Effect Of Water On Frost Heave Of Coal Containing Gas

With the increase of the mining depth of coal seams in the country,the control of coal and gas outbursts in the cross-cut coal uncover technology from rock strata and in geologic structure is becoming more and more difficult.At present,most measures to prevent coal and gas outburst are to reduce the energy storage by sacrificing the strength of coal,but the attention to improve the strength of coal is not enough.Refrigeration can reduce the energy of coal and inhibit gas desorption.Refrigeration and outburst prevention have attracted the attention of the industry.On the one hand,the gas in coal and coal will shrink due to the decrease of temperature,which has the effect of enhancing the strength of coal;on the other hand,the internal water and external water in coal bed will produce frost heave during the freezing process,which has the effect of reducing the strength of coal.Water content is one of the key factors causing frost heave.Therefore,it is of theoretical and practical significance to study the effect of water content on frost heave of methane coal.In this paper,laboratory tests,theoretical analysis and numerical simulation are combined to analyze the law of frost heave evolution of methane coal.The main conclusions are as follows:(1)During the freezing process of coal without gas and coal with gas,the decreasing process of coal sample temperature can be divided into: rapid cooling stage,constant temperature phase change stage,slow cooling stage and stable constant temperature stage;the strain of coal sample can be divided into: cold shrinkage stage,frost heave stage and stabilizing stage.(2)During the freezing process,the minimum moisture content of the coal samples with uniform moisture distribution shall be 7%.When the temperature of coal sample enters the water ice phase transition region,the coal sample will not immediately occur frost heave deformation,and the frost heave deformation has "hysteresis" effect.(3)During the freezing process,the duration of phase transformation at constant temperature and the absolute value of frost heave strain increase with the increase of moisture content;after the freezing,the residual strain of coal sample decreases with the increase of moisture content,and the coal sample is always in the freezing shrinkage state,indicating that freezing can make the coal sample shrink in volume and increase in strength.(4)Under the same water content and temperature conditions,the absolute value of frost heave strain of gas-containing coal is greater than that of gas-free coal,and the frost heave effect of gas-containing coal is affected by three factors,namely,water,temperature and gas.(5)The occurrence of the frost heave effect leads to the destruction of the pore structure of the coal body,the increase of the number of large and medium pores in the coal body,and at the same time,new micropores and small pores occur,which cause the contact area between coal particles to decrease,the friction resistance to change,and the strength of the coal body at the end of the frost heave deformation is lower than the strength of the frozen shrink coal body before the frost heave.(6)A three-field coupling model of water-heat-force frost heave of gas-containing coal is established and solved by COMSOL numerical simulation.The reliability of the model is verified by the measured data.The model can be used to predict the frost heave deformation of gas-containing coal by water.