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The end has come and gone, and the survivors of humanity have adopted a new religion - dismantling the sins of the past. Dismantling the cities, roads, towns and relics of the past by hand. The resources being returned to nature as best they can.

With no industrialised technology, how much work would it be to dismantle a city? Assume that the results are being taken away by canal barge for disposal.

Bonus points for suggesting things to do with a skyscraper of glass, or tonnes of tarmac that fit the atonement philosophy.

Assume knowledge of up to steam tech... But no desire to use it.

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    $\begingroup$ Depends very very very much on what exactly is meant by the phrase "by hand", and on the number of people doing it. You need to specify the size of the army doing the dismantling, and the list of equipment they have. (If "by hand" really means "with bare hands", then it cannot be done. Modern buildings are made of steel and concrete, and bare human hands cannot demolish them.) $\endgroup$ – AlexP Aug 15 '20 at 21:00
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    $\begingroup$ I suspect spending weeks on end laboriously sledgehammering the first set of concrete building footings back into sand and gravel would make many folks reconsider their zealotry. $\endgroup$ – user535733 Aug 15 '20 at 21:03
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    $\begingroup$ "Hand tools": do they have steel saws? Steel chisels? Steel drills? Are they allowed to use machinery, let's say, at the level of the late Antiquity? $\endgroup$ – AlexP Aug 15 '20 at 21:07
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    $\begingroup$ Possibly the real question is whether a small group of primitives (because they won't allow technology to support big groups) wielding sledgehammers will have any appreciable effect on the rate of destruction of a city compared to erosion and violent weather. I can easily imagine a situation where a group spends 100 years trying to demolish a skyscraper and finding that a major storm manages to bring the un-maintained building down overnight. $\endgroup$ – KerrAvon2055 Aug 16 '20 at 2:16
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    $\begingroup$ What do you mean by “dismantle”? If you want to leave no evidence that the city was ever there, then it’s basically impossible. Once a hole has been dug in the ground, you can’t fill it up in such a way as to make it undetectable to future archaeologists. $\endgroup$ – Mike Scott Aug 16 '20 at 16:40

15 Answers 15

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Background assumptions

To start, using cranes as a metric for roughly the level of sophistication that a steam-punk-era civilization can create, from what I've found, the disassembly process is around the same scale of magnitude as the assembly process. I think this is a good metric for the ratio of assembly to disassembly time on the level as a city as it has some complicated bits of machinery, but is largely just tons of material. Also, the disassembly process of a crane is non-destructive and made for it's pieces to be transported, as, intuitively, the parts of the crane are supposed to be used again elsewhere.

As such, perhaps the answer may just be roughly the sum of the time it takes to construct each building in the city (or at least on the same scale of magnitude).

Time for some estimates

Before I begin, clearly this is a very rough estimate with huge assumptions, but I reckon to be pretty close to an actual value within a scale of magnitude or two.

Mathematically, let's use $\frac{storey}{humans\times time}$ as our base unit of estimation. The goal is to estimate how many storeys ($S$) there are in each building in a city, then estimate how long it takes to build a storey in man-years $(Myr)$.

The Empire State Building was built in about $1.12$ years, took $3000$ workers, and is $103$ storeys tall. Thus, it was made in $\frac{103S}{3000M \times 1.12yr}\approx0.03\frac{S}{Myr}$. Let's take this value and run with it.

Let's assume the average residential home has $2S$, and the average 'skyscraper' has $50S$.

New York City has about $274$ skyscrapers and $761000$ residential homes. In total there are about $1000000$ buildings. Let's use that data to plot a normal distribution for the amount of homes with $n$ storeys. We find that there is a $0.0274\%$ chance of a given building having $50S$, and $76.1\%$ of a given building having $2S$.

With some extremely-back-of-the-enveleope calculations and fiddling with normal distribution, I conjecture the average building to have about $2.1S$

Final calculation:

The dismantling time for a city about the size of New York and a process similar to the precarity of crane-disassembly would be

$$1000000\times2.1S\times\frac{3000M\times 1.12yr}{103S}\approx70000000Myr$$

So it would take $1$ person about $70$ million years to dismantle a large city safely and package it all up for transportation,

or $70$ years for $1$ million people.


