I'm currently in the world-building phase of my book (hence why I'm here).

To add some perspective before I ask my question. The story is based in Britain, during the year 2032.

In my story, there is a large wall around the city of Milton Keynes (89 km²). The wall is around 300 feet tall and made of concrete. It's thick enough to keep whatever is inside, in.

Atop the walls are guards and people, who monitor the quarantined city. Surveillance and dorms are at the top as well, so rotations can take place with little downtime.

My question is, if this were to be built, how quickly could it be made?

To add some more information. The wall is a matter of emergency, and the UK government has contracted all available construction companies to work on it, putting any government projects on hold to see it completed as quickly as possible. Funding isn't a problem and the cash needed to see this done realistically is available.

Many thanks to those who reply. If you need more information, please let me know.


After several comments, I'm edging toward Hydro dam levels of thickness. To add some more context to what it is to be contained, there are creatures within the city that are dangerous to human life. Killing them isn't a viable option as new ones quickly appear. They aren't from our reality so, even if the whole city were bombed, more creatures would eventually turn up. The source is within the city and building the wall would protect the rest of the country from harm.

The source of the creatures also emits an infectious airborne virus that acts similar to gamma radiation. It can travel through surfaces and skin, instead of the typical airborne method we are used to. The infection attaches itself to cells within the body and begins replicating. Within a healthy population of people, around 40% are susceptible to this form. Those infected have an incubation period varying from 1 to 4 days. They are not contagious during this stage. After this, patients will become symptomatic and begin to show signs of burn-like lesions on their skin. The Virus will continue to grow across a hosts body over a week. Lesions are infectious to touch.

It's dependent on the person etc. But during the second week, they will begin to show signs of mental and physical degradation, as the infection starts to work its way through their nervous system and brain. This happens over several days. Then, the infected individual succumbs to the virus, becoming feral and highly contagious, emitting a close proximity airborne strain of the virus. This strain allows the virus to jump to humans with a 90% infection rate. Only those who are immune are safe from its effects, although feral infected individuals will still attack them on sight.

Second update

Thank you to everyone who's answered so far. You've helped me figure out the logistics of how such a thing could be built, and by what means. Ideally, I'd like the wall to be made within a year, or at least completed to a stage that's containing the infection and creatures within.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Aug 9 '19 at 15:21
  • 2
    $\begingroup$ For reference: a wall around the current perimeter of Milton Keynes would need to be about 30 miles long (assuming you include most of the Unitary Authority, such as Bletchley and Wolverton) $\endgroup$ – Chronocidal Aug 9 '19 at 15:27
  • 3
    $\begingroup$ Note if you pick a place with better geology you can cut your construction time in half. someplace that is basically on bedrock, or someplace like New York were glaciers carved almost to the bedrock. $\endgroup$ – John Aug 9 '19 at 20:52
  • 1
    $\begingroup$ The creatures would be enough. I think you are overthinking it with the gamma-like infectious airborne virus. Restricting the problem to physical monsters would justify much better the need for the wall. The virus will probably complicate much more creating a robust story, and -as a reader- it makes it much more hard to believe. $\endgroup$ – Ángel Aug 10 '19 at 23:50
  • $\begingroup$ If a government is sufficiently empowered or motivated things can happen pretty fast. China makes a great comparison point, since they routinely ignore public opinion in the same way that a democratic government might in an emergency. $\endgroup$ – jpaugh Aug 11 '19 at 8:22

12 Answers 12


4-5 years assuming near infinite money.

You are looking at building another Three Gorges Dam. The Three Gorges Dam is a concrete gravity dam which is about as close as you can get to your wall (see clay below). Now the Three Gorges dam is almost twice as tall as you need, but your wall will be 16 times as long. Your wall is around 34-38km long, so saying it is comparable to building another Three Gorges Dam is probably underestimating even accounting for far better site access. It is more like building 4 or 5 of them.

Where to get the material? Mobilizing materials is going to be difficult: The Three Gorges Dam consumed a huge portion of the globe's concrete and steel production. This is not like building a building: This is like building a city from nothing. Getting the material is a bigger limit than construction. It is comparable to war-time material mobilization. The UK only produces 20,000 cubic yards of concrete in a year. You need orders of magnitude more. For comparison, the Three Gorges Dam needed 37 million cubic yards of concrete and your wall is several times larger. You need about 1/60th of global concrete production for a year on a generous estimate, so I hope you have deep pockets.

