FAILURE CHARACTERISTICS AND INFLUENCE FACTOR ANALYSIS OF MOUNTAIN TUNNELS AT EPICENTER ZONES OF GREAT WENCHUAN EARTHQUAKE

The great Wenchuan Earthquake of Ms 8.0 grade took place in the Longmenshan fault zone of western Sichuan Province on May 12, 2008. Many tunnels on the highway from Dujiangyan to Wenchuan were damaged severely due to the site located near the epicenter. Through the on-site investigation and research, the data collection and analysis, this paper illustrates the failure types and characteristics of the mountain tunnels in the severe earthquake region. It further summarizes the basic types as the avalanche and sliding of tunnel opening, the rhegma of tunnel portal, the collapse of liner and surrounding rock, the crack and dislocation of liner, the uplift and crack of arch invert, the deformation and crack of preliminary bracing. Then, the influencing factors of the tunnel seismic damages are discussed by taking into account especially the controlling functions of the secondary faults of earthquake fractures, the contact zone between cover and bedrock, the unstable slope in the tunnel face, and the weak surrounding rock in high geostress environment on the strong seismic damages. Finally, some revelations of the tunnel earthquake-resistance are summarized based on the natural results of the mountain tunnel damages in Wenchuan Earthquake. It is suggested that in strong earthquake areas, a comprehensive aseismic design be conducted considering the slope of tunnel face, naked tunnel and tunnel portal structure as a system. For tunnel crossing the secondary faults of active fractures, the reinforced concrete structure should be used in the second lining along the faults and their bilateral some distances. For the contact zones between cover and bedrock, the change zones between weak and hard rockmasses, the jump zones of surrounding rock quality and so on, they should be treated by the methods of improving their mechanical property and making it gradual change in preliminary bracing. 