Study On The Response And Design Method Of Circular Tunnel Under Severe Earthquakes

A large number of subways have been constructed in big cites of China along with the development of the underground transportation promoted by urbanization. Meanwhile some of these cities are located in earthquake high-incidence area. In recent years, strong earthquakes occurred frequently. And the anti-seismic analysis of underground structures, such as subway tunnel, is becoming a critical topic in the study of urban disaster prevention and mitigation. Based on the summarization of domestic and foreign research achievement about the seismic response of circular tunnel, this thesis studies the progressive failure mechanism and the seismic design method of circular tunnel under severe earthquakes in the methods of theoretical analysis, numerical simulation and model test. The research results are as follows:(1) The features of shear deformation and the failure modes of the interface between soil and structure is revealed through the shear test, and the description method of relative displacement between soil and structure which indicates the interfacial deformation behavior is presented on the basis of interface mechanics. The connotation of relative displacement of interface is defined, and the corresponding cohesive zone interface constitutive model, which considers the property of strain softening and the interfacial failure in the form of normal detachment and tangential slip, is built. The zero thickness interface element is formed through the enhanced finite element method, and the progressive failure mechanism of interface in shear test is concealed. The average shear stress-shear displacement curves from shear test is pointed out not to be able to truly reflect the characteristics of stress and deformation of the soil-structure interface.(2) On the basis of solid elasticity theory and in the use of complex function series solution, both the pseudo-static analytical solutions of deep buried circular tunnel under "no-slip" and "'full-slip" soil-structure interface condition and the semi-analytical solution which considers the progressive failure process of the interface under high seismic shear stress are derived. Analysis shows that the plus or minus 45°position of the interface is the earliest to undergoing shear failure under severe shear stress. As the seismic stress grows, the shear failure zone extends to both sides. Due to the effect of the progressive shear failure of interface, the axial force of tunnel is much smaller than the one under the conditon of "no-slip", but higher than that under the condtion of "full slip". Meanwhile, the bending moment value almost coincides with the one under the condtion of "full-slip".(3) The seismic response analysis of the free field of saturated sand manifest that, the distribution of seismic stress reduction coefficient with depth not only have relation with seismic peak acceleration, but also have something to do with seismic waveform, vibration history and the properties of site soil. When peak acceleration is small, the empirical formula of rd brought by Seed etc can reflect the change law of stress reduction coefficient. However, the difference of calculated value of rd can reach to 20% because of the different softening degree of soil skeleton at different times. At the moment when the peak acceleration is big, the sand is liquefied, the variation rd with depth is greatly influenced by the liquefaction zone, and the empirical method can not express this character.(4) The seismic respose analysis of circular tunnel under single-phase and saturated sand site reveals that the internal force of structure is related to the dynamic time history of earthquake, and the seismic design specification only using the peak acceleration can not suitably reflect the structural response. The influence area of the common subway tunnel to the surrounding soil is about one time the tunnel diameter, and the deformation form of structure is restricted by the site. The tunnel design method based on free-field displacement will be much conservative due to the plastic deformation of site under high-intensity earthquake. In view of this, a pseudo-static design methond of circular tunnel based on modified free-field displacement is proposed, which is suitable to be used in high seismic intensity area.(5) After systematically evaluating the performance of geotechnical centrifuge ZJU400 and shaking table newly built in Zhejiang University, centrifuge-shaking table model tests of free field and underground tunnel in saturated sand site under different seismic excitations are carried out. Through monitoring pore water pressure, acceleration and structural strain, the distribution of stress reduction coefficient in free field under different earthquake excitation is obtained. The influence of circular tunnel to the dynamic respose of surrounding soil is studied, as well as the deformation form of the tunnel itself. The rationality of the design method based on modified free-field displacement is also verified.