Study On Stability Analysis And Control Of Subsea Tunnel Surrounding Rock

With the great development of transport, large quantities of river crossing channels will be invested into construction. And the underwater tunnel has been paid more and more attention. Compared with the mountain tunnel, subsea tunnels often have the characteristics of difficulties in geological exploration, long distance single-port continuous excavation, long-term static and dynamic water pressure acting on the lining structure, difficulties in waterproof and drainage, low arching effect, gushing water likely to happen when passing through unfavorable geologic bodies and so on. Therefore, there are many different key technical issues in design and analysis of the lining structure for the subsea tunnel compared with the mountain tunnel, which are urgent to be solved in design. In this paper, taking booming subsea tunnels in China as project background and stability of surrounding rock and its control technologies in rich water condition as the research focus, the following six aspects are carried out by using theoretical analysis, numerical simulation, indoor model test and field measurement methods.(1) Based on the Lame solution to thick cylinder under uniform pressure in elastic mechanic and Mohr-Coulomb yield condition, the subsea circular tunnel with liner is simplified as axially symmetrical and the effect of seepage field is simplified as seepage volume force acting on the stress fields. Elastic-plastic analytical solutions of stress and displacement are obtained respectively with considering the release of the displacement ahead of the cutting face. Other elastic-plastic analytical solutions which satisfy other yield conditions and flow rules can be deduced, according to this derivation. Combined with the results of a practical example, the deformation of host rock around the tunnel, the range of the plastic zone of rock, and the stress field and characteristic of seepage field in the rock around the tunnel have been analysed using the analytical formula, and the distruction law as well as the evolution features of these fields are obtained. Besides, the influence on seepage field and displacement of surrounding rock by some factors such as seawater tables, rock cover thickness, inside water head in tunnel, the mechanical parameters of the lining and the surrounding rock and the penetration relation between them have been discussed.(2) Based on previous research results, four models including upper bound solution, wedge analysis, two-dimensional logarithmic spiral model and slice method for estimating the support pressure required for the face stability were derived by utilizing upper bound theorem and limit equilibrium method. With considering of the pressure of overlying strata, theoretical analysis formulas have been modified. The existence of groundwater may seriously affect the face stability. Under the steady-state groundwater flow condition, most part of the total support pressure is owing to the seepage pressure acting on the tunnel face. Also, the coupled numerical analysis has been performed and the proposed estimation methods to obtain the support pressure have been verified. Combined with the results of a practical example, the results of the numerical simulation and the analytical formulas are comparied and analyzed. Meanwhile, influencing factors such as cohesion and friction angle of surrounding rock, advanced Injection, sea water level and depth have been discussed.(3) Taking Xiamen Xiang'an subsea tunnel as project background, by using in-situ measured data when passing through the completely weathered granite, the mechanism of integral deformation of upper strata due to tunnel excavation has been then expounded. It is shown by the in-situ deformation monitoring that how the settlement of crown and seabed, horizontal ground deformation and surface crack have distributed and developed with time and space. The relation between surface crack space and strata displacement is presented to offer guidelines to water inflow prediction.(4) Based on the coupled fluid-solid theory, numerical analysis is made to study the stability of surrounding rock during Xiamen Xiang'an subsea tunnel passing through the weathered slot under the sea. The distribution principle of displacement field and seepage field of surrounding rock, as well as the mechanical characters of lining structure are studied in various construction stages of subsea tunnel. The seepage of groundwater has great influence on deformation of surrounding rock, causing a large range of sea-bed subsidence. The excavation of pilot heading has the most direct and obvious influence on seepage field of surrounding rock, and the attenuation of surrounding rock deformation induced by seepage is small in its transfer process from surrounding rock to sea-bed. The surrounding rock deformation and sea-bed subsidence mainly concentrate on the excavation of both sides of pilot heading and the upper rock of the central core; And the advanced support and pre-grouting are the key technologies for improving the strength, stifness and the permeability of the surronding rock.(5) By using theory of analytical method, calculation model for subsea tunnel seepage field has been established with considering of the surrounding rock, grouting circle, primary liner and second liner. Taking Xiamen Xiang'an subsea tunnel and Qingdao Jiaozhou Bay subsea tunnel as project background, by using in-situ measured data, numerical simulation, indoor model test and theoretical analysis method, the basic regulations of the effects of grouting circle on tunnel gushing water and external water pressure on lining are studied. And it is potined out that there is an appropriate and economical parameter when designing the grouting circle. Based on the above-mentioned research results, the method and program for choosing appropriate parameters of grouting circle are proposed.(6) Based on load-structure model, the influence of the structural cross-section shape and design parameters on the stress characteristics of the lining structure are discussed. The lining structure section shape optimization calculation of Qingdao Jiaozhou Bay subsea tunnel water section is made. And taken the safety factor of the lining structural cross-section as the objective function, the optimum section is obtained.