ABSTRACT
The Seikan Tunnel is a 53.85-kilometer-long undersea railway tunnel in Japan. This tunnel is 240 m deep below sea level at the maximum. In 2013, heaving of the roadbed and tunnel convergence were observed at the Yoshioka pilot tunnel, which is used for the ventilation and the drainage of the ground water. As a result of geological surveys, it was found that the strength of the ground was low, the ground slaked easily and a large amount of ground water flowed through the central drainage. Therefore, the deformation of the pilot tunnel was considered to be attributable to a gradual decrease in the strength of the ground surrounding the tunnel and plastic earth pressure thereby acting on the tunnel. In 2017, struts and rock bolts were applied, and deformation was suppressed. This paper describes the deformation of the pilot tunnel, its causes, countermeasures and their effects.
