3. Seismic design for immersed tube tunnel at Soft sandy ground

The Niigata Minato tunnel is Japan’s first immersed tube tunnel constructed on the sandy ground and thus it was requested to take measures for seismic behavior and robust structure designing as technical requirements. The taken measures for designing are as follows.

1) Safety oriented seismic design on soft sandy ground where large sectional force and displacement are generated

• Application of the flexibility splice with displacement compliance improved rubber gasket and connection cables
• Application of Hachinohe wave (225gal) and Kobe Port Island wave (717gal) for the dynamic ground response analysis for safety checking of the tunnel.

2) Durability design to demonstrate steady performance

• Application of the measures against thermal cracks by proportioning low exoergic type concrete
• Application of cracking control by limitation of reinforced concrete stress intensity and prestress introduction in tunnel shaft direction
• Application of waterproof measures with waterproof steel panel watertight sheeting