Study On Practical Seismic Analysis Method For Underground Structures

Studies on seismic design theories for underground structures has lagged behind ground structures for the sake of minor damages and superior seismic performances of underground structures with respect to ground structures. Nonetheless, in Kebo earthquake (1995) serious damges occurred in underground structures, which overturned the tranditional concept. It was discovered that underground structures could encounter serious damages under large earthquake and their recoveries were of high degree of difficulty. Recently, with the repid development of underground structures, advanced seismic design theories are required and munerious problems are urgent for solutions. With the consideration of higher requirement of seismic design theories of underground structures, the dissertation focuses on seismic problems of underground structures. With this in mind, seismic response characteristics and correspongding influencing factors of underground structures are studied by virtue of theoretical analysis and numerical simulations; along the line, the calculation method for equivalent lateral forces is improved; and last but not least a three-dimensional simplified calculation method is proposed for seismic design of underground structures.The major research work and main achievements of this dissertation are detailed as follows:(1) Based on a sketchy summary of damages of underground structures in recent earthquakes, damage characteristics of underground structures are concluded. Also a systematic review on the existing seismic analysis and design methods and the problems encountered are provided. Moreover, the merits and drawbacks of these methods and their applicabilities are compared.(2) Impact of factors including buried depth, structural material properties and soil stiffness on seismic response of underground structures are discussed. Meanwhile, the seismic response of ground and underground structures are compared. It is indicated that interaction between soil and underground structure should be considered in the constraint condition of simplified seismic calculation method.(3) Based on the formula of coefficient of subgrade reaction, empirical formulas are proposed within the framework of the displacement reponse method and the equivalent lateral force method. The relevant coefficients of the proposed formulas are fitted according to the results calculated from finite element analysis. Besides, implementations of the proposed formulas are carried out on layered soil cases, in order to validate their rationality and accuracy.. The empirical formulas renders a favourable access to determine the spring constant of groundwork which otherwise requires several circles of finite element analysis.(4) The solution procedures of the equivalent lateral force method and the displacement response method are provided respectively, which are widely-used in seismic analysis of underground structures. And the main problems and the current application of these methods are presented as well. Moreover, the displacement response method is improved, including the calculation formulas of seismic load and soil shear force and the setup of constraint conditions, Lastly, numerical analysis are conducted to examine the riability and the applicability of the modified displacement response method.(5) Based on theoretical analysis, a three-dimensional simplified seismic calculation method is proposed for seismic design of underground structures. Furthermore, the method to expand the seismic load and spring constraints of plane model to three-dimensional model is studied, and empirical formulas are proposed for the normal and tangential spring constants of the groundwork. Through comparison with three-dimiensional finite element time-history analysis, the rationality and the accuracy of the simplified method are verified. The simplified method is suitable to the cases beyond the plane strain assumption.