Seismic Response Analysis Of Tunnels In Strong Earthquake Area

With the advancement of the Chinese modernization progress, the tunnel and underground engineering in many regions will inevitably suffer the problems of intersecting active faults or passing through high intensity seismic region. So it is of profound social significance to study stability response characteristics of the tunnel to seismic actions. In order to study the seismic impacts on tunnel stability. firstly, cases of seismic tunnel failures all over the world were compiled to analyze the failure modes and seismic influential factors on tunnels. Secondly, based on the regression analysis between earthquake magnitudes and coseismic displacements made by Wells, damages of Wenchuan earthquake on regional tunnels were discussed and analyzed. Thirdly, three-dimensional finite element software ANSYS was used to simulate the effects of seismic wave input angle on the tunnel. Fourthly, the change of in-situ stress before and after the Kunlun Earthquake, the Kobe Earthquake and the Wenchuan Earthquake were analyzed, and three-dimensional finite element software 3D-sigma was used to simulate the influence of stress variation before and after the Wenchuan Earthquake on the tunnel stability. Based on the above research, preliminary results and progresses are listed as follows:(1) Based on the compilation of research cases of seismic tunnel failures all over the world, and combined with the seismic tunnel failures during the Wenchuan earthquake, the failure modes can be categorized into failures of tunnel trunk and tunnel opening. The failures of tunnel trunk include shearing displacement of tunnel lining, longitudinal cracking of tunnel lining, circular cracking of tunnel lining, diagonal cracking of tunnel lining and upheaval of base floor. The failure modes of tunnel opening include collapse of falling rocks, compression of sliding slope, and cracking of end opening walls.(2) The research findings shown that three coseismic parameters were directly related to the aseismic design of tunnel and underground engineering, the three parameters include surface rupture length (SRL/km), the maximum coseismic rupture displacement (MD/m) and the average coseismic rupture displacement (AD/m). During the Wenchuan earthquake, the secondary faults between the central and frontal faults in the Longmenshan fault belt moved seismically, so the seismic failures of three tunnels between the two fault zones is very severe. Therefore secondary faults of near the seismogenic fault also can create coseismic displacement during earthquake, and can damage the underground structures and tunnels.(3) From the results of numerical modelling, in terms of principal stresses, it was different of the stress magnitude in various monitoring points of tunnel, and the maximum and minimum principal stress of crown arch and arch bottom were smaller. In terms of displacement field, the displacement at crown arch was bigger, and displacement of arch bottom was smaller, which indicate that impact of earthquakes on the crown arch was more severe, which was consistent with the actual situation of the tunnel failures.(4) By comparing the stress changes before and after Kunlunshan, Wenchuan and Kobe Earthquakes, it is found that earthquakes can cause a stress decrease in seismic area. Three-dimensional finite element software 3D-sigma was used to stimulate the influence of stress changes before and after the earthquake on the tunnel stability. Modeling results showed that the maximum and minimum principal stresses on the monitoring points of tunnel section after earthquake were smaller than those before earthquake. Seismic effects on the arch foot area of tunnel were the most obvious, on around spandrel were next, and on crown arch and arch bottom were the smallest.