ABSTRACT
The Shibayama tunnel is a new tunnel with a length of 616 m and an excavated cross-sectional area of 83 m2 under unconsolidated sandy ground and overburden of 3 to 17 m. A town road, buried water pipes, and a golf course exist directly above this tunnel. In the event of surface subsidence, the impact on stakeholders would be significant and a major challenge was to prevent quicksand during excavation. First, the optimal grouting material for sandy ground was selected through in-situ test construction. Next, grouting type long steel pipe umbrella was adopted, which minimizes the angle of steel pipe placement. Long steel pipe umbrella, long face bolting, and top heading leg reinforcement were implemented, and excavation was conducted by early closure with invert shotcrete. While constantly observing the surface settlement, the excavation was successfully completed without significant impact to the aboveground structures.