Edit: This doesn't account for parts of the cities that fall outside the scope of that which isn't a part of a "building". However, most of such infrastructure has much to do with attached buildings. Regardless, I imagine a "fully-disassembled" city would look precisely like spare bits of infrastructure that falls exactly into such building-less category.

Edit 2: This does assume that a storey for a residential home is as complicated as a storey in the Empire State Building. Taking vica-versa, having one story of a residential home being made by $20$ people in $0.5$ years yeilds $\approx 0.025\frac{S}{Myr}$, which is actually less fast than the metric for the Empire State Building. So luckily this hindsight doesn't nearly change my conjecture about the estimate being off by a scale of magnitude or two.


Sources

http://www.centralplainscranes.com/FAQ_Crane_Operations.html

https://www.britannica.com/technology/skyscraper

https://www.cnn.com/2013/07/11/us/empire-state-building-fast-facts/index.html

https://www.visualcapitalist.com/100-tallest-buildings-in-new-york-city/

https://urbanomnibus.net/2016/05/how-many-row-houses-are-there-in-new-york-city/

https://ny.curbed.com/2018/4/23/17271092/manhattan-buildings-data-visualization-taylor-baldwin

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    $\begingroup$ But a lot of that energy was delivered by electricity (through elevators and railcarts), and will have to be done by hand without the technology. That energy isn't accounted for in the calculation. $\endgroup$ – Erik Aug 17 '20 at 6:18
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    $\begingroup$ @Erik Good point! I mention at the top of my answer that I'm assuming a steam-punk-era civilization is capable of constructing that which is equivalent to a crane, which is albeit a little optimistic. Regardless, doing so inherently follows that they have the ability to move a large a mount of weight many storeys using machinery/mechanics. Though not industrially equivalent to an elevator or railcart, the energy required to have such ability is already taken into account by my initial assumptions. $\endgroup$ – Graviton Aug 17 '20 at 6:44
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    $\begingroup$ @Erik, so start with the bottom floor and by the time you're half done with that, a lot of the work for the upper floors will already heading for ground level. $\endgroup$ – Separatrix Aug 17 '20 at 10:06
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    $\begingroup$ I question the assumption that you can disassemble the Empire State building using only hand tools at the same rate it was erected using industrial technology - building materials like concrete and riveted steel are, by design, easy to put up and hard to take down. Even the logistics of just moving the debris will be a nightmare. $\endgroup$ – Nuclear Hoagie Aug 17 '20 at 14:51
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    $\begingroup$ Enter the mythical man-month. The reality is that some projects can only be done with a specific amount of people before it becomes detrimental to the project. There are also situations where added personnel will reduce the time taken by exponential amounts, rather than linear amounts. I've personally worked on projects that took 1 person 3-10 times as long as 2 people. en.wikipedia.org/wiki/The_Mythical_Man-Month $\endgroup$ – computercarguy Aug 18 '20 at 22:08
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Reinforced concrete!

Let's just consider one aspect of what they have to remove - reinforced concrete.

Recently the block I live in was fitted with a sprinkler system. This involved cutting 2" holes in concrete walls of 6" thickness or more.

The first contractor gave up. Their drill bits were making no impression. The second contractor charged much more and had to use diamond drill bits with huge and powerful electric drills. These had to be bolted to the wall because they were too heavy to lift and a human couldn't exert enough pressure. Even so they had to replace the drill bits regularly and each hole took a lo-o-ong time..

If concrete is properly made it is immensely strong and hard. Also it does not have fracture lines like most rocks so it won't split with hammer and chisel.

To demolish properly-made reinforced-concrete by hand with only picks and shovels is impossible. You won't even chip it.

The only thing to do would be wait for thousands of years for the weather to wear it away.

