How quickly could a country build a tall concrete wall around a city?

Question

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.

Update:

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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

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 ...

...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.

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:

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).

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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?

Answer 8

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.

Answer 9

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...)

Answer 10

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.

Answer 11

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.

Answer 12

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.