Cross-harbour Tunnel Seepage Field Analysis And Construction Of Numerical Simulation,

With the construction technology and theory of underground engineering gradually perfect, subsea tunnel will be the first scheme in realizing cross-sea traffic and transportation because of its special advantage of stable traffic volume and no influence by weather and climate. However, due to the special living environment of subsea tunnel, there are more difficulty and risk in design and construction compared with Land tunnel. Deepwater geological investigation has the characteristic of great difficulty, high cost and low accuracy. High pore pressure and seepage pressure will decrease the effective stress, arching effect and stability of surrounding rock, increase the bearing load of support structure and lead to a large amount of seawater flowing to tunnel by the high permeability Formation or the channel connecting with surface of water. When tunnel gets across poor geologic zone such as fault fracture zone, it will bear huge water pressure and even collapse. Therefore, this paper will discuss some problems of water inflow, external water pressure on lining and construction mechanics in the construction of subsea tunnel, and the main research contents are as follows:(1) Based on the characteristic of subsea tunnel and some assumed conditions, the calculation formulae are deduced for seepage field distribution in surrounding rock, grouting reinforced region and lining. Validity of the formulas has been verified by the comparative analysis of numerical solutions and analytical solutions.(2) The relationship between tunnel seepage volume and parameters of grouting reinforced region and surrounding rock is studied based on the deduced seepage volume formula. The relationship curve between tunnel seepage volume and the permeability coefficient of surrounding rock, the permeability coefficient of grouting reinforced region, the thickness of grouting reinforced region and the thickness of rock cover is analysed, and some suggestions on the selection of reasonable parameters of grouting reinforced region is given.(3) Based on the above deduced formula of seepage field distribution in lining, the relationship between water pressure on lining and various parameters is researched with the waterproof criteria of "block off absolutely" and "block off ground water and limiting discharge", and the relationship curve between water pressure on lining and the permeability coefficient of surrounding rock, the permeability coefficient and thickness of grouting reinforced region, and the discharge of tunnel is given. The results show that water pressure on lining can not be reduced during "block off absolutely" either with or without injecting grouting, and that water pressure on lining will decrease with the increasing of the discharge of tunnel during "block off ground water and limiting discharge". In the allowable discharge range, properly increasing the drainage capacity of tunnel drainage system can significantly decrease water pressure on lining.(4) After predicting tunnel seepage volume with three methods—theoretical analysis, empirical formulae and numerical simulation, the dangerous region where water bursting may occur is pointed out, and the suggested value for reduction coefficient of water pressure on lining of Qingdao subsea tunnels is given after the numerical simulation of water pressure on lining.(5) Based on elastic-plastic theory and seepage mechanics, analytical formulae of plastic zone radius, elastic-plastic stress and displacement are deduced with considering the combinative action of seepage and stress. The difference of stress and displacement of surrounding rock and the characteristic of supporting structure is analyzed and contrasted with and without considering fluid flow influence. And the results show that seepage action worsen the force situation of surrounding rock and increase support resistance force of support structure, which is adverse to the stability of surrounding rock and the force bearing status of support structure.(6) By using FLAC3D to simulate the construction of tunnel, the change process of surrounding rock displacement field, stress field, plastic zone and primary support internal force is obtained and some suggestions have been proposed on design and construction of similar projects.