Study On Safety Of Submarine Hydraulic Tunnel Resistant To Earthquake Loading

Shield tunnels have been widely used in varieties of infrastructure construction fields in China. However, the past damage records show that the loss is considerable once tunnels are destroyed during earthquake. Earthquakes frequently occur in our country, hence it's necessary to further study on seismic analysis for hydraulic tunnels. This thesis closely relates to the project - study on earthquake liquefaction and treatment policy of hydraulic conduit of Caojing electrisity plant. By virtue of cyclic triaxial test and FDM dynamic analysis, the potential occurrence of liquefaction of shallow sandy silt layer in which hydraulic tunnel lies is studied. Still inner forces of tunnel lining due to earthquake are computed and analyzed. The chief contents of this thesis are as follows:Firstly, physical and mechanical property indices of⑤₂ layer which is sandy silt in site layers are obtained by indoor geotechnical tests. The test items include: consolidated undrained triaxial compression test, through which strength parameters that is cohesion and angle of internal friction can be determined. Dynamic triaxial liquefaction test, through which variable relationship between deviator stress, axial strain and excess pore water pressure and number of cycles is studied respectively, still liquefaction strength of saturated undisturbed specimen of sandy silt under simulative earthquake dynamic stress is estimated. Resonant column test, through which the strain-dependent shear modulus and damping factors can be determined. These test result will provide reliable parameters for the following numerical analysis of seismic response of tunnel lining and evaluation of liquefaction susceptibility.Secondly, empirical method using SPT N-values, Seed simplified method and finite difference dynamic method are used to evaluate the liquefaction characteristics of sites respectively. The concept of safety factor against liquefaction F_l and Martin increasing mode of excess pore water pressure are introduced. Four different cross sections of typical possibly liquefiable stratum along pipeline are analyzed by finite difference method, thus liquefaction can be evaluated based on safety factor against liquefaction F_l and pore pressure ratio r_u. In the final, comparation and analysis among the results from the three methods are conducted.Thirdly, inner force characteristics of tunnel lining due to earthquake are studied by applying equivalent nonlinear finite difference dynamic time history analysis. The value and variation of inner forces including axial force, shear force and bending moment of tunnel lining under combined static loading and earthquake loading are studied, with a highly emphasis. The location where the maximum inner force may appear has been found through seismic analysis. This study will provide loading level for seismic design of tunnel lining, as well as exploring distribution and variation of inner force of underground structures with a circular shape subject to earthquake shear wave.