Experimental Research On Outburst Prevention Method Based On Liquid Nitrogen Freezing For Uncovering Coal In Crosscut

According to the statistical law of coal and gas outburst occurrence,outburst mostly occurs in heading face and geological structure zone,and uncovering coal seam in crosscut often involves these two locations.Therefore,during uncovering coal seam in crosscut,the coal and rock mass are very vulnerable to instability and outburst,which has a very high risk.Research shows that the strength of coal determines whether it can be destroyed under the action of in-situ stress and gas pressure,and whether it can withstand the impact of energy,so the strength of coal is the key factor to prevent the outburst.In the existing technology,measures to improve the strength of coal have achieved some results in outburst prevention,but they also have some limitations.Liquid nitrogen freezing technology is a mature construction technology,which can achieve rapid freezing of rock and soil,significantly improve the strength of rock and soil,and effectively ensure the safety of construction work.Therefore,liquid nitrogen freezing technology has been widely used in mine construction,building construction,tunnel excavation and other engineering fields.According to the above background,the technology of liquid nitrogen freezing can be applied in crosscut during uncovering coal seam.Before uncovering coal,the uncovered coal seam can be frozen rapidly,so as to improve the strength of the uncovered coal and rock and the integrity of the surrounding rock,thereby enhancing the ability of the coal body to resist the impact of outburst energy and reduce the risk of outburst.At present,the research object of liquid nitrogen freezing at home and abroad is mainly about rock and soil,but the study of coal seam is less.In view of the above problems,this thesis combines theoretical analysis with experimental method to explore the law of the influence of liquid nitrogen freezing on coal,and provides theoretical support and technical parameters for coal outburst prevention based on liquid nitrogen for uncovering coal seam in crosscut.This thesis analyses the characteristics and influencing factors of coal and gas outburst during uncovering coal seam in crosscut.Combining with the existing artificial freezing theory and technology,it expounds the prevention mechanism of liquid nitrogen freezing method to reduce the risk of coal and gas outburst during uncovering coal in crosscut,and verify with simulation software.The factors influencing the freezing effect of liquid nitrogen are also discussed.The unfrozen water which a key factor affecting the freezing effect of coal is studied.The dynamic change characteristics of unfrozen water content in coal during freezing and thawing process are measured and summarized by means of nuclear magnetic resonance technology.The distribution and change of pore water in different types of coal samples during freezing and thawing process are analyzed with T₂distribution curve.An experimental study on ultrasonic testing of coal under liquid nitrogen freezing condition was carried out.The structural changes of coal were analyzed according to the variation of ultrasonic wave velocity.The influence of liquid nitrogen freezing on dynamic parameters of coal was discussed in combination with elasticity mechanics knowledge.The experimental results show that the ultrasonic wave velocity of coal in liquid nitrogen freezing state is significantly increased,which indicates that liquid nitrogen freezing can significantly improve the integrity of coal and make coal more stable.At the same time,the dynamic elastic modulus,the dynamic bulk modulus and the dynamic shear modulus of coal have also been significantly improved.The mechanical properties of coal under liquid nitrogen freezing state were studied.The compressive strength,elastic modulus and tensile strength of coal under liquid nitrogen freezing state were measured by uniaxial compression test and Brazilian splitting test.The experimental results show that liquid nitrogen freezing can significantly improve the mechanical properties of coal.The elastic modulus,compressive strength and tensile strength of coal have been improved.It shows that the stiffness and strength of coal have increased,and the ability to resist extrusion and tensile damage has also been enhanced.All these changes indicate that liquid nitrogen freezing can effectively reduce the risk of outburst during uncovering coal seam in crosscut.