Study On Temperature Response Characteristics Of Gas-bearing Coal Body Frost Heave

During the coal excavation process in In uncovering coal seam by cross-cut,coal and gas outbursts are likely to occur in the tectonic coal body under the combined action of in-situ stress,gas pressure and physical and mechanical properties of the coal body.After the extraction reaches the standard,the freezing measures after water injection are taken for the coal excavation site.On the one hand,the strength of the coal seam can be enhanced,and the energy storage of the coal body can be reduced,thereby reducing the risk of coal and gas outburst during the coal excavation process in In uncovering coal seam by cross-cut;On the other hand,during the freezing process,the moisture in the coal body will cause frost heave,resulting in greater frost heave stress and frost heave strain,resulting in damage to the strength of the coal body.In this paper,the self-built gas-bearing coal frost heave simulation test platform is used to test the internal temperature,transverse and longitudinal strain,and frost heave force of gas-bearing coal during the freezing process under different low temperature environments,and a columnar gas-bearing coal frost heave model is established.The temperature response characteristics of gas-containing coal body frost heave were studied,and the following conclusions were obtained:(1)The pore and fissure structure of coal samples will change after frost heave.Frost heave causes cold shrinkage and frost heave of the coal matrix,and the coal microporous structure is connected to form large pores,and the coal fissures expand during the frost heave process.The mercury intrusion test found that the coal pore size increased after frost heave.(2)In the whole freezing process,the internal temperature changes of gas-containing coal samples are divided into four stages: initial rapid cooling,short-term stability in the middle,slow decline in the later stage,and final stability.The internal temperature drop rate of gas-containing coal samples decreased with the decrease of freezing temperature,and increased with the increase of gas adsorption equilibrium pressure.The lower the freezing temperature and the higher the gas adsorption equilibrium pressure,the shorter the time to reach the short-term stable stage in the medium-term and the shorter the stable time in this stage.This stage is due to the heat released by the consolidation of the moisture inside the coal sample,resulting in a short-term stable stage of the internal temperature of the coal sample.The larger the adsorption pressure,the smaller the gap between the coal molecules,the lower the temperature of the coal sample,and the conversion of free gas into adsorbed gas to release heat,so that the internal temperature drop rate of the coal containing gas is lower than that of the coal without gas,indicating that the coal sample adsorbs gas Afterwards,the temperature of coal samples is hindered to a certain extent.(3)The frost heave test of gas-containing coal body shows that the change of coal body strain during the freezing process is divided into a rapid decrease stage,a slow increase stage,and a slow decrease stage,which represent the phenomenon of cold shrinkage,frost heave,and secondary freeze shrinkage of the coal,respectively.With the decrease of freezing temperature,the greater the decrease of transverse and longitudinal strain of coal,the greater the amount of cold shrinkage.When the freezing temperature is 0°C,the internal temperature of the coal sample does not reach the consolidation temperature of water,and no frost heave occurs in the lateral and longitudinal strains;when the freezing temperature is-5°C,the coal sample does not undergo secondary freezing in the lateral and longitudinal directions;The frost heave deformation of the coal sample changes in an inverted V shape with the decrease of the freezing temperature.When the freezing temperature is-10℃,the frost heave amount of the gas-containing coal sample is the maximum value,and finally the gas-containing coal sample shows cold shrinkage.(4)The frost heave test of coal samples under different gas adsorption equilibrium pressures shows that the changes in frost heave of coal with gas and without gas during the freezing process are the same.The frost heave force has four stages: slow increase,rapid increase,stable and constant,and rapid decrease.The larger the adsorption equilibrium gas pressure,the smaller the cold shrinkage,the greater the frost heave deformation,and the greater the secondary cold shrinkage.(5)The frost heave model of gas-containing coal body was established and the frost heave characteristics of coal body were compared and analyzed,and the effects of different freezing temperatures and gas pressures on frost heave were obtained.The longer the freezing time is,the temperature difference between the inside and outside of the coal sample gradually decreases;the lower the freezing temperature,the faster the temperature drop,and the faster the equilibrium time;the greater the adsorption equilibrium gas pressure,the slower the temperature drop,and the longer the equilibrium time.