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The immersed tube method for underwater crossing involves the following basic construction steps:
(i) Prefabricating long tunnel units (steel shell or concrete) in a dry-dock or shipyard
(ii) Floating and towing the units with removable bulkhead to the site
(iii) Immerse the units in a pre-dredged trench
(iv) Connect the units one by one
(v) Covering the completed tunnel with backfill
Steel immersed tunnel is sometimes adopted because of the ease of fabrication and its relative lightness. Moreover, shorter construction time is required when compared with concrete immersed tubes.