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  Sidewall Reinforcement of Extremely Large Cross-sectional Tailrace Tunnel Passing Through a Fractured Zone

  In Amagase Dam redevelopment project, the stilling basin of tunnel-type outlet facilities is an extremely large cross-sectional tunnel (maximum drilling cross-sectional area: approx. 650 m2, maximum inner section finish area: approx. 500 m2) for dissipating the discharged water from the main gate before it flows into the Uji River. The target geology is mostly hard sandstone, but a large fractured zone (F-0 fractured zone) crosses at about 80 meters from the entrance of the tunnel. From the geological observation result during the sidewall drift excavation, we found that the zone’s width to be about 1.5 times wider than our initial prediction, and that the support structure designed was not appropriate. This report discusses the design change and construction of the sidewall reinforcement based on the review of the geological condition.

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