Urban planning Civil Engineering

 The immersed road tunnel is a system of constructing directly under a waterway whereby very large tube of prefabricated concrete or tube made of steel then filled with concrete elements fabricated a short distance from the site in the dry docks, or in improvised floodable basins sealed with bulkheads at each end and then floated out to be installed by sinking it into place inside the pre-dredged trench. The water between the bulkheads is then pumped out from inside the elements. The elements are then rubber sealed together to ensure a watertight tunnel and crossing. Tunnel cross-sections may have flat sides or curved sides

CONSTRUCTION PROCESS: For constructing an immersed tunnel is to establish one or more casting basins as open excavations, where the individual tunnel elements are constructed. Each tunnel element is composed of several segments. When the elements are completed, they are sealed this temporary bulkhead are the casting basin is flooded one by one to their intended location, immersed into their final position on the seabed, are linked together. The tunnel is normally placed in in a pre-dredged trench in the seabed of the waterway. The connection to the land surface is normally connected to a cut and cover tunnel to overcome the level between the water and the land surface. Casting basin and tunnel elements a. Constructing the casting basin for fabrication the tunnel elements 

b. Constructing the first batch of tunnel elements 

c. Construct the temporary water ballast system 

d. Seal the element with a temporary bulkhead 

e. Fill the casting basin (inundation) 

f. Remove the basin gate 

g. Floating, trimming and winching the elements out of the basin and transport them to the immersion site one by one or a temporary parking parking place 

h. Prepare the elements for the immersion process 

Immersion 

a. Dredge the trench for the tunnel 

b. Construct a gravel bed at bottom of the trench 

c. Immerse the tunnel element to its final place in the dredged trench 

d. Connect the element by pumping and emptying water from of space between the bulkheads (Water pressure on the free end of the new element compresses and seal the rubber joint between the two elements) 

e. Backfill material is placed on the sides and over the tunnel, to fill the trench and permanently bury the tunnel 

f. Ballast the element on-site 

g. Establishing the closure joint and the tunnel is open 

Tunnel installations Parallel to immersion process, refurbishing of the the tunnel can be carried out in the elements already in place.