What should I consider when making walls or fortifications that can stand against zombies, bandits, and enemy militaries?

Question

So recently I’ve decided to add a wall to Washington, DC and this is where the wall (named Federal Wall) would end up encircling, with the area being within the Wall being known as the Federal Defense Perimeter, or FDP. I’ve also decided to add zombies to my world due to being recently inspired by World War Z (both the movie and novel).

So that leads me to the title of this post.

Right now here’s my plan for the Wall:

The Federal Wall will be 20 meters tall in height and have enough width/spacing to fit artillery guns like the M777 howitzer. The Wall will also have enough space and infrastructure to support and operate helicopters, several VTOL aircraft like the V-22 Osprey, etc., which will all be stationed in different sections of the Federal Wall. Because it’s impractical to deploy Troopers, weapons, aircraft, etc. to cover every single inch of the Wall, military resources, personnel, and assets are distributed along 13 major areas called Station Patrol Outposts (STATPOPs, or “Stap-Pops”), all of which are designated a specific callsign that is related to their local surrounding (e.g. the Station Patrol Outpost covering Union Station is called “Union”, the STATPOP south of the US Capitol is called “Capitol”, etc.). On the map above, they’re shown by red circles.

The Federal Wall also has several “Gates”. These Gates allow for people, goods, and resources to flow into the Perimeter in a controlled and managed flow. Movement into and out of the FDP is free and open to everyone, however, the Troopers stationed at these Gates have the right to stop and detain you if it’s believed that you’re sick or are displaying suspicious/criminal behavior. In a declared state of emergency that affects the FDP, the Gates will automatically be sealed shut and/or will only be opened for a limited time of the day before being sealed shut.

That’s as far as I’ve gotten when it comes to thinking out the doctrine and strategies that goes with the operations of the Federal Wall. Even though DC has expanded in size, my idea was for the Wall to act as a protective measure for the core government infrastructure so that there is some level of physical protection for the White House, US Capitol, Supreme Court, etc. My idea is for the Wall to also be able to stand up to zombies, bandits/marauders/etc., as well as rival, near-peer enemy militaries like the Virginia Federation Army.

So would this work or be practical? What changes would you guys suggest? Like the title says, what else should I keep in mind?

(And yes, I know that walls are out of date due to aircraft, bombs, advanced artillery, etc. But suspend your belief for a second).

Answer 1

Zombies Require Layered Defenses

In zombie movies, when a defensive line falls, the place it defended falls with it. This is because there are never any decent internal barriers in place. Zombie movies do this because zombies are stupid, and intelligently designed defenses that compartmentalize damage (so a breach in the defenses doesn't spell certain doom) would so undermine zombies' ability to wreak havoc that writers would prefer not to have to deal with finding a way to make the apocalypse happen in spite of zombies being containable. So most writers that see the problem gloss over it, and the ones who don't aren't asking questions like this one :)

A well-regarded classic put it this way (quote paraphrased to hide spoilers): "Someone forgot to close a door. That's all it took for them to get inside. Now everybody's gone."

Don't do that.

You cannot get away with just one wall. That's a single point of failure. When anything happens to your wall - and it will eventually - zombies get in and Bad Things happen. If nothing else, you need a response to 'the enemy breached the wall, then deliberately attracted a zombie horde'. You need multi-layered walls. Not every one of these walls has to be full-strength, but it will need to be strong enough to hold long enough that the Army Corp of Engineers can reinforce it before zombies push through.

To really ensure long term survival, you'll also need the ability to cut off portions of the city from one another. Bonus points if most buildings don't even have entrances or windows on the ground floor and are only accessible via retractable stairs, or walkways from other buildings, etc.

Answer 2

Different threats, different walls.