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    $\begingroup$ Drilling is a pain. But breaking is a bit easier as you dont have to be so precise with your effort. My local hardware store has a DIY video showing how to destroy a reinforced concrete slab. Now they're using a Jack hammer and an angle grinder, but that is a substitute for a big guy with a hammer and chisel and another big guy with a decent saw. ( youtu.be/UYhDqtRlP90 ). $\endgroup$ – Ash Aug 16 '20 at 16:32
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    $\begingroup$ Good point. This reminds me of the Dora I U-Boat bunker in Trondheim, Norway. Built during WW II by the Nazis as a protected harbour for 55 of their submarines, it was too massive to demolish by explosives after the war and thus remains standing to this day. $\endgroup$ – Xi'an Aug 16 '20 at 17:56
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    $\begingroup$ @user2555705 The OP specifies "no industrialised technology". Can you imagine what it would take to cut through concrete and steel using a handsaw? It would be blunt within seconds. $\endgroup$ – chasly - supports Monica Aug 16 '20 at 21:05
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    $\begingroup$ I'll add to this. I once had to drill a 1/2" hole in my sister's basement wall. The concrete was over a century old (pre-portland cement, just OLD). I made the hole, but I had to sit with my back against furniture, which were themselves against the far wall, so I could push with my feet. I also know of a concrete structure near my hometown that's a spray paint mecca. Government tried to bring it down with explosives and failed. Reinforced concrete is a pretty good frame challenge. A big city with sledge hammers, shovels, and wheelbarrows. Yup. $\endgroup$ – JBH Aug 16 '20 at 21:59
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    $\begingroup$ @Solomon Slow - Yes, strictly speaking you are correct. In theory Mount Everest could be levelled with teaspoons, given long enough. If this demolition is used as a devotion with no thought for completion then the task can be commenced. However, after the first thousand or so years, the political landscape will be completely different. Technology will have re-emerged and the reason for demolition lost in the mists of time. Tourists will come to see the ancient cities as we do Stonehenge or the Pyramids. They will probably be returned to their former glory or at least preserved for posterity. $\endgroup$ – chasly - supports Monica Aug 16 '20 at 23:03
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Longer than it would take them to go extinct due to the ice age cycle... if nature didn't help.
Since nature will help, their combined efforts can cut the destruction time by 20-50%, compared to the buildings' natural lifespan.

Humans have already attempted to destroy the Colosseum during the Middle Ages. The wall plating was stripped, the clamps holding the stones together pulled out, everything wooden removed. In the end, it took multiple earthquakes to do meaningful damage to the stone and concrete structure, not humans.

Modern steel or reinforced concrete framed buildings are comparable in durability. Unlike wood and stone, their frames are continuous and can't be disassembled back. The steel inside the reinforced concrete is prone to rusting, but it's sealed in a way that prevents the ingress of water. Either the concrete or the steel is very difficult to damage without power tools.

The number of people you can employ will be limited by their primitive technology. Cities like Rome and Alexandria required technological civilizations to support their million-scale populations. Without technology, logistics and epidemics will limit your population to low tens of thousands, the traditional city size limit through the Middle Ages.

They can still do some damage - mostly by removing the windows, much sooner than they would break naturally, which will allow soil and water inside the buildings.

In a couple years, your population would be able to go through the NYC breaking all the windows, including the high-rises, the windows in which can otherwise last for many decades. The next step would be making holes in the roofs to help water enter the interiors.

In any case, their main tool has to be helping nature do its work. They'll get the most out of their effort by helping plant and animal life take over the buildings, which is essential to destroying them.

Smaller wooden and some brick buildings can, of course, be dismantled directly. If more destructive ways are appropriate, some larger buildings can lose their integrity from fires, although it may require accelerants. In most cases, interior fires will not destroy the stonework, but leave partially burned-out concrete structures. Just hammering on the concrete will only have very local effect.

90% of the work will still have to be done by natural processes. Rust will crack the concrete. Winds will carry the soil in. Plants can expand the cracks as they grow inside them. In colder climates, freeze-thaw cycles do the same. Eventually the buildings will be thoroughly filled with cracks and become vulnerable to earthquakes or in some cases to human effort.