It took 17 years to build the dam, but a lot of that was for concerns your wall will not have. Your wall will not take this long. The Hoover Dam took 4 years to build (the dam itself anyway). See L. Dutch's great answer for why you can't just pour concrete all at once. 2-3 years is probably much closer to what you are looking at. You have to clear and prepare the land and mobilize the materials, none of which is easy. You still have a river you need to build across as well, and no matter how you do that it will take time. This is assuming you have portions of the wall being built in parallel, and this will take years even if you utilize every able-bodied adult on the island as workers.

Your real problem is that the ground is awful. Much of the city is on clay which is among the worst materials to build large structures on. This wall is going to have to be wide to not topple over on such soft ground. They will spend a lot of time preparing the land and it require building a very wide deep base, so a dam is actually be a good model shape. You have to clear the land, then excavate a gigantic hole to pour a massive slab and sink billions of pilings in just to support the load. This is a big construction, and it will be subjected to huge loads just due to size. Hell, the wind alone on such a structure is going to subject it to massive loading; build it wrong and you will need to build it over again. Near the river you are going to have to sink hundreds of pilings and effectively build a dam across the river.

So 2 years or more preparing the land and 2 years or more building the wall. If everything goes perfectly, 4 years.

  • 1
    $\begingroup$ as you mentioned it doesn't have to be as tall, but also it doesn't have to withstand as much pressure, which would allow it to be made out of less concrete. $\endgroup$ – meaninglessname Aug 9 '19 at 6:08
  • 4
    $\begingroup$ @wpokdljnlnmn actually it needs a lot more concrete because it is 16 times longer and also needs to be wider. Most of the pressure a gravity dam withstands is the weight of the dam itself. $\endgroup$ – John Aug 9 '19 at 13:32
  • 1
    $\begingroup$ Is the OP attached to Milton Keynes? Is there another city of comparable size where the local geology would be more amenable to rapid wall-building? $\endgroup$ – Michael Seifert Aug 9 '19 at 19:10
  • 2
    $\begingroup$ @MichaelSeifert not nearby there is a lot of clay and chalk in england. $\endgroup$ – John Aug 9 '19 at 20:49
  • 1
    $\begingroup$ Getting to use a big share of global concrete production for a year may not be that hard if the rest of the world benefits from your containment of those creatures. Sadly, I'm sure some countries would consider it a "British problem", and think that it doesn't affect them (it's specially unlucky that it's an island) and try to profit themselves from their need instead. It makes for interesting context on the book, though. $\endgroup$ – Ángel Aug 10 '19 at 23:37

Modular Precast

While everyone is talking about times for bridges and dams, it's not correct for a wall. A brick wall in a house is built of thousands individual bricks stacked together.

There is no reason why the wall needs to be one solid piece like a dam. Individual self locking slabs/bricks could be cast at concrete plants around the country and trucked to site and lifted into place. No mortar would be needed as the weight of each block would hold itself in place.

If the fate of the country was on the line, you could do it in a month. Most of the time would be leveling the ground and compacting for the foundations

If you wanted even faster (depending on what you want to contain) empty stacked shipping containers could do a wall at least 100 foot in a pinch or maybe even higher. Containers are designed to be stacked and they already exist. There is an estimated 43 million shipping containers in the world and enough empty containers could be shipped to the UK in a week.

  • 3
    $\begingroup$ How do you compact the earth in just a month? There's going to be a 100 meter high wall on it. I'm not even certain if you can build that on compacted earth, or whether you have to strip the top soil down to bedrock. $\endgroup$ – MSalters Aug 9 '19 at 8:49
  • 1
    $\begingroup$ @MSalters the whle thing is going to roll along - so someone compacts the earth, and someone else comes along and slaps concrete slabs down behind him, continue until you get back to where you started. I think 300ft walls are overkill, but also will last in a temporary way for a while, even if they later find sections are tilting and collapsing - but that kind of storyline is what sequels are for! $\endgroup$ – gbjbaanb Aug 9 '19 at 12:20
  • 3
    $\begingroup$ except the wall is 300ft tall. precast concrete sections are tiny by comparison, so precasting them does not really save you any time, better to truck the raw materials and cast on site remember these need to be precisions cast or it will not work. Road construction is a really poor comparison the load a road takes is miniscule a 300ft tall building will sink into clay No matter how much you compact it. You are talking about a wall twice the height of the tower of pisa. $\endgroup$ – John Aug 9 '19 at 13:49
  • 1
    $\begingroup$ Shipping Containers are only designed to stack up to 6 high, which is the equivalent of 51 feet -- possible to reenforce them to support more weight, but that would only add a little height. It is not a long term solution, but might work as a temporary structure while a more perm solution is being built. The fact that they stack vertical shouldn't matter as long as you connect (weld or bracket) the side-by-side ones. Disadvantage... enough grip on them that they might be climbable. $\endgroup$ – Phil M Aug 9 '19 at 21:26
  • 1
    $\begingroup$ Shipping containers are designed to be stacked six high fully loaded. Cargo ships stack them twelve high with lightest at the top. Empty containers could be stacked higher. $\endgroup$ – Thorne Aug 10 '19 at 12:59

