Study On Characteristics Of Gas Movement And Construction Safety Risk Assessment In The Highway Tunnel

With the proposal of national policy,” Western Development” and” The Belt and Road initiative”,the number of mountain tunnels in China is increasing,and more and more tunnels encounter gas occurrence coal seam in the process of excavation,which greatly increases the construction difficulty and risk of tunnel engineering construction.Tunnel gas disaster is usually affected by two aspects.Firstly,the gas content in the coal seam is high and the pressure is high.In addition,in the process of tunnel excavation,due to mechanical disturbance,the structure of the coal seam changes,resulting in the redistribution of the internal gas pressure.In this paper,the gas occurrence form,gas diffusion and seepage development characteristics in coal and rock are studied,and the gas-solid coupling equation of "air gas" two-component migration law in coal and rock tunnel with gas is derived.Based on the site conditions of Tongzi tunnel,the gas-solid coupling model is constructed by using COMSOL multiphysics finite element software.The temporal and spatial evolution law of gas migration in coal seam during tunnel coal uncovering is analyzed.The risk assessment of gas tunnel construction is realized,and the risk assessment system of gas tunnel is improved.And the corresponding measures for prevention and control of gas disaster in tunnel are put forward.(1)Based on the theoretical analysis of gas occurrence,gas diffusion and seepage,the gas-solid coupling model of "gas air" two-component coal and rock gas is established.(2)Based on the above gas-solid coupling equation,a multi physical field coupling analysis model is established to realize the simulation of coal gas migration in the process of tunnel uncovering coal.(3)As time goes on,gas pressure and gas content decrease rapidly at first and then slowly;After 10 meters away from the face,the gas pressure and gas content are almost unchanged;The pressure and content of gas at the bottom,center and top of the tunnel face are roughly the same.The pressure gradient and velocity of gas increase first and then tend to be stable with time;The gas pressure gradient and gas velocity near the face are the largest,and the farther away the face is,the smaller the pressure gradient and gas velocity are.(4)Based on the analysis of the influence of four factors on the law of gas migration in the tunnel,it is known that the initial gas pressure > buried depth > coal uncovering length > coal seam thickness.Therefore,in the subsequent risk assessment and corresponding measures,the influence factors of initial gas pressure and in-situ stress should be considered.(5)The gas pressure gradually decreases and the contour of low pressure area expands to the surrounding area over time,.Similar to the distribution of fracture gas pressure,the initial high pressure is concentrated around the face.With the excavation,the pressure around the face gradually decreases,the pressure gradient decreases,and the low pressure contour gradually expands to the surrounding.In the initial stage,the high air concentration area is mainly distributed near the tunnel wall,and the coal uncovering in the tunnel leads to a large amount of air flowing into the area.The farther away the tunnel face is,the lower the air concentration is.Based on this,the importance of tunnel ventilation is known,and the ventilation scheme of Tongzi tunnel is designed.(6)Based on the field situation and numerical simulation results of Tongzi tunnel,a risk assessment model of gas tunnel in coal uncovering process based on multi-level extension system of rough set is established,which is applied to the actual project of Tongzi tunnel,and the evaluation results have been verified in practice,which proves the effectiveness and scientificity of the assessment model.(7)Finally,combined with the results of numerical model analysis and risk assessment,the paper puts forward the targeted measures for prevention and control of tunnel gas disaster,and analyzes the impact of tunnel on geological environment in road engineering construction.The research results can provide some guidance and theoretical basis for gas control in the process of coal uncovering in highway tunnel.