UnderWater Construction

* See also Construction and Rapture_Construction_Domes
* See also Prefabricated
* See also Rebar
* See also Concrete_IS_Your_Friend
* See also Undersea_Windows
* See also Real_Physics
* See also Watertight_Doors_and_Bulkheads
* See also Airlocks
* See also Automation
* See also TechnicalDrawings (Diagrams)
Big Difference Between Constructing a City on The Surface and Below The Ocean. 
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The reality of an underwater city ( no matter how deep it is ) is the Force of the Water trying to crush the air space of the living zone within. That is Assuming it is not kept AT the sea water's pressureOne of the aspects which Rapture would have had - specified in its early game development - likely to facilitate going back and forth from the water by its weird tentacled denizens. -- Which has separate problems affecting physiology, and with it varying significantly in pressure with depth ( as in 1 atmosphere of pressure changing for only about 33 foot added difference in depth of water ).
So how do you structurally handle Pressures like this ( at ~600 feet deep) to preserve 'normal' air pressure inside ? You have to build a structure which holds out all this pressure - on all sides/every surface, every window, crack and cranny and connecting interface. Bulk Metal ( like on a submarine ) could be used, but it is VERY expensive for large volumes/structures. A material like reinforced concrete resists compression forces very well, and is far cheaper - both for the materials and for the construction process.
So THAT is what Rapture ( as large as it is shown to be ) has to be mostly made of to plausibly exist.
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CONCRETE CONCRETE CONCRETE : 
Reinforced Concrete is the only Plausible Solution short of Infinite BS type Bad Fantasy. And Rapture was NOT supposed to employ Careless/Bad Fantasy to justify itself.
Many LARGE Buildings and Structures paid for with a few mere Billions of dollars and largely using CONTEMPORARY Technology of the mid-late 1940s.
Things might be pushed a little with Ryan's Surface Companies doing research into Underwater Technology - Deep Sea Drilling hadn't started yet in the Real World ( but could have been under development long before that ).
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THICK WALLS :
A key Element affecting buildings in Rapture is THE WATER PRESSURE of that depth. Where buildings above the water's surface need only face the force of gravity and some minor weather issues, those deep in the Ocean have to be built like a Safe/Bank Vault - solid enough to withstand the immense weight of water above - which pushes in on ALL sides ( and tiniest cracks ). The tiniest unsecured hole means doom.
Thus these nice high buildings ( which they liked portraying, and short ones too ) need THICK walls whose mass needs to be held up against gravity. Sideways leverage of ocean currents is multiplied, the taller the building is. Thin also is a problem, as the pressure containing walls need to be structurally thick uniformly, and their useful interior space dwindles away as the building narrows towards any pointiness.
Fancy Protrusions have a similar issue - they still have to be thick and strong in their pressure containment, and any protruding ornateness which contains living spaces requires extra/excess surrounding structure for that smaller interior. Extraordinary structure might have such waste, but the more numerous ordinary/mundane should not ( and could not afford to have ).
If you look at MY revised level maps, they reflect this with a MORE monolithic design -- Which maximizes the interior by having a flat-surfaced/blocky overall shape ( Most of the game levels interiors were squat, irregular and sprawling -- No matter having all those views of predominantly tall thin building seen outside through the windows. ( You are never IN one of those while in the game. )
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Building Water Seal Methods :
Individual buildings would not usually be significantly seal-partitioned within themselves, because of the complexity of there being too many passages which need keeping open : Elevator Shafts, Stairways and Ventilation/Utility Shafts otherwise would all require numerous bothersome closeable/pressure-proof doors and isolation mechanisms. Extra tall/large buildings would have intermediary transfer points, which WOULD be seal-partitioned ( ie- requiring transfer between elevators separated by the seal-partition mechanisms ). Airlocks would be used where required.
Regular free-traffic seal points in buildings need to be normally kept open ( main thoroughfares can't have people waiting for big heavy doors to open and close repeatedly along their paths through a building ).
Securis Doors ( used between seal-partitioned areas ) are more complex than implied in-game (and expensive), and still are not very large. ( Note that My Design for these Securis Doors do NOT have the massive pressure door actually Open and Close constantly. The 2 facing door mechanisms (seen) are largely cosmetic, BUT keep the space between them clear for emergency deploying the thick heavy pressure seal door in their middle - between them ).
Windows have to be immensely strong and well anchored ( ALL the water force pressing against their thin flat surface must be transmitted to the fabric of the building ).
Foundation areas with limited/restricted access can have lower-tech metal 'submarine' doors which are normally kept closed. Building Foundations are built with multiple watertight sections ( as they interface directly with the seabed ).
Transitways ( where they enter buildings ) have LARGE (simple) Emergency Section Seal doors, normally left open, but designed to be closed in moments ( can take longer to be reopened once the threat is eliminated ).
Integrated Transit areas ( like stations ) would be separate seal-partitioned volumes from the immediate building.
All Viaducts have regularly Spaced Emergency seal-partitioned Doors.
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Melted Basalt As A Construction Material :
Using Ryan's own high-power technology - and cheap (enough) power from Hephaestus, and the availability of this material -- the type of volcanic/seabed rock from Rapture's geographic region ( Tectonic plate spreading zone is generally Basaltic ).
This is basically what the Plasma Tunnel Boring/melting machines would create ( though the material is melted in place and becomes the water-proof lining and support for those tunnels - allowing them to be constructed cheaply ).
Such a material could also be used for prefab sections for new above-seabed buildings ( manufactured in well controlled interior conditions - facilities at the Port of Rapture ).  This material could be used for various shaped building struts/slabs and other components which otherwise would be made of Concrete.
A fiber like material can be made from melted Basalt which is used as insulation (like fiberglass).
Aggregate for high strength concrete can be better made with the qualities required - Concrete's compression strength largely comes from the strength/quality of the aggregate (filler rock) used, and how well the cement sets.
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Secret to Underwater Construction is doing AS LITTLE AS POSSIBLE UNDERWATER.
Mass manufacture on the mainland, then construction AT the Surface of the Ocean. The parts going together as simply as possible with minimal work required 'down below'. Once the building shell in built up, then more conventional construction methods can be used inside.
There is NO ENDLESS Money, Effort or Time to build this City.
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