Experimental Study Of The Effect Of Freezing On The Mechanical Properties Of Raw Coal

China's coal seam has high gas content and is low-permeability.There are many mines have high gas content and are prone to have coal and gas outburst,which seriously hinders the safe production of coal in China,and the increase in mine depth further exacerbates the danger of coal and gas outburst.Coal and gas outburst disasters are one of the major disasters in mines,prevention and control of coal and gas outbursts in coal mining face have become the key to mine safely.Some experts have proposed new technical measures such as adopting freezing technology to dig stone gates and liquid nitrogen cracking coal to increase the gas drainage rate to prevent and control coal and gas outburst.It is necessary to conduct in-depth research on fundamental issues such as the effect of freezing on the physical and mechanical properties of raw coal,and fully understand how raw coal freeze at low temperatures in order to apply these new technologies to solve the coal and gas outburst.Therefore,this paper carries out the research on the influence of freezing on the physical and mechanical properties of raw coal and its changing characteristics based on the method of theoretical analysis and laboratory test.The main contents and achievements of the research are:(1)Taking two kinds of raw coal(Yibin coal samples and Yulin coal samples)from the engineering site,the coal quality of the two kinds of raw coal was tested,and the industrial analysis indexes such as moisture,ash,volatiles,and density of the two kinds of coal samples were obtained according to the technical specifications,and the raw coal was processed into test samples 25 mm in diameter and 50 mm in height according to the rock mechanics test standards.(2)The change characteristics of strength,elastic modulus,failure modes,wave velocity,and expansion of crack of the coal samples with or caused by different freezing temperature(0?,-10?,-20?),different water content(dry,natural and saturated)and freeze-thaw cycles were studied by uniaxial compression tests,ultrasonic wave velocity tests and NMR imaging techniques.1)Freezing temperature and water content have a great influence on the strength and failure mode of raw coal.The strength and elastic modulus of the saturation and freezing coal samples decreased and the dispersion was large.The failure mode of uniaxial compression of Yibin coal samples was shear failure,its failure mode was more complex(including shear failure and brittle failure)while saturation and freezing.The failure mode of uniaxial compression of Yulin coal samples was brittle failure,and the uniaxial compression of saturation and freezing Yulin coal samples occurred damage along the bedding plane while freezing.It was obvious that the distribution of water in the raw coal is uneven and the pore penetration was poor after measuring the distribution of pore water in two kinds of saturated coal samples based on NMR imaging technology.Combined with the relevant test results,it found that the volume expansion of water while freezing causing the local damage and the integrity of coal samples decrease was the root cause of the decrease of saturation and freezing coal samples' strength.2)The longitudinal velocity of the freezing coal samples increased with the increase of the moisture content,and the longitudinal velocity of the saturated coal samples increased with the decrease of the freezing temperature.3)The effect of freezing on coal samples with relatively complete structure was limited,but the freezing makes fracture areas of coal samples more broken,not only can expanded the existing cracks,but also produced new crack.(3)By nuclear magnetic resonance test technology,this paper studies the damage characteristics and its difference to coal samples saturated by water and different concentration of sodium sulfate solution under the freeze-thaw cycles condition,and the mechanism of the damage of two kinds of hydration environment(pure water and solution)to coal samples under freeze-thaw cycles condition was analyzed.1)By analyzing the changes characteristics of porosity,effective porosity,permeability,pore size distribution and other parameters of coal samples after different freeze-thaw cycles,the damage to the coal samples saturated by pure water and sodium sulfate solution under freeze-thaw cycles condition were compared,and the latter caused heavier damage to coal samples.The damage to coal samples saturated by water under freeze-thaw cycles condition gradually weakened after 10 freeze-thaw cycles,and the damage to coal samples saturated by sodium sulfate solution with the same freeze-thaw cycles increased firstly and then stabilized with the increase of solution concentration.2)According to the results,the cause of damage characteristics of saturated coal samples in two hydration environments(pure water and sodium sulfate solution)under freeze-thaw cycles condition were analyzed theoretically.Damage in coal samples saturated by pure water under freeze-thaw cycles condition was due to the increase of water volume while freezing,while damage in coal samples saturated by sodium sulfate solution under freeze-thaw cycles condition was due to the crystallization of sodium sulfate solution while freezing.The damage effect of freeze-thaw cycles on small size pores of coal samples saturated by pure water was poor,while sodium sulfate solution could increase the proportion of large pore size of coal sample under freeze-thaw cycles.These results could provide a basis for the application of freezing technology and liquid nitrogen fracturing technology to the prevention and control of coal and gas outburst disaster.