Research On The Dynamic Response Of Portal Section Of Mountain Tunnel Induced By Intense Earthquake And Shock Absorption Measure

Lots of tunnels have been built in high seismic intensity regions in western of Chinawhere earthquake frequently happens. Many mountain tunnels have to be built in highmountains and gorges due to the limitation of the complicated topography. However, in ourcountry, the quasi-static method has been widely but unreasonably used for anti-seismic andshock absorption calculation in the the standards of roads and railways tunnels which can notmeet the requirements of key parts of mountain tunnel, such as portal sections, shallow-seatedsections and unsymmetrical loading sections. Supported by the Western traffic ConstructionTechnology&Science Project in China, based on the case study of Ma-Yazi tunnel of theWu-Guan Highway, the dynamic response of the shallow and unsymmetrical loading tunnellining was analyzed by using FEM. The characteristics of earthquake-induced dynamicresponse of shallow and unsymmetrical loading tunnel were obtained and the anti-seismic andshock absorption measures suitable for reducing the earthquake-induced dynamic responsesof the portal section of this tunnel was proposed. The main works are as follows：1) Based on the characteristics of elasto-plasticity of ground, and by using the theory ofdynamic finite element, the equations for back-evaluating the seismic acceleration of thebedrock from the seismic acceleration of the ground surface were derived by using themethod of Wilson-θ. It is indicated that the ground has an obvious amplifying effect forground motion. Therefore, the seismic acceleration of the bedrock should be input foranti-seismic and shock absorption calculation of underground structures in order to obtainmore reasonable results.2) With the consideration of the characteristics of elasto-plasticity of surrounding rock,the formulas of nolinear equations were deduced by using the method of Newmark-β. Themodels of the shallow and unsymmetrical loading tunnel lining were established and thedynamic responses of the tunnel portal section induced by intense earthquake were analyzed,the most unfavorable input direction of ground motion was determined too. Meanwhile, therelationships of the lining displacement, inner force and stress with the burial depth and thedifferences of those of the symmetrical loading&symmetrical loading were discussed. Thekey anti-seismic parts of the tunnel portal section in such kind of surrounding rock wereascertained.3) The effects of different grouting areas of ground and different lengths of anti-seismicprotected section to the anti-seismic and shock absorption of tunnel lining were studied byusing numerical simulations. The proper grouting areas of ground and suitable length ofanti-seismic protected section for the shallow and unsymmetrical loading tunnel lining wereobtained respectively. γ4) The transverse planar model of unsymmetrical loading tunnel portal was establishedto analyze the earthquake-induced dynamic responses of tunnel lining with different thickness.The conclusions show that the increasing of lining thickness can effectively reduce the axisstress in tunnel lining, but the bend stress in tunnel lining will increase with the increasing ofthickness, while the combined stress will gradually reduce with the increase of liningthickness. Thus, an integrated measure of increased grouting area of ground at tunnel portalsection, strengthened length of anti-seismic protected section, enhancing the intension byadding reinforcing bars in concrete and adopting a smoothed transition shape to reduce thestress concentration in the tunnel lining was proposed to ensure the safety of tunnel portalsection with unsymmetrical loadings under ground motion.5) The vibration equations for simplified mechanical model of the tunnel lining systemcomposed of the primary lining, layer of shock absorption and second lining were derived byusing the vibration theory. Based on the case study of Ma-Yazi tunnel, the conclusionscalculated by analytical method show that the tunnel lining system can effectively reduce theearthquake-induced dynamic responses in the tunnel lining where the stiffness ratio of layerof shock absorption and second lining (γ2) is0.01~0.5. And the calculated results by usingfinite element method indicate that the shock absorption effect is much better if the stiffnessratio of layer of shock absorption and second lining (γ2) is0.05~0.15.