| Mountain tunnels, constructed in rock conditions, have not suffered severe damage during earthquakes, except for ones in extremely poor ground condition, such as fault fracture zones or ground at portal areas. Consequently, they have been empirically considered to be structures with great resistance to earthquakes.However, the road tunnel, constructed by conventional method, with a length of approximately 2km, was heavily damaged by the Kumamoto Earthquake, a hypocenter depth of 12km, a moment magnitude of 7.0, in April 2016. The cracks over permanent concrete lining and the openings of joints between linings were found in the most areas of the tunnel. The kinds of defect and deformation were various. Especially, the collapse of concrete lining occurred in a position of about 400 m from the eastern portal and finally it fell inside the tunnel, though the temporary support and the ground material themselves did not. The compressive failure of concrete lining occurred and its surface with a certain size spalled in a position of about 100m from the western portal and the part of pavement was heaved in longitudinal direction of the tunnel due to compressive force.This tunnel locates about in 17km from the epicenter, in nearly 300m from active fault zone. The construction of the tunnel was difficult in some areas because the various ground conditions from hard to soft rocks such as andesite, sand gravel and tuff, were found and many of them were autobrecciated ones. Ash from volcano as deposition materials induced by past pyroclastic flow was also found and the strength of each stratum was varied. This tunnel was closed due to the heavy damage, however, the altitude of the alternative road was relatively high and its closure during winter was also expected. This tunnel and the route was needed to reopen before the season of snowfall and the prompt decision of the countermeasures such as repair and rehabilitation was needed.It is essential to grasp the mechanical behavior of permanent lining such as the conformity to the movement of surrounding ground, when large outer force or deformation acts on it, because some examples of road tunnels which the concrete blocks fell after collapse during big earthquake in the past were reported. Plain concrete is generally adopted for mountain tunnel as permanent lining in normal rock conditions. The mechanism of behavior not only in the normal condition but in the post collapse stage should be examined when reinforced concrete or steel fiber reinforce concrete is used against the collapse.The paper presents the situation of road tunnel damage in the Kumamoto earthquake. The results of elemental experiment of permanent lining simulating collapse stage is also introduced. Also the concept of of countermeasures against earthquake is discussed. |