• High walls against zombie hordes.
  Zombie are generally understood to be stupid, arrive in large, lemming-style swarms, and build a ramp with their dead bodies to flow over the wall. They do not use sophisticated tools.
  Against that, you need a high concrete wall. The higher, the better. It would help if every foot of wall is covered by rifle-caliber automatic weapons (7.26mm, 5.56mm) with a high rate of fire and plenty of ammo.
• Fortified checkpoints against bandits.
  Bandits attacking such a city are probably dumb for a human being, but much smarter than zombies. They arrive in cars or trucks, they try to blend into the traffic until it is too late, they might try to fool the gatekeepers. But they are very much not suicidal. They want the loot, they don't want to die to get it. Bandits would never try to run a machine gun emplacement out of bullets with their own dead bodies.
  Against that, you need a wall or fence that is two or three times as high as a man, topped by barbed wire, and with defensible guard posts at the ground level. Think Berlin Wall, or perhaps the Green Zone. Weapons are again rifle-caliber machine guns, and perhaps heavy machine guns or automatic grenade launchers. If bandits come with Mad-Max-style armored vehicles, perhaps 20mm-40mm autocannon.
• Hidden gun bunkers in depth against artillery and tanks.
  If the attacker is a real military, any upright walls are just upright targets.
  The much-maligned Maginot Line actually worked pretty well against tanks and field artillery, it is just that the Germans went around it and attacked it from both sides. Such a fortification would still have problems against a modern army and air forces, but it would work much better than any curtain wall.
  A fortification line needs heavy, mobile counterattack forces. In WWII, the Americans had a hard time defeating Japanese island defenses, and that was with sea and air superiority. Imagine how the leathernecks would have suffered if the Japanese could have thrown armored divisions at the beaches.

Answer 3

You're in for a bad time. I've lived in Washington DC for the past fourteen years. (I won't take offense that my house is outside of your walls.) We have an aggressive system of defense. Every major federal institution has its own police department and there's an entire military base on the south side of town. The Capitol Police alone has more than half as many officers as the Metropolitan Police Department. That means you have thousands of armed professionals ready to defend. Your proposed wall would require huge resources to build. And if Virginia is an enemy, their forces can take shots at the wall from their state and hide before you can return fire. Over time, the wall would have big gaps from these attacks. It's better to pursue one of two options: either abandon the District and relocate to a more easily defensible bunker or follow a counterinsurgency approach to find and destroy zombie nests and local enemy forces.

If you want to learn more about the actual plan for keeping the government running in the end times, check out the book Raven Rock. Spoiler: the government went with the first option.

Answer 4

Bad luck on being influenced by the film; but well done for using the book.

The problem with defensive walls is that everyone thinks "Great Wall of China". There's an inside and an outside, and that's all that matters. Medieval and Renaissance fort builders knew better, and they came up with the bastion fort design, and the related star fort design. The wall is not simply an inside/outside dividing line - it's a weapons mount system, and like any weapons mount it considers fields of fire. To ensure you get that field of fire, also don't forget that you'll need to 100% clear the ground for at least the aimed range of a rifle, and ideally the aimed range of a sniper rifle. So at least 1km of totally razed buildings outside that wall.

For zombies, as a previous answer noted, the problem you have is the ramp of bodies building up. Your best solution there is to imitate World War Z - you transport your troops to a nice big open space, you lure all the local zombies there, and you kill every last one in a patient, ammunition-efficient way. ("Zack ain't in no rush, why are you?") If you do this periodically, you can ensure the wall defenders only need to take out occasional stray zombies, because the ravening horde has already been wiped out.

Sure, the wall isn't going to stand up to modern artillery. If you're going to be faced with that kind of thing though, your answer should be air power. And in practise there is no strong reason why the Virginia Federation Army should want to attack Washington, nor vice versa. Zombies are enough of a threat, and we are no longer in the Middle Ages where wars were started by kings just because they wanted a little more territory. An educated population won't stand for it. The closest we got recently was Vietnam, and we all know how that went. By all means plan for defending against minor incursions, but there's little need to plan for a full existential-threat invasion. The cost in lives would be too high, and for survivors of a zombie holocaust there wouldn't be any appetite for that.

Answer 5

Forgive me, but I don't think you want a "wall" at all in the traditional sense.

