Mountainous tunnel portal section is one of the most easily damaged parts during strong earthquake. Failures of this part may block roads, which greatly influences the transportation of the local area after earthquake. Model tests on shaking table of tunnel portal section are carried out. A rigid testing box is adopted in the tests. The rock condition around tunnel portal section is designed rather weak as the rock in this part is often weathered and loose. A new similarity material with a main component of fly ash has been successfully made to simulate the surrounding rock. Test with a model scale of 1:50 are designed. Slope angles are set to 35? according to the slope range limited by the rock type. By the testing results, the rule of tunnel portal failure under the condition is studied. The results can be references for other similar projects and for seismic damping measure design.

1.1 Research background

Underground structures have long been assumed to have good earthquake-resistance during earthquakes. Tunnel is a kind of linear underground structure, which was also been considered to be rather safe under earthquake effects. But recent site investigations and research have shown that tunnel portal sections and tunnel body run across displaced faults are the two possible parts of a tunnel that may be damaged severely during earthquake especially strong earth quakes.

There are already many examples of tunnel portal damages during earthquake. 10 of 11 tunnels on Duwen highway are damaged on the portal section during Wenchuan earthquake with a Magnitude of 8.0 (Li, 2008). Some tunnels of the 8 old Sanyi tunnels were damaged at the portals during Hsinchu-Taichung earthquake with a Magnitude of 7.1 (Hwang & Lu, 2006). 16 tunnels were found portal failure and 3 tunnels collapsed due to slope failure out of 57 tunnels during Chi-Chi earthquake with a Magnitude of 7.3 (Wang, et al. 2001).

Tunnels portals are the only part exposed outside the surrounding rock as shown in Figure 1. If failure happens on this part during an earthquake the road pass would be blocked, which greatly influences the transportation of the local area (Figure 2). So it is important to guarantee the stability of this section. Also, understanding on interacting mechanism of tunnel and slope helps repairmen and retrofit of this section.

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