Study On Deformation And Seepage Characteristics Of Coal Under Repeated Loading

In the process of coal mining,underground coal-seam is often occupied with repeated disturbance induced by mining activities.This periodic load leads to coal progressive deformation and damage,and changes the flow behavior of gas in the coal seam,and then induces dynamic disasters such as rock burst,coal and gas outburst.Cyclic loading is not only a common form of disturbance in engineering practice,but also an important loading method in laboratory research.In this paper,the mechanical properties and seepage law of gas containing raw coal under repeated disturbance were considered as the objects of study.The axial and confining pressure asynchronous cyclic loading and unloading stress paths which were consistent with the field were designed.The influences of different mining method,coal seam depth,working face advancing speed and bedding direction on coal deformation and gas migration were simulated and analyzed.The research results are as follows.(1)The coal deformation and gas-seepage characteristics in the axial and confining pressure asynchronous cyclic loading process were obviously different from those in the traditional cycle process,and showed significant confining pressure effect: The deformation of specimens was mainly controlled by tensile and compressive alternating load.The curves of permeability and axial stress varying with strain were no longer intersected as "X" type.(2)Considering the asynchronism of the input-energy process of the axial and confining pressure,the energy calculation formula under three-axial stress was revised.The energy accumulation in the loading process can be roughly divided into three stages,which were the initial stage,the isokinetic stage and the deceleration stage respectively.The axial-load energy in the total energy accounted for about 70~85%.The 80~90% of the total energy was converted to elastic-strain energy.Correspondingly,only less than 20% of the energy was exhausted for the plastic strain and fissure development.Moreover,the energy dissipation ratio of the confining pressure was generally higher than that of the axial pressure.(3)The deformation and permeation characteristics response of coal samples were different under different designed cyclic stress parameters.When the stress concentration coefficient was relatively low,the effect of confining pressure on the deformation of the specimen was obvious,where dilatation were dominating.As the upper limit of the axial pressure cycle increased,the deformation of the specimen was gradually controlled by the axial pressure.Furthermore,when the axial pressure was larger,greater plastic strain and more damage were produced.The greater the total energy input to the sample by the external stress,the greater the rate for dissipating energy.The higher the initial hydrostatic pressure level,the more deformation and plastic deformation accumulates in the test sample,and the more energy input by the tester to the test sample.When the coal sample entered into the particular deep stress level,the strain response of the coal sample,the deformation modulus and permeability change were quite different.The larger the loading and unloading rate,the more the plastic strain of the test piece accumulates and the greater the degree of damage,and the less the strain potential energy stored in each load section.Under the same stress cycling condition,the deformation of vertical bedding specimens was generally greater than that of parallel beddings.The ratio of the total energy converted to dissipated energy was also generally higher than that of parallel bedding while vertical permeability was 1 to 2 orders of magnitude higher than the vertical bedding direction.(4)By range analysis,it was found that mining method was the most important factor affecting the damage degree of specimens during cyclic loading,followed by initial hydrostatic pressure and loading rate.Initial hydrostatic pressure was the most important factor affecting the permeability evolution,followed by bedding direction and mining method.After systematic comparison,protective coal-seam mining was considered as the most effective mining method in preventing coal and gas outburst,which should be given priority if field conditions permitted.