gas pressure were proposed.By performance analysis and evaluation,the experimental data is in good agreement with the modified strength criterion.So it can be used to judge the strength of sandstone under the effect of gas pressure.(4)Influence of the rate of unloading confining pressure on the mechanical properties and seepage law of sandstone were conducted.The results show that: The deformation of sandstone under different initial confining pressure and different unloading confining pressure rate was basically same.When the sandstone was destroyed,the stress suddenly dropped under different initial confining pressures,so the failure of sandstone showed obvious brittle failure characteristics.Through the experiment results,the time of the platform stage of stress and the unloading rate of confining pressure can be expressed by a power function.It is to say that the greater the unloading rate of confining pressure is,the more likely the sandstone to be destabilized and the faster the permeability of sandstone increased in the stage of stress platform.The relationship between permeability and unloading time in the stage of stress platform could be expressed by exponential function.