Seismic damage assessment of mountain tunnel: A case study on the Tawarayama tunnel due to the 2016 Kumamoto Earthquake

The Kumamoto Earthquake with magnitude of 7.3(Mj) on April 16 and 6.5(Mj) on April 14, 2016 have triggered numerous damages to the Tawarayama Tunnel in Kumamoto Prefecture, Japan. Distribution and characteristics of these seismic damages were investigated and summarized to assess potential influencing factors. Seismic damages are categorized into five patterns as follows: lining cracks, spalling and collapse of concrete lining, construction joint damage, pavement damage and groundwater leakage. Lining cracks can be further classified into ring crack, longitudinal crack, transverse crack and inclined crack. Site investigation showed the primary seismic damage was lining crack, especially ring crack. In special, an interesting phenomenon was observed that ring cracks occurred with an estimated average spacing of 10.0 m in 23.4% spans of the Tawarayama Tunnel. This results from the interaction between seismic wave and special geological conditions that dense Andesite and crushed Andesite around the Tawarayama Tunnel appear in tilt alternately with space between 10 m and 20 m. Following these analysis, some recommendations were proposed for future tunnel planning.