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I am building a world where the new world is built on top of the old one, and the old world is kept under a layer of "immigrated" dirt from other worlds via portals.

If there was a portal (or multiple) that was spilling dirt onto our planet, how much would be needed to cover most buildings and vehicles? Skyscrapers and major buildings would be uncovered, and that would be fine. I don't need exact measurements, but rough estimates like "The equivalent of the Moon" would be precise enough.

In addition, what real locations in our world would likely not be impacted as much as others? (Civilizations high up, or surrounded by mountains preventing them from being covered)

Thanks in advance!

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  • $\begingroup$ Perhaps you can describe the dumping more. Are you filling based on sea level (or closely related but slightly different distance to center of gravity) or ground level? $\endgroup$
    – user25818
    Commented Jul 9, 2018 at 22:50
  • $\begingroup$ Loving the relevant username. Are you asking me where the dirt is filling FROM? Because I imagined these portals as being above the planet itself, in the sky. But please let me know if that wasn't an answer to your question. $\endgroup$
    – Man_Over_Game
    Commented Jul 9, 2018 at 22:53
  • $\begingroup$ The "from" on Earth. If you drop dirt from a height mountains and elevation don't seem to mater, but you imply they might, so I was a little uncertain. Would say Denver and Miami receive similar orders of magnitude of dirt? $\endgroup$
    – user25818
    Commented Jul 9, 2018 at 23:01
  • $\begingroup$ Hello, Daniel Zastoupil, and welcome to Worldbuilding! Please take our tour and visit the help center to learn more about the way the site works. Have a nice day! $\endgroup$
    – Gryphon
    Commented Jul 9, 2018 at 23:04
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    $\begingroup$ Hydrology is a problem: Coastal cities are usually built up to the water's edge - to prevent the sea from uncovering the waterfront districts, you must also fill in bays and harbors, and perhaps build barrier islands. Also, blocking every river that flows through every city will create many thousands of unintended, un-engineered, and unstable dams. Each of those dams will eventually collapse, often quite catastrophically to the worlds both above and below, and often devastating many miles downstream. $\endgroup$
    – user535733
    Commented Jul 10, 2018 at 14:34

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Assuming by "major buildings" you mean anything over three stories then you need 14.5m of dirt to cover everything three stories or smaller in a reasonable soil cover such that casual digging doesn't uncover a chunk of roofing. This leaves the "skyscrapers and major buildings" at least partly visible above the new soil layer. In order to be sure of getting everything you'd need to coat the continents and the continental shelves to some distance offshore to prevent erosion stripping things back too quickly. So you need to cover about 155 million square kilometres 14 and a half metres deep or possibly a bit more on the coast. That's 2.25 million cubic kilometres of material, or just a little smaller (18% less volume) than Neptune's moon Galatea.

Sorry I had the wrong surface area number in originally, I have corrected it.

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  • $\begingroup$ Here in Berlin the average building looks to be maybe 20 metres tall (5-6 stories). You would have most of the buildings still exposed. $\endgroup$
    – Criticizing Israel not allowed
    Commented Mar 13, 2020 at 10:56
  • $\begingroup$ @user253751 So maybe go deeper over some cities and shallower across area like the Great Plains and Eurasian Steppe, although I would except that the average across all of Berlin including suburban sprawl might be lower than you think. $\endgroup$
    – Ash
    Commented Jun 25, 2020 at 14:59
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Click here for an amazing site full of tons of data and I'm not even scratching the surface, but from that site:

  • As of 2015, there were 1,041 buildings 200 meters or taller.
  • 13 of which were 300 meters or taller.

So the real question is, how much civilization do you want to cover? 200 meters of dirt covers all but 13 buildings. 100 meters of dirt covers all but a couple of thousand buildings.

Regrettably, that site is dedicated to 200+ meter buildings and not all buildings.

So, at a guess, 15-20 meters of dirt would get rid of about 50-70% of all signs of humanity. 100 meters would get rid of 90-99% and 200 meters would make finding evidence kinda fun.

Your bonus question doesn't make sense. Low population areas need little or no dirt. High population areas need lots of dirt. Geography should only be an issue if your portals have trouble opening at high altitudes.

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About 3% of the planet's land is urban, the vast majority of that being within 100miles of the coast. Though about 45% bears human footprint.

If you're trying to bury everything at once with an even layer except the skyscrapers, then you're looking at maybe 100m of soil.

If you're willing to do it in stages, then around 5m (just over a London bus), followed by another 5m once the bulk of buildings have collapsed under the first round. The vast majority of the world's buildings are either low rise or low quality.

Given that roof collapse is around 150$kg/m^2$ and you've just given it around 5 times that, it could well take out a lot of the skyscrapers as well. The second batch is really just to level things out and make it look pretty for your new residents.

You're still a few orders of magnitude short of the volume of the moon at only 1489390631330000$m^3$ or so.

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Mostly sounds like a physics question. Area of land: 148,326,000 km2 (57,268,900 square miles) multiplied by how high you wish to go. The volume of the Moon is about 21.9 billion cubic kilometers. So the total volume of the Moon ground up and spread over just the landmasses of earth would cover it 147.65 kilometers deep.

If you're just dumping material over continents the distribution general follows a bell curve with the majority directly under the source of the dirt. So near the edges, near the oceans would be the place to be. In many areas this massive dumping of dirt would expand coastlines and survivors could just follow it out away from source of incoming dirt.

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