How: enormous cement towers with large bases, pyramid-like for their solidity. They would be several hundred meters high, built in many steps (even 1000m or more) and covered in greenery. There would have to be enough inhabitants (over 1 000 000) to able to live in complete self-sufficiency.
In the beginning materials will certainly need to be bought and traditional construction methods will have to be used. In time, Utopia should be able to build itself with robots.
The towers will be built with high resistance cement with synthetic fibers and cables. They will have ‘old-school’ structures: arches, domes, etc. (the support strength of the slabs acting in compression on the supporting structures); or modern structures such as ‘beehives’.
– sand and ballast will be gathered by dredger vessels
– cement will be made in floating factories
– plastic waste floating on the ocean surface will be gathered by sea-water filtration. It will be used to make fibers, cables, framework for the cement and will be used to heat rocks to make cement
– metals will be gathered from the seabed (nodules, sunken ship remains, underwater mines, etc.)
Why: these towers will be able to be closed. They will have to be resistant to big, predictable disaster and provide suitable living quarters for their inhabitants during and after the disasters.