question: How are major tunnels made under water?
comment: Sometimes they use giant boring machines that work like a mole, digging the tunnel out, and sometimes they will pre-build the tunnel in sections, sink it, then connect it together / cover it.
passages: Section::::Underwater tunnels.
  
  There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes, examples are Bjørvika Tunnel and Marmaray. Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date.
  
  Section::::Landtunnels.
   Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.
   Underwater tunnels are either bored or immersed: tunnel boring is common for deepwater tunnels longer than 4 or , while immersion is commonly used for tunnels which cross relatively shallow waters. Immersion involves dredging a trench across the seafloor, laying a foundation bed of sand or gravel, and then lowering precast concrete tunnel sections into the excavation and covering it with a protective layer of backfill several metres thick.
   Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.
   Section::::Construction.:Underwater tunnels.
  
  There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes, examples are Bjørvika Tunnel and Marmaray. Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date.
  
  Section::::Construction.:Temporary way.
   There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally a tube can be sunk into a body of water, which is called an immersed tunnel.
  
  Section::::Construction.:Cut-and-cover.
  
  Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.
  
  Two basic forms of cut-and-cover tunnelling are available:
   BULLET::::- Bored tunnel, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section. Some concepts of underground mining apply. Modern techniques include Shotcrete used in the New Austrian tunnelling method, use of a tunnel boring machine or tunnelling shield. But still tunnels are constructed which are secured with pit props and shoring and then are steined or timer supports are set. Techniques known from barrel vaults are helpful.
  
  BULLET::::- Immersed tube tunnel, sunk into a body of water and laid on or buried just under its bed.
  
  Section::::Cost.
   Additionally, many water tunnels are sealed and can reduce or increase the internal static pressure, to perform cavitation studies. These are referred to as cavitation tunnels.
  
  Section::::Methods.
   Section::::Construction.
  
  The concept of submerged floating tunnels is based on well-known technology applied to floating bridges and offshore structures, but the construction is mostly similar to that of immersed tunnels: One way is to build the tube in sections in a dry dock; then float these to the construction site and sink them into place, while sealed; and, when the sections are fixed to each other, the seals are broken. Another possibility is to build the sections unsealed, and after welding them together, pump the water out.
   Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
   Section::::Engineering.:Surveying.
  
  Marine soundings and samplings by Thomé de Gamond were carried out during 1833–67, establishing the seabed depth at a maximum of and the continuity of geological strata (layers). Surveying continued over many years, with 166 marine and 70 land-deep boreholes being drilled and over 4,000-line-kilometres of marine geophysical survey completed. Surveys were undertaken in 1958–1959, 1964–1965, 1972–1974 and 1986–1988.
   BULLET::::- The 1927 Holland Tunnel was the first underwater tunnel designed for automobiles. The construction required a novel ventilation system.
  
  BULLET::::- In 1945 the Delaware Aqueduct tunnel was completed, supplying water to New York City in the US. At it is the longest tunnel in the world.
  
  BULLET::::- In 1988 the long Seikan Tunnel in Japan was completed under the Tsugaru Strait, linking the islands of Honshu and Hokkaido. It was longest railway tunnel in the world at that time.
  
  Section::::Examples.:Longest.
   Tunneling will generate soil that has been excavated and this can be used to create new land, as was done with the soil of the Channel Tunnel to create Samphire Hoe.
  
  Section::::Advantages.:Compared with ferry links.
   The tunnels were constructed using the technique refined by Ole Singstad with the Baltimore Harbor Tunnel, whereby a large ditch was first dug for each tunnel, into which was lowered pre-fabricated tunnel sections cable-suspended from overhead barges. Interior chambers were filled with water to lower the sections, the sections then aligned, bolted together by divers, the water pumped out, and the tunnels finally covered with earth.
   The initial nickname of the Teredo tunneling protocol was "Shipworm". The idea was that the protocol would pierce through NAT devices, much as the shipworm (a kind of marine wood-boring clam) bores tunnels through wood. Shipworms have been responsible for the loss of many wooden hulls. Christian Huitema, in the original draft, noted that the shipworm "only survives in relatively clean and unpolluted water; its recent comeback in several Northern American harbors is a testimony to their newly retrieved cleanliness. The Shipworm service should, in turn, contributes to a newly retrieved transparency of the Internet." 
   The trench is then backfilled and any necessary protection, such as rock armour, added over the top. The ground beside each end tunnel element will often be reinforced, to permit a tunnel boring machine to drill the final links to the portals on land. After these stages the tunnel is complete, and the internal fitout can be carried out.
   Tunnel valleys have similar characteristics, irrespective of whether they are formed on land or in a submerged environment. This is because they are formed by high pressure water under a thick ice sheet – in a submerged environment they still have sufficient pressure to erode tunnel valleys into configurations comparable to those generated on land.
   Tunnel construction
  
  Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
  
  There are three basic types of tunnel construction in common use:
  
  BULLET::::- Cut-and-cover tunnel, constructed in a shallow trench and then covered over.
   CNN's Michael Cary went into the tunnels with O'Brien and described him as "an expert on the more than 300 miles of underground flood channels and its tunnel dwellers."
   Micro tunnel shield method is a digging technique used to construct small tunnels, and diminish in size of general tunnelling shield. Micro tunnel boring machine is quite similar to general Tunnelling shield but on a smaller scale. These Tunnel boring machine generally vary from , too small for operators to walk in.
  
  Section::::Back-up systems.
   Section::::Construction.:Riverside Badlands Tunnel.
   Section::::Test facilities.:Cavitation tunnel.
   Section::::Under construction.
  
  The following tunnels are under construction, but are not yet completed or taken into use.
  
  Section::::Proposed.
   Some tunnel stories turn out to be very plausible, such as the tunnel at Hayle in Cornwall which really does seem to have been built specifically for smuggling. More modern tunnels can be found along New England's coast. In other instances the tunnel either doubles as a storm drain or some other functional channel, or else is an extension of a natural fissure in the rock, as at Methleigh and Porthcothan.
  
  Beith in North Ayrshire was a notorious haunt of smugglers at one time, and legend has it that a tunnel ran from the town centre down to Kilbirnie Loch.
   BULLET::::- "Bottom-up method": A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
  