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    $\begingroup$ Increase all this by at least an order of magnitude due to the part "With no industrialized technology" in the question. With no industrial technology, over 90% of their time will be spend just by growing enough food to not starve to death. This means they won't have all that much spare time to do the ritualistic deconstruction. Or if full-time, then it will be done by a small part of population who does nothing else, and their day-to-day needs have to be supported by the peasant class. $\endgroup$ – vsz Aug 17 '20 at 9:02
  • $\begingroup$ Toxic remnants won't prevent plants for long. (That's why railroad tracks are re-poisoned regularly, to prevent them from being overgrown with toxin-resistant plants.) Given that houses don't usually have large amounts of herbicides in them, and fires tend to produce other toxins, I don't think that fires will prevent plant life in any significant way. $\endgroup$ – toolforger Aug 17 '20 at 10:09
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    $\begingroup$ They might be able to use nature as a tool of their destruction. If they purposefully went through and cultivated plants with deep and winding root systems in the structures by bringing in soil and water they could help with the infiltration of cracks by roots and moisture to speed up the process. It'd still take a very long time but it would probably be faster than waiting for nature to work on it's own. $\endgroup$ – Neberu Aug 18 '20 at 22:11
  • $\begingroup$ @Neberu True, it would help. It would be a tiny fraction of what wind and rain will bring in (that's why breaking the windows is a priority), but they could focus on iconic buildings. And it would definitely give them something to do that's more effective than just beating on the concrete. $\endgroup$ – ZOMVID-20 Aug 19 '20 at 4:56
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Don't break down and return the city to nature - return nature to the city!

As others have detailed, dismantling the modern cities would be so great a project that a primitive tribe would never succeed - they would die out long before completion either quickly by focusing all their resources on this instead of survival or more drawn out by the simple passage of time. They might realize this fact fairly soon.

So to remove the scars of the sins of the past, use nature as your guide, engineer and god.

Do not break and dig up the tarmac and asphalt - cover it with soil (atonement work) and/or plant on it or around it with plants with strong roots that reproduce by suckers. Poplar suckers will make quick work on a road if planted in a strategic pattern. If the rest is covered with soil and seeded with dandelions, elytrigia repens, horse tail and others with strong roots, it will soon be indistinguishable from a field or forest. Throw in some fast growing species that cover a great area quickly: periwinkle, phlox etc.

Do not waste time by completely tearing down urban housing. Manually tear down the roofs to expose the walls and let water inside and the structure will quickly come into disrepair. Plant strategic around it with ivy, japanese knotweed, bamboo and other fast growing, destructive and climbing species (sacred plants that cleanse the unholy ruins of the past).

The high rise is of course the big problem. Manually tear out the windows and interior. Carry enough soil up on each floor to plant trees at every window opening. This will speed up the natural degradation of the building, but maybe more importantly the columns will be green and wildlife will return.

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Symbolic destruction

In the same way humans being did during different revolts, the people didn't look for a way to destroy/dismantle all the structures. Most of the times, revolutionary groups focused on lay down those elements who played a symbol of the power/hegemony of the previous ruler (person or entity, whatever).

enter image description here

If the aim of this religious group is to give back to Earth everything we took for our capitalist/homocentrist system, maybe they could decide only to get down some structures: big monuments and/or the highest/recognisable building of a city.

In order to destroy them, they can:

  • localise and sabotage the weak points of them,
  • dig around the basament of them (mining rock should be easier for hand work than breaking a reinforced concrete block),
  • set fire to the objective building before intend additional dismantling work (supposing the furnituring inside is intact),
  • colapse big buildings from inside in medium-high floors,
  • flood underground parkings or subway tunnels after putting away part of the aislant covers to lead the water to impregnate the ground around, getting this way the worst damage you can do to the complex.

In the worst scenario, the sacrifice of one or two chosen during these missions (even using explosives, fuel for the fire or thermite) could serve as a kind of tribute from the "sinner sons of the perverse society" that ruined the world.

enter image description here

As we can see nowadays, a sinked boat or an abandoned building/town is quickly recovered by the nature, with the vegetal cover spreading over the walls and finding ways for new trees to grow up from under the concrete. Even before that, the most curious animals will find in this structure a place to hide themselves or to storage their food, for example.

enter image description here

After several years or decades, weather, sysmical activity and lack of maintenance will lead to the fall of these giants that humans were not able to tear down.

Maybe then these people would see their objective accomplished to redempt for the past aspirations of their predecessors.

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In Star Trek II: The Wrath of Khan Spock says:

SPOCK: I was not attempting to evaluate its moral implications, Doctor. As a matter of cosmic history, it has always been easier to destroy than to create.

http://www.chakoteya.net/movies/movie2.html[1]

Therefore, if Person or Group A manages to destroy what person or Group B has created, that doesn't prove that Person or Group A are as powerful as Person or Group B. It is easier to destroy something than to create it, so those who destroy something may be far weaker than those who created it.