The closest real life example I can find is the Hoover dam

Height 726.4 ft (221.4 m)

Width (crest) 45 ft (14 m)

Width (base) 660 ft (200 m)

It is made of concrete.

Since concrete heats and contracts as it cures, the potential for uneven cooling and contraction of the concrete posed a serious problem. Bureau of Reclamation engineers calculated that if the dam were to be built in a single continuous pour, the concrete would take 125 years to cool, and the resulting stresses would cause the dam to crack and crumble. Instead, the ground where the dam would rise was marked with rectangles, and concrete blocks in columns were poured, some as large as 50 ft square (15 m) and 5 feet (1.5 m) high. [...]

Each five-foot form contained a set of 1-inch (25 mm) steel pipes; cool riverwater would be poured through the pipes, followed by ice-cold water from a refrigeration plant. When an individual block had cured and had stopped contracting, the pipes were filled with grout. [...]

The first concrete was poured into the dam on June 6, 1933. Concrete pouring ceased on May 29, 1935

As you can see, the main problem is not the logistic of having the material in place, but dissipate the heat from the curing concrete.

Since you don't mention the thickness it's hard to make calculations. But the above should give you a rule of thumb.

  • $\begingroup$ I'm not sure that the curing issue would occur in his scenario though, since that was a feature of the thickness of the dam which was a feature of having to resist ~45,000 PSI of pressure at the base of the dam. Unless he's trying to stop Godzilla or something, that might be overkill here. $\endgroup$ – Morris The Cat Aug 8 '19 at 17:11
  • 15
    $\begingroup$ @MorrisTheCat, I don't know what's inside that wall. If it needs a 300 feet tall wall, it's not a canary. $\endgroup$ – L.Dutch - Reinstate Monica Aug 8 '19 at 17:12
  • 2
    $\begingroup$ The Hoover dam is not completely cured yet. $\endgroup$ – Simon Richter Aug 9 '19 at 9:11
  • $\begingroup$ @MorrisTheCat The wall is containing infected humans who've become feral and dangerous, while also containing creatures that have come out of a rift from another reality. The creatures vary in size, strength and ability (with new ones turning up all the time). The wall was built to keep any of them in, assuming they try and break out. There are weapon platforms on the wall to attack anything large enough to cause considerable damage (were it to get close). $\endgroup$ – Meterious Aug 9 '19 at 10:56
  • 1
    $\begingroup$ Hover dam is built on bed rock, which skips the hard part: substrate capable of supporting 150 pounds per square foot of a 300ft tall wall. That's 45,000 pounds per square foot print. That's 22.5 tons per sq. ft. That's like... a lot. $\endgroup$ – Mazura Aug 10 '19 at 3:21

Very quickly!

There are approximately 315,000 construction firms in the UK with almost 3 million workers. You're on, basically, a wartime mobilisation of at least this sector of the economy (not to mention the food truck and port-a-john industries!).

The city's area is 89 km2 with a perimeter of 356 km (assuming the firm of B.S. Johnson LLC have done the maths right!)

The proposal for your Great Wall of Milton Keynes is to not pour a conventional dam but rather to build and modify a standard car park all the way round the city's perimeter.

This firm can build a pre-fabricated car park of about 1/3 the required height in 9 to 12 months. Their car park is about 20 m deep, and with recommended buttressing can easily rise to the approximately 30 storey / 300 foot requirement. The structure will not require full parking decks, but some kind of access ramps and ring roads will be required for maintenance and observation of whatever's inside. Also, emergency facilities, lifts, sprinkler systems and so forth won't be required, so finishing touches should not take any extra time. Electric & surveilance equipment can be installed while the structure is being pieced together. The only alteration to the basic car park framework will be reinforced concrete slabs to fill in the usually open windows.

