Towers in the ocean; How deep can they be built?

Question

Many questions on this site mention how difficult it is for merfolk to develop technologically, as there's almost nothing but rocks and kelp to work with. My solution to this is that they use these rocks to build sea-spires that put them right in the center of maritime trade. These spires would essentially act as a port between continents for kingdoms lacking the economic might, navigational skills or shipbuilding know-how to sail all the way across the ocean. The towers would provide landmarks, trade, shore-leave, ship repair and other services on the jetties ringing the lighthouse like tower.

For what the tower is built of, materials acquired from the ocean or close to the shore are preferable. Merfolk can breath & walk, but to them it's like crawling is to us. This means they'll heavily prefer to trade for materials from further inland, making them more expensive and harder to acquire.

The deeper and wider the ocean, the more valuable these towers become but the harder they are to build. Can these towers get up and out to a useful height and depth or would the currents and weight topple them before they get out of site of shore?

Answer 1

Currently, static oil rigs built on reinforced concrete towers are built in sea depths of up to 350 metres. This isn't the limit of the structural mechanics, but more of an economic limit.

The deeper concrete structures are built using concave towers (thicker at the bottom and the top). The thicker top actually provides some buoyancy reducing the force on the structure vertically and the slender middle reduces the horizontal force on the structure created by the ebb and flow of the oceans.

I very much doubt your Merfolk have the maths, materials and merman-power to create a concrete structure such as this.

But, the interesting thing about looking at oil rigs in regards to this question, is that the deeper the water, the simpler the design.

After concrete structures, you get into steel frame structures, these go up to 1 km in depth. Still not a material readily available to Merfolk and I doubt the could do the underwater welding required.

But next, comes the simplest design. Moored floating platforms.

So what is required for one of these?

• A floating structure - and us humans have a habit of leaving lots of wooden boats at the bottom of the sea.
• Heavy weights to sit on the bottom of the ocean - plenty of big stones in the sea.
• Long enough ropes - this is probably the trickiest thing for the Merfolk to get hold of. But those shipwrecks would all have had anchors right? And those anchor ropes could be spliced together. You'd just need enough of them to keep your floating platform in roughly the same position

TL;DR - Don't build giant underwater towers, build a floating platform and anchor it in place.

Answer 2

Use Coral for the Spires

You could have a giant or fast growing coral that you either use as building material (coral can be as hard as concrete) or use it as the building itself, that could look cool as well.

A joint Building Project

The mer could also get help from the humans to build the top of the sea-spires with the mer building the bottom. They could also send building materials that the mer could not get themselves. You could have symbiotic relationship with the humans, they get a trade partner and a safe haven at sea, the mer get cool buildings that will bring in much needed industry and trade (like smiths and forges) the trade will get the mer items that the mer could never make in the sea

i would like to point out the mer have to be careful where they build theses sea-spires as the saying goes location location location. don't build it where the sea is highly active like storms, king waves, and earthquakes. be smart where you build them.

Answer 3

Ancient Rome's Underwater Concrete

More than 2000 years ago, the Roman Empire invented a unique marine concrete that allowed for the construction of enormous, durable structures – even underwater. Incredibly, the exact chemical properties of this concrete mixture have eluded scientists to this day – but now, researchers from the University of Utah believe they may have finally cracked the code.

https://www.archdaily.com/875212/scientists-uncover-the-chemical-secret-behind-roman-self-healing-underwater-concrete/

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The chemical secrets of a concrete Roman breakwater that has spent the last 2,000 years submerged in the Mediterranean Sea have been uncovered by an international team of researchers

https://newscenter.lbl.gov/2013/06/04/roman-concrete/

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Here you see the Merfolk trading with humans from the top of their tower

Answer 4

I want to frame-challenge the notion that merfolk could not manufacture materials that require being on dry land to make.

• humans run refineries that are hundreds of feet tall, yet we do not have wings
• humans run fish hatcheries, yet we do not have gills
• humans run steel mills, yet we are not fire elementals

Just like all those things have catwalks or safety areas for the humans... a dry-land factory would have water-channels for factory workers.

So yes, they would simply dig up lime and set up a landside factory to kiln their own portland cement.

Of course if they trade with humans, all bets are off.

Another interesting wrinkle is nuclear power. Merpeople would struggle to build some of the machinery, but underwater is literally the perfect place for a nuclear reactor. It takes 99% of the "cooling problems that lead to meltdown" right off the table. You would have to labor to keep the reactor core, steam lines and turbine insulated (e.g. By keeping them in air spaces). At first sign of trouble, simply wet the insulation or flood the air spaces, rather easy if you are already underwater. Also, mining thorium/uranium is a fundamentally different task for seapeople; you don't search the world for veins, you simply extract it from seawater, and when you deplete your local sea, currents bring you more.