There is a story that one of the Abbasid Caliphs wanted to demolish the ruins of a Sassanid palace at Ctesiphon to get materials for a building project at Baghdad or Samarra.

And his advisor said he shouldn't do it unless he could used the materials to build something that was just as splendid as the Sassanid palace, or else people would say that the Caliph had the power to destroy but not to build anything as great as the Sassanids had built.

The Caliph proceeded with the demolition for a while, and then said that the palace had been built so well that it was too much trouble and expense to keep on demolishing it.

And the advisor said the Caliph had to keep on demolishing the palace or else people would say that he couldn't even demolish what the Sassanids could build.

I don't know if the Caliph finished demolishing the Sassanid palace at Ctesiphon. There were at least two great Sassanid palaces at Ctesiphon. One was the "White Palace", known only from literary descriptions, in Ctesiphon proper, and another was the one that modern people call the Taq Kasra or Iwan of Chosroes or Arch of Ctesiphon, in one of the other cities of the Ctesiphon metropolitan area. And many people often confuse the two palaces.

Also see the similar story mentioned by Alex P in his comment.

The point of the story is that after the fall of civilization, primitive people who try to dismantle a large modern city are likely to end up with egg on their faces because they were unable to demolish what the ancients could construct.

In Victorian era Britain countless thousands of cheap and shoddy buildings were built, but important buildings were built very solid an durable. Mark Girouard, in The Victorian Country House 1979, Introduction, Section 10 "Materials Old and New" says: "The solidity of Victorian houses has always been remarked on, especially by those who have had to demolish them. It was a by-product of Victorian seriousness; everything had to be what it seemed to be, and made to last."

Demolishing a vast modern city full of many thousands of buildings, some extremely large, and many built very durably, would be an immense project for a primitive society without modern technology. It would be quite possible for them to devote so much time and energy to such a vast project that they wouldn't devote enough time and energy to surviving, and they would die off while a large part of the city still remained.

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    $\begingroup$ In the late 1100s, Malik al-Aziz, the Ayyubid Sultan of Egypt, put his mind to demolish the pyramids of Giza, and started with the smallest, the pyramid of Mykerinos. After eight months of toil and sweat the effort was abandoned, when the Sultan realized that he won't live to see it completed. The pitiful result of the demolition attempt remains as a visible reminder of the folly of men. $\endgroup$ – AlexP Aug 15 '20 at 22:45
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Millennia.

It's not just that there's a vast amount of material and every bit of it has to be freed, it's that it's impractical to break down things by hand. Concrete, in particular, and other hard materials. Witness that one of the Pyramids has a gouge in it where there was a twelfth century attempt to take down the Great Pyramids. Even that little took eight months.

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Frame challenge

Assume that the results are being taken away by canal barge for disposal.

What makes the disposal site a better place to store all the rubble? It seems as though they are returning one place to nature only to contaminate an even bigger area. Why bigger? Because they can't build so high. Therefore there will be a huge area of wasteland even bigger than the original city. They have simply moved the problem.

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  • $\begingroup$ They need to fill in the Grand Canyon for some reason? $\endgroup$ – NomadMaker Aug 16 '20 at 9:57
  • $\begingroup$ Concrete and brick being dumped into the ocean, allowing it to return to it's true spiritual function - being bedrock. $\endgroup$ – user2702772 Aug 16 '20 at 14:28
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    $\begingroup$ @user2702772 That is extremely destructive to the aquatic environment - so these people only care about visible environmental harm on land, with littoral areas turned into rubble-strewn poisoned dead zones an acceptable cost for clearing land? $\endgroup$ – pluckedkiwi Aug 17 '20 at 15:05
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    $\begingroup$ @pluckedkiwi If these people had a reasoned, rational view they would be horrified by the environmental damage that they were causing. But if they had that, they'd either not be doing this... Or they would be protagonists.... $\endgroup$ – user2702772 Aug 17 '20 at 17:17
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ignore my mistake. This underestimates by a factor of about 12. Misread the source

I dismantled an old rusty 10m x 4m metal shed with only hand tools (A crowbar, socket set, ladder, ropes, hammer, chisel, and hacksaw) during the covid19 lockdown. It took 1 man about 4 days to do 40 square meters. Walls, roof, and cieling.