Each "job" should therefore take a little less than the nine months suggested, where a "job" is a 100 m (10 storey) by 20m deep by 20 m wide car park with maintenance & access roads and necessary electric / electronic systems installed. Each segment therefore comprises 3 "jobs".

(35600 m perimeter / 20 m width) x 3 = 5340 jobs

315,000 construction firms doing the work, approximately 60 firms per segment, could get your job done within the one year figure. Once the actual perimeter is determined, and the affected land owners are turned out, the first crews will engage in a massive bulldoze operation: a 50 m swath around the city will be levelled of all buildings; gas, water, sewer, electrics will all by cut to the city area and the ground will be filled, tamped and levelled for foundation work.

During this phase, pre-fab concrete parts will be manufactured to a standard shape: upright segments, horizontal segments, decking and ramp segments plus the fill-in segments. (If you require observation stations, extra heavy duty plexiglass within a reinforced concrete frame can be supplied. Emergency access stairs (so MOD can send up or withdraw personnel) will also be manufactured at this time of standard steel parts.

As the foundation work is completed, other crews will begin trucking in all the prefab parts to staging areas.

As the parts arrive in their staging areas, third wave of crews will begin assembling the car parks. As one segment is finished, those crews will simply leap-frog ahead of any other segments in process and begin afresh at the next available segment.

As preparation and foundation crews become available, these will circle around and either be assigned to augment building crews or else will work on whatever outer perimeter work needs doing: access roads, rubble clearance, fencing, gates, guard stations, etc.

  • 6
    $\begingroup$ This; by the numbers. No real world example exists that has ever even come close to tapping that kind of capacity. My question would be how many extra pile drivers would they have to order and what's the time frame on that? You can have concrete block shipped to you from all over the world, but it is only on-site that you can drive piles to support 300ft of concrete. $\endgroup$ – Mazura Aug 9 '19 at 3:46
  • 1
    $\begingroup$ If the city is 89 km2 then how can be the perimeter of 356 km? A 10km x 10km square is already 100km2 with a perimeter of 40km. Is the city "longish" like 170km x 8km? $\endgroup$ – user9 Aug 9 '19 at 7:15
  • 5
    $\begingroup$ 89 square kilometres is not 89 kilometres square, which you’re assuming. In fact, it’s exactly 1/89 as much. So as another commenter has said, your perimeter will be more like 40 or 50km. $\endgroup$ – Mike Scott Aug 9 '19 at 9:18
  • 1
    $\begingroup$ 9-12 months on already prepared ground, you are going to need to excavate huge amounts of earth and pour a gigantic slap just to support the weight. $\endgroup$ – John Aug 9 '19 at 13:57
  • 4
    $\begingroup$ This is a delusional timeline. Getting enough raw materials on site alone is going to be a nightmare. Then the people on site need food, sleeping quarters, and, above all else, security from what appears to be a nigh-on magical radiation-plague thing, and the infectious monsters it creates. Which of those 3 million construction workers (not all of whom know how to work with concrete, btw), do you think will volunteer to go work under rushed, unsafe conditions, do you think? Plus, you're brushing away the fact that foundations need time to settle, concrete to dry, etc. Think decades. $\endgroup$ – AndreiROM Aug 9 '19 at 14:21

Consider how quick the Berlin Wall went up, but also how long it took to replace the initial, provisional wall with the permanent fortifications.

Building a 5 metre or even 10 metre wall is well within the experience of commercial building companies. Building a 100 metre tower would be something unusual, the job of specialists. Building a 100 metre high and mile-long wall would be more unusual yet.

Also compare the Chernobyl containment systems. The radiation conditions made a difference, of course, but your wall is guarding against something nasty as well, right?

For that reason I would expect the wall to take years.

  • $\begingroup$ The "something nasty" in Chernobyl was not highly mobile, whereas it seems like some kind of animal is at play here; although I think the OP might have added that detail after your post. $\endgroup$ – jpaugh Aug 11 '19 at 8:44

People compare to the Hoover Dam or the Three Gorges Dam. These are maybe not reasonable, depending on what has to be kept in. These dams are built to withstand a large column of water. A column that can possibly move from all the way empty to all the way full over difficult to predict times. And in situations in which a failure could possibly kill large numbers of people and cost $100s of millions of damage. And where the dams are designed to have projected life expectancy in centuries. And these structures are built in a river gorge with lots of possibility for failure.

