Research On The Minimum Rock Cover Thickness Of Underwater Tunnel In Alternating Layers Of Hard And Soft Rocks

The minimum depth of rock cover plays an important role in the construction of underwater tunnel. It must take safety factors and economy factors into consideration during its construction and usage period. How to determine reasonable rock cover thickness is still an intractable research subject. It is quite different for the deformation and failure features of the underwater tunnel between meeting the alternatively layers of soft and hard rocks and a uniform one. Meanwhile, the seepage flow has great influence on the stability and the determination of the minimum rock cover thickness of underwater tunnel. The paper studied these problems mentioned above based on ChangJiang tunnel of LiangJiang tunnel in ChongQing.â‘ From research on physical circumstance of ChangJiang tunnel, features on location design, cross section selection and supporting of underwater tunnel are discussed.â‘¡On the basis of data collection of built underwater tunnel at home and aboard, the key factors of the minimum rock cover thickness of underwater tunnel as well as empirical research methods are generalized as follows: Norwegian empirical methods of submarine tunnel construction, domestic empirical formula for underwater coal mining, empirical methods of water insulation by rock pillars and Japanese empirical methods. Take the water depth, construction methods and geological conditions of ChangJiang tunnel into account by importing the four methods mentioned above, the range of the minimum rock cover thickness is determined as 21 to 32 meters.â‘¢The basic principle of underground engineering analysis using Finite Element Method is elaborated. MIDAS/GTS is applied to the elastic-plastic simulating analysis of ChangJiang tunnel under eight different rock cover thicknesses. The results show that there is some discrimination in deformation, stress and plastic zone of surrounding rock. Although distinctions of deformation and stress of surrounding rock, axial force of anchors, stress of shotcrete and distributing areas of plastic zone are found under different rock cover thicknesses, but the variation regularities and distributing tendency is quite consistent. By taking the criterion of principle stress effect range and displacement convergence of surrounding rock into account, the minimum rock cover thickness of ChangJiang tunnel is ascertained as 25 meters.â‘£On the basis of the minimum rock cover thickness which is ascertained by geological conditions and elastic-plastic simulation, this paper is further studied through hydromechanical coupling analysis. The affection of seepage flow on displacement and stress of rock, axial force of anchor,stress of shotcrete and plastic zone in the tunnel is researched. It indicates that seepage flow have large influence on sandstone while little on mudstone. On the conditions of partial support, advance water exploration, anti-seepage and waterproofing in time, the minimum rock cover thickness of 25 meters will ensure the safety of tunnel construction.