Experimental Study On Coal-rock Mechanics And Gas Desorption Characteristics Under Stepped Cyclic Loading And Unloading

With the expansion of mine scale and the increase of mining depth,the " three highs and one disturbance" phenomenon has become more serious,and the suddenness and destructiveness of dynamic disasters have increased.Rock bursts,mining disturbances caused by mine mining,and friction and slippage between coal-rock,etc.make coal-rock always be in a complex stress environment of repeated loading and unloading of stress,and the mechanical properties become more complicated.The combination of the gas stored in coal-rock and the stress state of repeated loading and unloading may cause accidental desorption of gas,resulting in abnormal gas emission,which is very easy to cause gas disaster accidents,and seriously affect the production safety of mining.The mechanics and gas desorption characteristics of coal-rock in the cyclic loading and unloading stress environment formed by the reciprocating loading of displacement are not yet clear.For this reason,stepped cyclic loading controlled by displacement amplitude is designed to simulate the real stress state of coal-rock,and experimental study on coal-rock mechanics and gas desorption characteristics are carried out under such stress state to study the mechanical characteristics and gas desorption characteristics of coal-rock from the initial complete state to the failure state under cyclic loading and unloading action.Main conclusions of this article are as follows:(1)The evolution law of deformation,compressive strength,elastic modulus and Poisson’s ratio of coal-rock specimens under stepped cyclic loading and unloading stress path is obtained: The overall deformation process and deformation trend of coal-rock specimens under stepped cyclic loading and unloading stress path are similar to those under conventional triaxial stress path,but compressive strength,elastic modulus and Poisson’s ratio are different.The effect of stepped cyclic loading and unloading can reduce the compressive strength of the coal-rock specimens,and the compressive strength decreases with the increase of the displacement amplitude;the elastic modulus and Poisson’s ratio of the coal-rock specimens under stepped cyclic loading and unloading stress path show a synchronous step-up trend with the step-up of applied displacement.The elastic modulus is higher than that under conventional triaxial stress path overall,and decreases with the increase of the displacement amplitude;the Poisson’s ratio is lower than that under conventional triaxial stress path before passing the yield point,and higher than that under conventional triaxial stress path after passing the yield point;before passing the yield point,the Poisson’s ratio decreases with the increase of the displacement amplitude,and then increases with the increase of the displacement amplitude after passing the yield point.(2)The irreversible strain of coal-rock specimens under stepped cyclic loading and unloading stress path is affected by the displacement amplitude.The axial and radial irreversible cumulative strain curves of coal-rock specimens show a synchronous step-up trend with the step-up of applied displacement.The irreversible cumulative strain of the coal-rock specimens increases with the increase of the displacement amplitude.(3)The desorption law of coal-rock gas under stepped cyclic loading and unloading stress path is obtained: Stepped cyclic loading and unloading can promote the desorption of coal-rock gas.Overall,the desorption curve of coal-rock gas under stepped cyclic loading and unloading stress path is similar to that under the conventional triaxial stress path,which is similar to the logarithmic function curve.The desorption curve of coal-rock gas under stepped cyclic loading and unloading stress path is lower than that under conventional triaxial stress path before passing the yield point.After passing the yield point,the desorption curve shows an upward trend,and finally exceeds the curve under conventional triaxial stress path;the amount of gas desorption before passing the yield point decreases with the increase of the displacement amplitude,and increases with the increase of the displacement amplitude after passing the yield point;the cumulative gas desorption increases with the increase of the displacement amplitude.(4)The influence of stepped cyclic loading and unloading on gas desorption is analyzed: Stepped cyclic loading and unloading increases the compaction effect of gas-containing coal before passing the yield point,hinders gas migration,and reduces the permeability of gas-containing coal;after passing the yield point,the development and connection of cracks open up gas migration channels,a large amount of desorption gas accumulated before passing the yield point gushes out,and a sudden surge happens.Afterwards,the coal-rock gas continues to desorb and desorb due to the continuous action of stepped cyclic loading and unloading.