300 feet is roughly 30 stories. Lots of office towers and apartment blocks this tall. If the equivalent of an office tower with no windows, solid concrete all the way, is enough, it tells you how long each segment takes. Such buildings go up in 2 to 3 years providing there's adequate ground for a foundation. Since they'd all be the same, and have no decoration or artifice, you could make a factory to build the very similar components.

Considering that you don't need any plumbing, electricity, or air ventilation, probably not even stair wells, they'd be relatively quick to build. No windows, no balconies, no parking garage, no landscaping. There are construction companies in China that put up 30 floor apartment blocks at a pace of a floor a day. The interior stuff goes in after the first few floors are constructed. And they build groups of 50 of them at the same time. I should think the equivalent of 50 apartment buildings would be able to reach 1 km. That's only 20 meters each.

If it's necessary to dig very deep to get a foundation it might take a lot more effort.

So I'd say, if you got 90 construction companies from China, you'd probably be able to put the whole thing up in 3 years. It would be ugly as hell. And it would be hard as hell on the local environment.

  • $\begingroup$ A building is mostly composed of air, which means the load is much, much less on the bottom than it would be if it were required to hold up solid sections. As a comparison, each tower at the World Trade Center weighed about 450,000 tonnes, in a tower over 400 meters high and about 64 meters on each side. Using the same footprint, 450,000 tonnes of concrete at 2.4 tonnes per cubic meter gives you a building less than 46 meters high. The difference between the two situations is left as an exercise to the reader. $\endgroup$ – Keith Morrison Aug 9 '19 at 21:42

For that size, I think you should forget about high walls, and go with a retaining wall instead:

simplified typical retaining walls

In a nutshell, you don't build the whole thing out of concrete - instead, you prepare foundations and then move earth around.

Start by conscripting all available piling rigs ...

pile driver

...and lay down cylinders of reinforced concrete as foundations. Current piling rigs can do up to 1.5m diameter and 90m deep, which should do just fine for your scenario. Make sure to leave some rebar at ground level, so that another gang can pour concrete on top of the foundations and start laying prefab concrete slabs on top. Lay anchors as well, to be covered in the next step.

Right behind, have digger rigs and a truck convoy move earth from the lower-to-be part of the wall to the higher-to-be part of the wall. Maybe you want to have a purpose-built bucket-weel excavator that can crawl along the safe side of the wall during construction and reach down to the lower side.

bucket-wheel excavator

The end result should be that the lower side is 50m deeper than the original ground level, and the higher side is 50 meter higher than the original ground level.

It should look like an ugly-as-heck soldier pile wall at the lower half, and prefab at the upper half, a bit like this:

pile wall

I kinda expect the final wall to be tilted about 10° instead of vertical, to better support the load.

Oh, and make sure to ask the geophysical engineers about water drainage. And do whatever they say in regards to geology (kinds of earth/rock/clay/sand underground) and topography (you want to build this in a slope anyway).

All this is fine, but how long would it take?

My rough estimate is that an engineering project like this should take two to five times the effort of laying down a subway line that long. I'll assume that you can pool all geophysical engineers for this, put on hold any construction projects, and do it in the same time than a subway line.

Funnily enough, I can take the Line 12 of Madrid's subway system as a reference. It's 41 km long, and it took 4 years and 1640 million € (41 millon €/km).

So I'll guesstimate a 40-km-long retaining wall at 4 years, and 10.000 million €. That's about 1km per month (total), building several segments in parallel.

The bottleneck here IMHO is gonna be the purpose-built heavy machinery. I guess britain would send France a request for piling rigs?

