ABSTRACT
Otonaka Tunnel (L=4,686m) is one of the most difficult tunnelling projects under squeezing ground condition in Japan. Some supports were heavily deformed and tunnel section was completely closed or obstructed by ground heaving. To overcome this condition, it is necessary to design the triple layered perfect circle support structure. During excavating, it is necessary to constrain initial displacement by short distance ring closure which is around 5m behind tunnel face. As a result, these construction methods have ensured the stability of tunnel support structure under extremely high squeezing ground conditions.