It was my first time doing something like this, and I was taking breaks and relaxing throughout the project.

According to https://www1.nyc.gov/assets/sirr/downloads/pdf/Ch4_Buildings_FINAL_singles.pdf , New York has 375 million square feet of floor space. In usable units that's 34838640 square metres.

Extrapolating my experience to all of New York city, thatll take 3483864 man days to dismantle all of New York city's buildings.

1000 people working in parallel could do it in 3483 days. Or a little under 10 years.

Now, there are lots of quibbles with this calculation that say it should be larger, steel frames are easier to dismantle than bricks, tall skyscrapers are harder to dismantle than single story sheds. But there are also quibbles that say it should be smaller, teamwork should speed things up, and they'd learn tricks to speed things up on the job. A religious cult also wouldnt stop for TV breaks. I'm handwaving here and saying they cancel each other out.

Now this will leave street, foundations, subway lines, signage, highways, water and power infrastructure, etc. This is very hard to estimate, I dont see cities spend more on common infrastructure than residents spend on buildings (based on relationship between tax and property cost), so I can guess an upper bound; I'd expect this would be no more than another 10 years.

Some earthworks are below sea level. Eg undersea tunnels, or foundations below sea level. The empire state building for example has 17m of foundations, 9m below sea level. This would need to be done by dismantling from the basement down, chiselling and hand-drilling away until they reach bedrock, then removing the side walls with the water proofing membrane. Hand pumps / windmills / steam powered pumps will have to be used to pump water out as they go.

Returning the materials to earth will be tricky without the city, you need power to melt steel down, etc. You may need to add a 5 years of breaking small concrete blocks down into gravel and land filling.

Add 5 years for unexpected difficulties that come up as a buffer, and add a support network of 500 to feed, shelter, bandage any injuries, transport materials, and create and maintain the hand tools.

That adds up to 30 years. Maybe leaving a bit of time left in their lives to dismantle the asphalt highways leading to a neighbouring city, or scatter plants or topsoil to start regrowing the landscape.

A religious sect with 1500 very motivated people could plausibly dismantle New York city in their lifetime.

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    $\begingroup$ I think you're underestimating how much effort it would take to "undo" major earthworks like subways or tunnels. Many cities have rivers or bodies of water near them. Damming or diverting the rivers to dig out the tunnels is not feasible without power tools. $\endgroup$ – Dragongeek Aug 16 '20 at 8:34
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    $\begingroup$ Most definitely not. Even 1500 very motivated people will need a lot of explosives - probably more than 1500 people can produce in their lifetime, working at a modern explosives factory - to dismantle a significant part of the NYC. Nuclear devices excluded, but these also take a lot more than 1500 people to produce. $\endgroup$ – ZOMVID-20 Aug 16 '20 at 12:41
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    $\begingroup$ I don't think you can extrapolate from an old rusty tin shed to a steel and glass skyscraper by comparing their floor area. $\endgroup$ – Michael Kay Aug 16 '20 at 18:12
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    $\begingroup$ "1000 people working in parallel could do it in 3483 days." - NYC has over 1 million buildings. You're assuming a single person can dismantle a single building every 3.5 days, like you did your shed? Even assuming every building is just a normal home (so, discounting skyscrapers and shops), I suspect you're off by two orders of magnitude. $\endgroup$ – marcelm Aug 16 '20 at 19:05
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    $\begingroup$ @user2555705 Re-read your source; "it defined the 100-year floodplain—the area that has a 1 percent or greater chance of flooding in any given year—as an expanse that today includes approximately 35,500 buildings with more than 376 million square feet of space" - Your 375mln sqft is only small part of NYC. $\endgroup$ – marcelm Aug 16 '20 at 20:19
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Fred Dibnah style.

Fred was a British TV personality and steeplejack. He is famous for demolishing 100 metre high red-brick chimneys by supporting the base with wooden props. He then removed the bricks below the base, and set a fire in the base, burning away the props. The chimney would then collapse, hopefully in the direction he wanted.