  • 1
    $\begingroup$ The world's tallest retaining wall is about 74 feet. geoprac.net/2008/01/north-americas-tallest-mse-retaining-wall Going four times as high probably isn't impossible. But probably isn't worth it either. $\endgroup$ – puppetsock Aug 9 '19 at 13:25
  • $\begingroup$ Not to mention that the builders would, presumably, be under constant attack by the creatures in the city, not to mention working with the fear of the "magical radiation" turning them into monsters themselves. $\endgroup$ – AndreiROM Aug 9 '19 at 14:32
  • $\begingroup$ @puppetsock That's a MSE wall, but I'm talking about soldier pile walls, which can get higher. $\endgroup$ – IvanSanchez Aug 9 '19 at 15:22
  • 1
    $\begingroup$ @AndreiROM That's an issue for most of the other answers here, though. $\endgroup$ – IvanSanchez Aug 9 '19 at 15:22
  • $\begingroup$ Citation needed for "1.5m diameter and 90m deep, which should do just fine for your scenario." ... but they didn't give us a width, so it's kinda moot. Screw concrete; I want the pile driving contract because every one of them on the planet is about to be shipped to the UK. Also, +1 for drainage, being tilted, and every other brass tack you listed, such as how to rebar dis-continuous pours. The rebar contract is probably pretty lucrative too. $\endgroup$ – Mazura Aug 10 '19 at 1:08

If you have skilled laborers guiding the work, and if you have enough people to work on the entire wall at once (rather than doing it in parts), and if you have all your needed materials readily available... I think you could do it pretty quickly. Concrete takes 28 days to cure, so it's conceivable that you could make a 100ft wall in a month, and then add a month for every subsequent 100ft, and add an extra month for trussing &c. Here are some options for construction:

  1. Co-planar Layer Separation: Make several large (w,l,h) 100'x100'x5' frames, lay them with rebar along the long-axes, and pour them full of concrete. Leave holes in them to run I-beams and brackets through various places for support. When the slabs are dry, you simply pull them upright and support them. You could conceivably make several layers of concrete wall, as thick as you want, as long as you have enough space for to make the frames and let them dry. You could potentially make the frames much taller and try to do 300ft all at once, but the bigger you make these, the more difficult it will be to raise them up without breaking them. 100ft is already going to be difficult. Once you've done the first 100ft, you can do the same thing for the next 100ft if you want, but it would be easier to use smaller frames for the higher layers, and just crane them up and bracket them onto the top of the wall to add height.
  2. Stacked Layer Separation: Make several 5-10ft tall frames for the whole base of the wall and slowly pour it from the ground up. You have to make sure you run water pipes through the wall so that they don't get too hot while they dry. Concrete gives off a lot of heat while drying, and it can cause all kinds of problems. You'll want to pour only 5-10ft at a time, and allow at least a week to cure before adding the next 5-10ft frame. I think your practical limitation will be the number of concrete pouring machines you have available. This method will be slower than the first, but you'll be layering from the bottom up, which is gives you more control over the shape, and will probably mean a better wall.

So, anyway, if everyone in this city is on-board, and the materials are readily available, and you've got plenty of concrete pouring machines, and all the framing material you could ever want, so you can do the whole circumference of the city simultaneously, I say it's going to take you a year. You could put a more aggressive timeline at 4 months, and sacrifice the safety of your builders and/or the quality of your wall.

  • 1
    $\begingroup$ The time it takes Concrete to cure is very dependant on how much of it there is. The Hoover Dam, for example, is still curing. $\endgroup$ – Michael Richardson Aug 9 '19 at 21:11
  • $\begingroup$ @MichaelRichardson Strictly speaking, concrete never fully cures. It's got like a half-life. For a normal 5ft tall concrete pour, it's my understanding that it cures to about 75% strength in a week, and is considered structurally stable after 28 days. $\endgroup$ – boxcartenant Aug 14 '19 at 18:24

If you're really pressed for time, you could maybe pull off a Volcano (1997)* move and blow up some tall buildings in such a way as to surround the city with toppled over skyscrapers. (I know, unlike L.A., modern Milton Keynes doesn't seem to have any suitably tall buildings for this, but if you future it up a bit, you could maybe imagine that such exist. Or you could move it to a bigger city if you want this idea to work better.) Any kind of new construction is going to take a lot of time and work, but a little creative destruction can get the job done in a matter of hours. Granted, it won't be a perfect, seamless wall, but you can use that as the basis to form a more permanent barrier.

*(Yes, I know. Terrible movie. Not realistic in any fashion. Doesn't mean you can't steal ideas from it...)


For a rough estimate, we'll assume a 2 meter wide walkway at the top. So let's assume we build a 2 meter thick wall 100 meters high. Since speed of construction and quantity is the most important, you can't assume it will be the very best high-strength concrete, so we'll go with 50 MPa structural grade concrete. That means the concrete will start failing if the pressure on it gets beyond 50 million Pascals (50 million Newtons per square meter). That means the high-strength concrete for our wall will support a maximum of 5100 tonnes per square meter.

