Sci-fi author needs advice on building a multi-layered city

[Thomas Hewlett]

Many cities, such as Rome and Mexico City, have been built over older, preexisting cities. From what I've read however, this tends to happen in stages, especially after disasters like fires or earthquakes.

I'm wondering if it would be theoretically possible to build a completely new city over a still standing modern city and how high you could go. That is, if you somehow kept layering city atop city, what would happen? Would the bottom city layers sink lower and lower into the ground? Would they be crushed? Would the support structure for such a project even be possible?

I realize this is an absurd question, but if I can get even a *tiny* basis in reality for this idea, I could avoid the dreaded "handwaving over the details" so many sci-fi stories rely on.

Thank you in advance for any input!

 

[Rive]

Many cities, such as Rome and Mexico City, have been built over older, preexisting cities.

Sorry to correct you, but those cities were built over the rubble pile of the older, preexisting cities.
So if you want to that tiny basis of reality, then you can keep only the half-ruined tunnels, underground parkhouses and such: also, you should add a layer of rubble too.
You can have the catacombs in Paris and Rome as an example.

Ps.: also, since modern buildings needs extensive underground parts, the next layer would be the same too. That means that before anything new the old under the ground level would be thoroughly destroyed/filled.

 

[BWV]

https://en.m.wikipedia.org/wiki/Seattle_Underground

 

[Ophiolite]

Sorry to correct you, but those cities were built over the rubble pile of the older, preexisting cities.

There is the occassional partial exception: this interesting instance from another capital city.

@Thomas Hewlett If you envisage advanced materials then you can penetrate lower levels with 'pillars' going down to bedrock and supporting the upper city, including a combination roof/floor. This can contain all electrical and fluid conduits thereby overcoming one of Rive's objection.

As to the height the new city could rise, keep in mind you are writing SF and that has a range of "hardness".

 

[DaveC426913]

Think about why, when and how they're building over top of existing structures.

Are the existing structures occupied?

If the next layer is being built in a way that is meant to preserve the lower layer, that would be very different than if the constructors didn't care about preservation, safety or keeping the lower layer operational.

Do they just need the geography? Do they want to build the next layer on stilts or columns, so that the new city is not dependent on the stability of the old, except for the pillars.

 

[diogenesNY]

London has some surprisingly complete buried ruins, in layers that go down through medieval to Roman.

Again, these are ruins, not functional structures, but there are some amazingly complete structures that have been found. There is a lot on the subject out there... let your favorite search engine be your starting point.

diogenesNY

 

[Thomas Hewlett]

Sorry to correct you, but those cities were built over the rubble pile of the older, preexisting cities.
So if you want to that tiny basis of reality, then you can keep only the half-ruined tunnels, underground parkhouses and such: also, you should add a layer of rubble too.
You can have the catacombs in Paris and Rome as an example.

Ps.: also, since modern buildings needs extensive underground parts, the next layer would be the same too. That means that before anything new the old under the ground level would be thoroughly destroyed/filled.

Thank you for pointing that out!

 

[Thomas Hewlett]

There is the occassional partial exception: this interesting instance from another capital city.

@Thomas Hewlett If you envisage advanced materials then you can penetrate lower levels with 'pillars' going down to bedrock and supporting the upper city, including a combination roof/floor. This can contain all electrical and fluid conduits thereby overcoming one of Rive's objection.

As to the height the new city could rise, keep in mind you are writing SF and that has a range of "hardness".

Yes, I like the idea of pillars. Especially complex ones that maybe get buried/fused with the bedrock.

 

[Thomas Hewlett]

Think about why, when and how they're building over top of existing structures.

Are the existing structures occupied?

If the next layer is being built in a way that is meant to preserve the lower layer, that would be very different than if the constructors didn't care about preservation, safety or keeping the lower layer operational.

Do they just need the geography? Do they want to build the next layer on stilts or columns, so that the new city is not dependent on the stability of the old, except for the pillars.

As the story now stands, I need them to treat the old cities as support for the new layers. I think whether they're operational is of no consequence to them. Thank you for your reply.

 

[Thomas Hewlett]

London has some surprisingly complete buried ruins, in layers that go down through medieval to Roman.

Again, these are ruins, not functional structures, but there are some amazingly complete structures that have been found. There is a lot on the subject out there... let your favorite search engine be your starting point.

diogenesNY

Ha, yes. No shortage of reading/viewing material. I'm buried in it, no joke intended. But I always like hearing from the people who can actually build things in the real world before I start building them in my head. Thanks for your reply!

 

[snorkack]

When you build existing structures upwards, you have several options to bear the added weight:

1. The original structures were overengineered to begin with, and can take the extra weight
2. The engineers adding extra weight add extra columns/loadbearing walls through the lower layers to bear the extra weight
3. The lower rooms are filled in, and this is the extra support that prevents the bottom of the columns from collapsing sideways.

The engineers adding the extra weight will want to prevent collapse in any of the above ways, or combination of them. Of course, they might fail.

 

[stefan r]

You could build down from the dome. The columns in the building work under tension. they hold each floor up the same ways that a compression column wood.

You upper city can have a web of graphene tube bike and pedestrian movers. The lower city is under water because of rising seas. you could do quiet a bit of submarine developement.

 

[some bloke]

IIRC in the TV series Futurama, old-New-York is mostly intact, with New-New-York built over the top of it.

I think that yes, it is possible for this to happen in a sci-fi setting - practically it would not be likely to be done, but that's why we have fiction!

How are you envisioning this looking? are you anticipating ever-increasingly tall structures, linked by bridges, or an entire "ground" somehow laid over the top of the previous city?

If you're going dystopian, you could justify the reasoning that, with the invention of flying cars, new laws were put in place dictating that only so much space above a property belongs to the owner, and so some juggernaut company comes in and builds a platform over the top of the city, making the whole area above belong to them, without infringing on the "owned" airspace below them.