Study On Failure Mechanism Of Typical Seismic Damage At Mountain Tunnel Portal Section Under Strong Earthquake

For the mountain tunnels built in the high-intensity seismic region, it is inevitable to be affected and even damaged by earthquake. As the only access into and out of the mountain tunnel, portal section is of poor geological conditions. Once seismically damaged, the portal section will be possibly traffic-unaccessible. Obviously, it is extremely necessary to research the failure mechanism of mountain tunnel portal section so as to essentially figure out the reason why the seismic damage at portal section is of large severity and probability, aiming at improving the seismic resist performance of the portal section. At present, although the studies on seismic damage of mountain tunnel portal section have made some progress, systematic failure mechanism analysis of the typical seismic damage is still scarce. In view of this, a mountain tunnel-Longxi Tunnel located in high intensity seismic region being a research object, combined the numerical simulation with shaking table test, a systematical research on failure mechanism of typical seismic damage at mountain tunnel portal section is carried out in the paper. Not only can this research provide theoretical reference for the future research on earthquake damage of mountain tunnel portal section, also can be a guideline for the seismic resistance and absorption design of mountain tunnel portal section, with the important value of engineering application.Main work and results obtained are shown as follows:1. Based on the collection and sorting of damage examples at mountain tunnel portal section during Wenchuan earthquake, the typical seismic damages (including crack, rock fall, collapse and slide of slope; cut-and-cover section and portal structure damaged; crack and bulge of inverted arch; crack, dislocation, spalling and collapse of lining, etc.) of tunnel portal section are comprehensively analyzed from many aspects involving the characteristics, influence factors, development conditions, damaged location and failure mechanism, laying the foundation for further mechanism analysis of typical seismic damages of portal section.2. Using FLAC3D software, a numerical model was established to simulate the portal section of Longxi Tunnel. With the input of Wolong wave, the dynamic response rules of acceleration, displacement and lining structure stress of the mountain tunnel portal section are finally revealed. Main results obtained include that with the increase of buried depth, the dynamic response of acceleration is gradually decreased and obviously affected by unsymmetrical pressure, and that Lining displacement deformation is priority to horizontal and vertical, namely the closer to the tunnel portal, the greater the displacement.3. Through the special research on the key technical difficulties of shaking table model test, shaking table model test is successfully accomplished; Dynamic response rules of acceleration, soil pressure, structure strain at mountain tunnel portal section is also revealed. Main results obtained include that with the increase of buried depth, the dynamic response of acceleration is gradually decreased; with the increase of the vertical model height, the dynamic response of acceleration is gradually increased and the magnification coefficient is also gradually increased; the internal force distribution of lining structure is obviously affected by unsymmetrical pressure and is apparently concentrated at the base-cover interface.4. Combined numerical simulation test with shaking table test, comprehensive failure mechanism analysis of typical seismic damage at the mountain tunnel portal section is conducted, involving slope cracks, rock fall, collapse and slide, lining cracks; lining ring fracture of shear-tension. The results show that seismic slope damage is caused by huge horizontal seismic inertial force exceeding the tension and shear strength limit of the slope itself; and that seismic lining damage is caused by internal dynamic accumulation strain exceeding shear and tension strain limit of the lining concrete itself.