For reinforced concrete buildings, this would be slightly harder, as it's a lot more difficult to remove the lower metre or or so of the building. Fred also relied on a draught through the chimney to get the fire going - for a skyscrapper you'd have to open all the stairwell doors (but not the ones to the floors), and either the roof door, or the one topfloor door, and knock out the windows.

You'd then have to clear the twisted remains of the building, which will probably be hard to do manually, as the remaining chunks could well be over what humans can lift.

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How long depends on the technology used, how many people are working, and how big the city is. I think this is an overly broad question.

I think a better question would be, what sort of technology could be used to demolish most of the structures in a city given a steam engine level of knowledge?

Once you've defined the tech, then you can define a typical city size and a typical work force.

Two types of technology available I haven't seen in any of the other answers are, obviously, steam, and, if the city is located in a "4 season" location like New York or London or any other city that gets cold enough to freeze every year, ice. You said they have no desire to use the steam tech for living purposes, but I'll assume they are willing to use it to achieve their ends.

Steam explosions can be quite powerful and could be used to demolish a building's support structures.

Freezing water can induce significant stress on a structure to cause it to collapse.

As others have noted, chemical explosives, solvents, and nature itself can be used to weaken structures. Re-introducing nature into a city is an excellent way for the workers to do something to aid in its return to a natural state.

Here's one last tool for demolition if you wanted to write it into your story. Having a large dam upstream from your city could kill two birds with one stone - return the river to its natural state and wipe out the city downstream. Releasing the water could be the main goal for your characters to start with. Bursting a dam is no small feat. But again, explosives, ice, steam, etc. can all be used to help break through that tremendous amount of concrete and release a stupendous amount of potential energy downstream. It would help to close all the normal water release mechanisms and fill up the reservoir so that the dam was as close to its stress capacity as possible.

I think a motivated group of 1000 or so could breach a dam in a couple of years or less - depending on how they did it. And the city could be nearly gone in an hour or two depending on how big the lake or reservoir was.

If there wasn't already a dam, maybe your people built one expressly for the purpose of destroying the city.

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For this question, I will use New York as the example of a major city. By finding the average living space per person to be about 300sqft with 37.5% of the city zoned as residential and the total population to be about 8.3 million we can guess that this means you have a total of about 6.64 billion square feet of real estate that need to be reclaimed. Dismantling the whole city is beyond stupidly hard, but you may not need to.

What it takes to help nature reclaim the city

If the goal is the reclamation of Nature, leveling the cities is a very wasteful way to go. As chasly points out in his answer, you are doing more harm to nature by tearing the buildings down to nothing, than you are by leaving them standing; so, instead of leveling the cities; you only need to help nature reclaim what is there. Many kinds of grass and small trees can grow well in thin soil with little light and water; you just need to knock out a few windows, carry in a some soil containing the seeds of various weeds, and let nature do the rest. Rain and light will come in through the windows to water the plants. The plants will begin to slowly break apart the concrete with the roots and in a few years the entire facades of these buildings will be completely overgrown with life. Much like the struts of Oil Rigs, they will make great shelters for all sorts of wild life too. Birds can roost inside and out, reptiles and rodents can nest wherever they wish. In the end, New York could host more wildlife in its mostly preserved ruins than it could by ripping the whole thing up.

So, instead of asking what leveling the city will take, I will first answer what helping nature reclaim the city might take. The first and most important step is going to be smashing out windows and propping open all the doors. Smashing all those windows and propping open doors would be relatively easy; so, the bulk of the work will be the time it take you to haul in all that soil.

New York is a very easy city to reclaim since the average distance from any point to the nearest waterway is only about 1 mile meaning you can bring your barges up and just wheelbarrow the soil to where you are going. Since I will assume your worker can only maintain a slow walk while pushing that weight, that there is a return walk, that the wheel barrow has to be both filled and unloaded, the window has to be smashed, doors propped open, and that you often have to go up many flights of stairs, that your average time to transport soil will be about 2hrs and that each load can transport enough soil for about 1 window garden.

