Seismic Response Analysis For Mountain Tunnel Portals In High Steep Slope Based On Wave Theory

Because of the seismic wave amplification effect produced by weak surrounding rock and the local topograhphy, earthquake damage occurred frequently, tunnel portals are the weak part of mountain tunnels during earthquake. Strong earthquake observation show that the acceleration along the elevation has obvious nonlinear amplification characteristics. With the increase of the slope height and angle, the rise of the tunnel portal position, the seismic response of the tunnel lining will be more intense, and a higher risk of tunnel structures damage induced by strong earthquake. Based on the tunnel portal project of Chengdu-Lanzhou railway, the seismic response of tunnel portals in high-steep slope are carried out by means of field investigation, theoretical analysis and numerical simulation. The main research results are as follows:(1) The vibration theory and wave theory for the seismic nummreical analysis of tunnels were summarized. A lot of numerical examples are carrried out to discuss the difference and applicability of the two theories. The results show that the difference between the wave theory and the vibration theory is that seismic input in different ways. In the vibration theory, the accuracy of displacement input is the worst, the acceleration input value is less stable which is influed by the boundary effects. When the surface is flat and no local topograhphy, the acceleration input can approximately replace the wave input. For the mountain tunnel entrance influenced by local topograhphy, the wave input is proposed to use.(2) By the investigation of 64 tunnel portals of Chengdu-Lanzhou railway, the main characteristics were summarized. Based on statistical results, the mountain entrance slope is simplified as a single-side uniform slope, parametric study is carryied out on amplification effect by using numerical method. The influence of the relative stiffness of lining and surrounding rock, the dynamic parameters of surrounding rock (damping and dynamic Elastic modulus), the local topography (slope height and angle), and the amplitude and frequency of seismic wave on amplification effect are analyzed. The results show that the acceleration amplification factor is influenced by the slope height, the shear wave velocity of the surrounding rock and seismic wave frequency, and shows two kinds of distribution law along the elevation. When the slope height is smaller than the critical slope height, the acceleration amplification factor increases linearly along the elevation, and when the slope height is greater than or equal to the critical slope height, the acceleration amplification factor slowly decreases and then increases rapidly along the elevation. The critical slope height is related to the shear wave velocity of the surrounding rock and the excellent frequency of seismic wave. According to the numerical results, the determination method of the acceleration amplification factor was proposed. This method can provide a reference for the seismic design of linings by pseudo static method.(3) Based on the viscous-spring artificial boundary, the calculation formula of the 3D equivalent node force for tunnel portals was derived under the P wave and S wave vertical incident. In the ANSYS, the APDL was used to compile the assistant program, which can realize the automatic loading of seismic wave. Taking a tunnel entrance as the research object, the seismic response characteristics of tunnel lining was studied and the damping measures was discussed by 3D numerical analysis.The results show that the internal force of tunnel lining presents three dimensional characteristics.The influence of the tunnel portal topography on the longitudinal internal forces is more significant than the transverse internal forces.When the P wave and SV wave input,the vault and invert are the weak parts, while the SH wave input, the arch shoulder and arch foot are the weak parts.The SH wave which is polarized perpendicular to the axis of the tunnel has the greatest influence on the seismic response of the lining.The location of tunnel entrance has a certain influence on the internal force amplitude. When the location of tunnel entrance rise from slope foot to slope waist, the maximum internal force of lining increased about 30%.(4) Based on time domain wave numerical simulation theory, the input method of the P wave and S wave obliquely incident was established for mountain tunnel portals. In the ANSYS, the APDL was used to compile the assistant program, which can realize the seismic wave oblique incident, and the validity and accuracy of the method and program were verified by numerical examples. Based on this method, taking Liutongzhai tunnel portal as research object, the influence of the seismic wave incident angle on the dynamic response of linings was studied by numerical analysis.The results show that, the longitudinal oblique incidence of P wave and SV wave has little influence on the distribution law of internal force of lining, while the transverse oblique incidence of SV wave has significant effect on the idstribution law.The linings bear vertical mechanical behavior druing the transverse oblique incidence of SV wave. This results are consistent with the vertical damage features of some tunnels inWenchuan earthquake.s...