| High water pressure and unlimited water supply is the distinctive feature of subsea tunnel. In particular, when tunnel gets cross the worse geological section such as fault fracture zone and soft ground, groundwater often causes the instability of surrounding rock, and there will be inrush and other geological disasters. In the end, the tunnel structure will be damaged and there will be threat to builders'safety. It is believed that the accurate prediction of tunnel water inflow and the measures of pre-grouting are not only the key of controlling stability of surrounding rock but also the urgent question of design and construction of tunnel drainage. Therefore, this paper will discuss some problems of the prediction of water inflow, surrounding rock stability and the measures of pre-grouting in the construction of subsea tunnel, and the main research contents are as follows:(1) Based on the theory of shaft in infinite aquifer, the calculation formulae of the steady seepage field were deduced for two cases, considering or without considering grouting circle. It was also given analytical expression of external water pressure on lining and tunnel water inflow, and the relationship between water pressure on lining and the discharge of tunnel, parameters of the grouting circle was given. At the same time, it was described the theoretical basis of the waterproof criteria of "block off ground water and limiting discharge"(2) The methods of predicting tunnel water inflow and conditions of their application were introduced firstly. Based on the theoretical formula of the paper, the relationship curve between tunnel water inflow and the permeability coefficient of grouting circle, the thickness of grouting circle was analyzed, and the conclusions obtained could guide the selection of the parameters of grouting circle.(3) Based on the hydrogeological conditions of Jiaozhou Bay subsea tunnel, empirical formulae, theoretical analysis and numerical simulation were used for predicting tunnel water inflow, and comparing three results obtained above in order to point out the dangerous region where water or mud bursting may occur during constructing.(4) Based on Drucker-Prager rock yield criterion, and taking into the effect of sea water seepage account, the numerical solution for radius of plastic zone, stresses around circular opening and radial displacement of tunnel wall were derived. The relation curve between hydraulic head pressure difference and radius of plastic zone around circular opening for different types of rock masses, ground reaction curve, and distribution curve of radial stress based on Drucker-Prager yield criterion were obtained with seepage being considered.(5) According to the pre-geological survey data of Jiaozhou Bay subsea tunnel, it was taken to analyze the geological risk of subsea tunnel. Based on the analysis of geological risk, the relative technology for geological forecast in advance was adopted for ensuring the reasonableness of the survey method and the reliability of the result. Considering the survey result and the water discharge and pressure, the criteria for scheme selection and the evaluation method for grouting effect were put forward to safely treat the unfavorable geological body with money saving.(6) By using FLAC 3D to simulate the fault fracture zone F4-4 of Jiaozhou Bay subsea tunnel, while considering coupled fluid-solid theorem, the affection of the parameters of grouting circle, the structure of lining and the mode of drainage on the seepage field, the stress field, the displacement field, the radius of plastic zone were analyzed. At the same time, the result was carried out to verify the correctness of the theoretical formula. |
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