| Currently, the research and analysis of tunnel gas includes formation mechanism, explosion hazards and prevention technology. There are few people discussing the theory of tunnel gas migration. Therefore, people can not accurately analyse the migration mode and accumulation direction of gas in the tunnel construction process, which bring the uncertainty and serious damage to the tunnel construction. Combined with information of Xuan Zhen Guan tunnel, this thesis researches the formation mode and the distribution range of cracks in the rock in the process of tunnel blasting and surrounding rock stress change, and analyzes the gas migration patterns in the process of tunnel excavation.The main results of this paper show as follows:(1) According to the data obtained in the course of geological survey and tunnel construction, this essay analysed the gassy level, which suggested that a tunnel is the low gassy tunnel during the geological survey and the tunnel is the high gassy tunnel during the construction. Three reasons are analyzed to explain the difference of gassy level in the course of geological survey and tunnel construction. First, gas in the rock entered and gathered in the tunnel through those cracks which came into being in the process of tunnel blasting. Second, gas entered and gathered in the tunnel through those cracks which came into being because of surrounding rock stress change. Third, the absorbed gas in rock changed into free gas, migrating to the tunnel and gathering in it. All these make the tunnel high gassy tunnel.(2) Based on the data of tunnel blasting, the radius of blasting cracks in the surroundings rock was calculated as504.7mm. According to the relationship between the orientation of blasting and the orientation of joint surface, this essay analysed the scale of blasting cracks of different parts in surrounding rock, which suggested that it was not easy to form large cracks in surrounding rock of tunnel vault and the bottom of tunnel, where as it was easy to form larger cracks along the joint in left and right side of surrounding rock.(3) Using the finite element software, the writer simulated the surrounding rock stress in the process of tunnel excavation. The results were that after above step excavation, compressive stress concentration occurs in the left and right surrounding rock of tunnel. At the same time, tensile stress occurs in the surrounding rock of tunnel vault. After under step excavation, compressive stress concentration occurs in the surrounding rock of tunnel vault, and tensile stress occurs in the middle and bottom part of surrounding rock of tunnel.(4) Combined the formation mode and the distribution range of cracks in the rock in the process of tunnel blasting and surrounding rock stress change and used the gas migration rule, this essay analysed gas migration patterns in the surrounding rock in the process of tunnel excavation. The results were that in the crack’s area, under the influence of concentration stress or diffusion force or these two forces, free gas in rock would migrate to the area of lower pressure and lower gas concentration along the cracks quickly; in the none crack’s area, under the influence of concentration stress or diffusion force or these two forces, free gas in rock would migrate to the area of lower pressure and lower gas concentration along the original pores and joints at a relatively slow speed.(5) Based on the volume of the crakes of rock and the reservoir’s physical properties, the gas overflow rate was calculated as1.99m3/min. By the concentration of the gas in air return, the gas overflow rate was calculated as2.565m3/min. The results showed that the tunnel was high grassy.
|
PDF Full Text Request