In soft or squeezing ground, early section closers by means of invert shotcrete and invert concrete are often carried out to restrain deformation and stabilize the ground around the tunnel. However, while advancing the terrestrial face, there is a risk of damage, such as cracking, occurring in the invert due to redistribution of plastic ground pressure and the intrinsic characteristics of the ground. During excavation of the Ozaso Tunnel on the Tohoku Chuo Expressway, ground deformation caused the propagation of numerous cracks in the invert. This paper presents an outline of the investigation into the cracks, the major cause, and the design and construction management ofthe countermeasure work.

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