ABSTRACT
In a discharge tunnel of the Amagase dam redevelopment project, a stilling basin section is an ultra-large cross-sectional tailrace tunnel. This tunnel passes through a weak fracture zone intervening in sandstone and mudstone with small overburden. Therefore, it is excavated through multiple bench cut with advancing side and center drifts, and observational construction is adopted during excavation. This section is under very difficult conditions and has only few similar design examples, so safety was prioritized. Safety, workability, profitability, and inner smoothness of tunnel lining has been secured by prior corner casting and upper/lower-divided-type travelling form in a 40% steep slope section. Tunnel behavior was controlled as estimated; an ultra-large cross-section tailrace tunnel was constructed reasonably.