One source suggests that New York City has a total of about 232 million windows. This means it would take ~464 million man hours to do a reclamation project even without tearing the whole city down... oh and don't forget harvesting all that soil and operating those barges is not free either. Harvesting and shipping soil would probably double the scale of your operation here too close to 1 billion man hours. This is about the same amount of labor as it would take to build the great pyramid of giza twice over making it a huge undertaking, but theoretically possible if you could afford to commit ~20,000 people to the project for a few decades.

What it takes to level the city

If you wanted to break down entire buildings, you could assume you'd need to make 1 trip per 100lbs of debris you need to haul away. This is the equivalent of 2 sqft of an average building's floor plan, but you'll probably be closer to 1sqft per load when you consider walls and foundation, and other brikerbrack, plus added demolition time would probably equal closer 3hrs per trip. That is about 20 billion man hours to disassemble the city, plus however many billions of additional man hours it takes to haul it all back off the barges... so I would not suggest that unless you have a population the size of New York to commit to it...

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to dismantle a cargo vessel of 40,000 tons it takes about three months and are used electric tool. To dismantle an entire city i think it takes many years, it depends on the number of people doing it.

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  • $\begingroup$ Not an answer to the question as written. The question restricts the scenario to pre-steam technology, and electricity is definitely post-steam. $\endgroup$ – toolforger Aug 15 '20 at 21:36
  • $\begingroup$ yes, the question was in a pre-industrial and post-apocalypse world, without electricity but they can use steam, my answer was to understand with our tech what time are required to dismantle a big stuff like a ship $\endgroup$ – DG79 Aug 15 '20 at 21:44
  • $\begingroup$ @toolforger -- OP says they're "free to use found tools", which, I'd argue means they can use rock crunchers and acetylene torches and various petrol and electricly powered thingies. $\endgroup$ – elemtilas Aug 16 '20 at 3:53
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    $\begingroup$ I know of no way to reasonably transfer experience with ship demolishing using electric tools to building demolishing with manual tools (the question posits that steam is known but not used). Electric tools won't work because the power plants don't operate. Acetylene torches and such would help, but they would run out of gas very quickly and not make a serious difference. $\endgroup$ – toolforger Aug 16 '20 at 8:22
  • $\begingroup$ @elemtilas neither blowtorches nor diesel generators will last long enough to make a noticeable dent into the overall task. $\endgroup$ – toolforger Aug 17 '20 at 9:59
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The advantage that future primitives have over our primitives is that they would be starting with the technology existing that will be destroyed. Our previous had to create from relatively nothing to slowly get us to where we are now. The future primitives would be able to use what was leveled to assist with leveling more.

If we have understanding of steam technology then we know how to make heat.

If we can focus that heat then we have a better chance of bringing down buildings than with mechanicals.

Bringing down a building this way would also allow for less of the workers to be involved for risk of injury. Tumble them into each other for maximum breakup potential, let gravity do the bulk of work.

Once structural members are exposed, use pieces dislodged as levers to split the larger complete sections apart. Use the bulk of your workers to hall away pieces. Leveled materials can be used as fuel for the heat source. This problem is paradoxical in that the very process of dismantling the buildings would increase the primitives knowledge therefore their capabilities for destruction will increase. As the ability to produce and focus heat would improve, so would the ability to break down components. There would be no reason to haul away materials, these would be used to house the generations of workers encamped outside the city, and to fill in the holes in the earth previously mentioned.

Lastly, I would suggest that steam power understanding might coincide or be predicated by an understanding of how to make explosives. These would obviously allow for the most rapid reduction to dust. Another issue is with materials designed by current moderns that aren’t nor will never be biodegradable. Let’s hope at least our future prime have a thorough understanding of chemical processes to help break things down into easier to handle constituent forms.

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  • $\begingroup$ note if ship dismantling in India is any indication a lot of these people will die as things unexpectedly collapse. $\endgroup$ – John Aug 17 '20 at 14:58
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If the 'End' of society as we know it does involve a huge use of weapons that do destroy the structure of concrete and steel buildings, your new 'primitives' may stand a chance.
If not, current building methods are too well build to take down by hand (see most other answers, they have the details.)

So you may want to start your story by a recounting of the horrors of the 'End', the weapons used. And in that way explain how your people are able to take down the concrete and steel buildings.

How long it then still takes depends on the amount of damage those weapons have already done.

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