Study On Seismic Response Characteristic And Damage Evaluation Method Of Tunnel Portal

Portal of mountain tunnel located at high intensitive earthquake zone is prone to seismic damage. Two types of tunnel portal damage, including crack damage of tunnel portal liner induced by propogation of seismic wave and landslide or collapse damage of tunnel entrance slope induced by dynamic instability, significantly affect the safety of tunnel during the operation stage. In this dissertation, a comprehensive study is carried out to analysis the damage evolution of tunnel portal liner and dynamic stability of tunnel entrance slope under the effect of earthquake by theoretic deduction, numerical simulation and large scale shaking table test. The main work and its conclution is described as following:(1) Conventional absorbing boundary condition and seismic wave input method are summarized. It is found that all these seismic wave input method are essentially a spectial case of a coupled wave input method. An uncoupled wave input method based on the domain reduced method is proposed. Such approach converts the conventional exogenous wave problem to a new endogenous wave problem by migrating the interface on which equivalent loading is applied from the artificial boundary to an internal zone of the nummercial model. The equivalent loading is convenient to calculate and sufficient accurate. Arbitrary absorbing boundary condition can be used conjunction with such wave input method. It also makes it possible to produce a uniform static-dynamic boundary condition for the seismic analysis of tunnel.(2) A new seismic wave input method is introduced to simplify the generation of shear wave field for dynamic analysis of tunnel under the effect of shear wave traveling upwards. The comparison of results between one-dimensional soil column model, free field model and tunnel model shows that the result produced by such shear wave input method conforms to theoretical prediction and is thus reliable.(3) Variaty of concrete damage and crack models are reviewed and the concrete plastic damage model is selected and introduced to dynamic analysis of reinforced concrete liner. Such model is derived from continuum damage mechanics and the classic elastoplastic theory and is suitable for predicting the development of crack of liner under the effect of complicated seismic wave. A numerical model after the Shaohuoping Tunnel is established to analysis the damage mechanism of tunnel portal liner under the effect of shear wave. The crack pattern in the simulation result agrees with that observed in the field investigation, which verifies the applicability of concrete plastic damage.(4) In order to overcome the shortcoming of the conventional evaluation index based on elastic stress, two damage indices, e.g. the crack opening width and the overall damage index, are proposed to assess the damage state of liner during the earthquake. The crack opening width provides theoretical prediction of extend of crack that appears on the surface of liner and can be used as a design criterion for the design level Ⅱ and level Ⅲ during the design of liner. The overall damage index is a straightforward indicator to show the variation of overall liner damage state with earthquake time, and also points out the time interval within which the damage of liner onsets or aggrevates.(5) A Rayleigh wave input method for three dimensional numerical model is developped based on the domain reduced method and the derived displacement solution of free field during the propogation of Rayleigh wave. The comparision between result from such approach and that from conventional elasticvisco boundary input method shows that the Rayleigh wave input method based on domain reduced method gives sufficient accuracy. The decoupling of seismic wave input and absorbing boundary condtion and the independency on stress solution of free field significantly reduces the difficulty of seismic wave input and arrangement of absorbing boundary condition.(6) A three dimensional nummercial model after the portal section of Longxi Tunnel is established to investigate the damage evolution of tunnel portal under the effect of Rayleigh wave. The results of damage analysis showed that all the cracks appeared within a scope of 0-93m of the tunnel portal, and that the length of each crack decreased as its distance from the entrance increased. The cyclic and accumulative effects of successive earthquake loading over time, rather than simply, the effect of loading with a maximum amplitude determined the final damage state of the lining structure. The comparison between the unreinforced and reinforced cases showed that, the strengthening effect of reinforcement remarkably reduced the overall damage extent of the lining structure, but could not prevent hairline cracks. Moreover, the presence of reinforcing steel prevented micro-cracks from expanding to wider cracks, which is essential to maintain the waterproof capacity and mechanical stabilization of the lining. The damage pattern of the pavement at the portal and the tunnel lining revealed in the simulation closely matched the field observations for the Longxi Tunnel during the Wenchuan Earthquake.(7) A large-scale shaking table test was conducted to study the dynamic behavior of entrance slope and its interaction with lining structure of mountain tunnel under earthquake loading. Test result showed that the acceleration response of tunnel entrance slope exhibit obvious amplification effect and surface effect along both vertical and axial direction. Significant nonlinear behavior was observed when the earthquake loading amplitude is larger than 0.6g, and after that, amplification factor decreased with the increasing input loading amplitude and additionally the distribution of acceleration became more even in the slope body. It is also found that the dynamic response along axial direction of entrance slope did not affect much by existence of tunnel structure, thus it could be evaluated by treating the entrance slope as a natural slope for simplification. On the other hand, the potential instability of entrance slope had much influence on the safety of tunnel structure. When the loading amplitude is relatively small, internal force induced by vertical acceleration is larger than that caused by horizontal acceleration. As the loading amplitude became lager, the horizontal component of earthquake play a dominant role in affecting the tunnel structure.