Study On Calculation Method Of Safety Thickness Of Rock Slate In Karst Tunnel And Mechanism Of Water Bursting

Water inrush disasters have caused great losses and casualties frequently in the process of karst tunnel construction in China in recent years. The purpose of this project is to prevent the water inrush disaster in karst tunnel. Combining domestic and foreign water inrush disasters cases; Depending on the project "Research about the stability of surrounding rock and mechanical behavior in tunnels construction in transition section of karst tunnel" which come from State Administration of Work Safety, and project "The mechanism and prevention measures of water inrush disaster in karst tunnel" which come from science and technology plan in sichuan province department of education. Through literature researching and ragarding Yuan-liangshan tunnel as engineering background. This paper have summarized the characteristics and main influencing factors of water inrush disaster in karst tunnel. And safety thickness calculation formulas have been deduced respectively when concealed holes distributed on the top, side and bottom of karst tunnel. The mechanism and prevention measures of water inrush disaster in the karst tunnel have been further studied through numerical analysis method, which can solve the shortages and the difficulties in the prevention and control of water inrush disaster in design. The main research results are as follows:(1)Water or mud inrush disasters at home or abroad were summarized and classified. So the great harm of water inrush disasters to engineering was confirmed by these facts. The research status of water inrush mechanism in karst tunnel have been summarized. Which point out the current problems in scientific research and clear the purpose, content and method of this paper. The most important is that characteristics and the influence factors of water inrush disasters have been analysised. Related research helps to deepen people’s understanding and cognition of water inrush disasters in karst tunnel.(2)Regarding the rock plate simplified as structural mechanics model. When the conceale-holes locate in the top, bottom and side of the tunnel. Analytical expression of the safe thickness of rock plate can be deduced according to the bending and shearing strength. Comparising the theoretical rock plate thickness with the actual values can obtain similar results, which can verify the rationality of the theory.(3) On the basis of FLAC3D numerical software, established three calculation models which are the concealed cavity respectively located in the top, bottom and side of the tunnel. And simplify the cavity water pressure to the equivalent normal stress. According to the curve of water stress and displacement of the growth can determine the critical water pressure of the thickness of safety rock plate. And then calculate the curve relationship between the safe thickness of plate and water pressure. Comparising with the numerical and theoretical results can find that first results is slightly smaller than the second one, but the the growth rate of them almost the same.(4)Combined with the characteristics of the second cave of Yuan-liangshan tunnel. The main messures to prevent water inrush for the cave are summarized. To examine the effects of grouting on the stability of the tunnel, the grouting effect has been tested by FLAC3D. Comparison the maximum shear stress, the maximum principal stress and displacement of the rock before and after grouting which can obtaine the following conclusions:The maximum shear stress, the maximum principal stress and the maximum displacement of the surrounding rock are reduced in some degree after grouting. Which indicates that the overall strength of surrounding rock can be greatly increased after grouting. The ability of surrounding rock to resist the high water pressure is strengthened obviously.(5) The monitoring data of the second cave of Yuan-liangshan tunnel shows that the compression stress of lining is obviously bigger than the numerical results. There are two possible caused the results:First is that the water pressure of the cavity is greater than4MPa; The second factor is that FLAC3D does not take the actual rock structure face into consideration in the process of calculation. The parameters of the rock in the calculation process is better than the actual parameters of surrounding rock.