At roughly 2.4 tonnes per cubic meter, a column 1 square meter in area 100 meters high is going to mass 240 tonnes. A solid concrete wall could therefore support itself if vertical stress was all you were worried about.

Now we come to practicality: At 38 kilometers long, the total amount of concrete, assuming it's 2 meters thick, will be 7,600,000 cubic meters, or 18,240,000 tonnes of concrete. Concrete can be 15 percent cement, so 2.7 million tonnes are needed. That's doable, the UK's annual production is 9.7 million tonnes.

An average concrete truck is 6.1 cubic meters, so, say 1.25 million truckloads.

Now this doesn't take into account foundations, engineering for crossing rivers (well, diverting rivers), materials necessary to erect the wall, material for reinforcement, and so on. But if money were absolutely no object, a few years is certainly possible. But the cost would probably be heading toward hundreds of billions of dollars.

Note that this is something of a variation of the Great Pyramid question: why don't people build the Great Pyramid of Giza today? The answer is, of course, it could be, and built much faster than the Egyptians managed, but no one sees the point of doing so.


Pretty quickly. Nobody is building big walls these days, so the closest analogy I think to your question would be bridges. A few years ago a bridge in Iowa was replaced in two weeks, and the builder said they could have done it much more quickly.

This wasn't a particularly large operation and they didn't spend very much money. The answer to your question really depends on how much in the way of resources the government has to throw at the problem. Any given section of wall, say 100' wide, could probably go up in few days. At that point it's a question of how many 'teams' you could have simultaneously digging the foundations, building the frames, and pouring the concrete.

Your given area of 89km2 translates to something in the area of 30-35km of wall. Estimates for Donald Trump's border wall came in at about 3km/week. Now, that's a considerably SMALLER wall than what you're proposing, but it's also not thousands of miles long way out in the middile of Nowhere, Texas either.

So, given all that, your shortest possible time is probably no shorter than three months, and that would require a MASSIVE expenditure on the part of the government of the UK. Like, conscripting most of the construction equipment and skilled builders in the country. A more reasonable effort could be complete in more like six to nine months, so it sort of depends on how badly you want to quarantine the Red Bull F1 team.

  • 2
    $\begingroup$ A dam would be a better fit, bridges are mostly open, a 300ft wall will need to be solid and fairly reinforced. Bridges are also often built offsite in sections and moved into place. $\endgroup$ – John Aug 8 '19 at 16:46
  • $\begingroup$ @John a dam has to be built to withstand a LOT of extra force that this wall wouldn't though. It's not like there's going to be ten thousand PSI of lateral pressure against the base of the wall like you'd get with a dam that tall. $\endgroup$ – Morris The Cat Aug 8 '19 at 17:02
  • $\begingroup$ The lateral pressure on a dam is fairly inconsequential to construction cost compared ot how little concrete is used to build a bridge. $\endgroup$ – John Aug 8 '19 at 17:21
  • 1
    $\begingroup$ That "bridge" article is highly misleading - the time on-site can be very short, but that's because of pre-fabrication. That works exactly because bridges are light-weight enough to be moved. This wall isn't $\endgroup$ – MSalters Aug 9 '19 at 8:54

Nobody mentioned 3D printers. There are films in YouTube of 3D printers building walls and houses.

The only limit on the speed is how fast you can feed the printers with material, so theoretically if you have enough printers and some red hot logistics you could build it overnight.

  • 3
    $\begingroup$ Are there 3D printers that produce material that could support a structure 300 feet high? Also, what do you do about the foundations? Reinforced concrete in forms would seem to go in vastly faster and produce drastically stronger structures. $\endgroup$ – puppetsock Aug 9 '19 at 13:21
  • $\begingroup$ Well, this is in the future, so hand-wavium. If the hole for the foundations were already dug, print them too. I'd go for machines injecting foundation material straight into the ground. Well, like pile-driving machines do, but bigger and more of them. $\endgroup$ – RedSonja Aug 9 '19 at 13:24
  • 1
    $\begingroup$ To paraphrase Arthur C Clark, you could make it out of marshmallows if you made it wide enough. $\endgroup$ – Michael Richardson Aug 9 '19 at 21:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for?Browse other questions tagged or ask your own question.