For this I am going to concentrate on zombies/near-peer enemies. Bandits that can get through the zombies are going to be about as nasty as any peer SpecOps infiltration team, and if they can't get through the zombies they're not your problem!

The first thing you should do is clear-cut and level an area at least 800 yards out. Preferably this would extend all the way out to a few miles to give enemy artillery nowhere to hide, but 800 yards would mean enemy infantry have no safe area to engage you with small arms. (Yes small arms can have ranges up to and exceeding 800 yards. No most people won't even try to hit somebody that far out. That's why the US Army's WWI Rifle could accurately hit targets at ranges of about 3x the current standard.) 2-300 yards is about as far away as most infantry will attempt to engage the enemy and have a chance of hitting somebody, 800 yards gives you a beaten zone more than twice that distance to notice an attacker and bring them down.

Next a series of ditches, 30 yards apart for the first two, couple hundred yards for the 3rd and 4th ring, along with another few hundred yards between the 4th ring and the city proper. Smooth sided and at least 20ft deep and varying width with each successive ring. (First ring maybe 8ft wide, 2nd ring 12, 3rd ring 30, final ring 17 for instance) Hard to build in DC as it's got a low water table, but still. Smooth sided to stop people from climbing out. Have the first ring or two be able to be periodically flushed with fire or acid or whatever to clean out zombie buildup. Preferably at alternate times, so at least one ring will always have some Zs to interfere with people sneaking in/assaulting.

Have retractable bridges at different points in each ring. Think the gates of Gondor, where you enter one gate, then have to ride halfway around the city to the next gate. The principle here is even IF you get caught with your pants down by zombies and have every bridge extended, you have time to demo the bridges or retract or whatever as they meander around. Same for attacking infantry. Have a couple hardpoints on the far side of each bridge so in case of zombie attack these chokepoints could be better defended. Probably useless against near-peers, but still. The ditches are variable width to force any peer enemy attempting ground attack to either bring 30ft-long bridging gear for all of them, or adjustable gear, or maybe even not realize the width difference and have unusable bridging gear for one section.

Between the 3rd and 4th rings is your defense in depth against near-peer enemies. bunkers, camouflaged artillery pits/SAM sites (preferably far more than you actually need so the enemy can't just plaster all of them and guarantee a kill) tunnel systems connecting only small groups of bunkers/arty pits together (so if one is overrun your whole ring isn't compromised). Basically take the Maginot line, crank it to 30, and make sure there's no way around it.

I would also suggest ankle/knee high razorwire belts between the 1st and 2nd trenches, and that along with claymores/other mines in belts for the 3rd and 4th rings.

Now, is such a defense impregnable to zombies? Basically. "Walls are useless without men to defend them" and all that, but it'd be a cold day in hell.

Is such a defense impregnable to a near-peer enemy? I would say tentative yes, if your artillery was equal to or superior to the enemy in range, and your SAMs could reliably down enemy bombers and fighters. Your post seems to assume DC is one polity, Virginia another. At that size such defenses, if they were fully manned, would represent to tough a nut to crack. Even the modern US military probably wouldn't want to risk it if the SAM sights were sufficiently advanced to nail F-22s or high-flying bombers before they could drop munitions. In WWII the germans went AROUND such defenses rather than through them. With no way around, you have a classic "not worth the price in blood" scenario for an attacker. You'd have to starve them out.

Of course putting all this together in a post-apoc zombie wasteland seems like a waste of time. You could probably get by with just the two trenches, the gates, and the zombie-guns. Make yourself impregnable to Zs, congratulate yourself on those defenses also being of some use against your neighbors, and move on with life. Not perfect, but also not bankrupting your economy and hopefully enough of a deterrent that the VFA would rather talk than fight.

Answer 6

The state of this FDP would depend on whether it was specifically designed to handle zombies, how much time the government had to prepare for the attack, and what minds are behind these defenses. Did the zombie outbreak first occur right next to DC and the feds said "Dang, glad we have this wall!" Or was the wall built after the zombie outbreak to prepare for their arrival?

I've never watched zombie shows, and I don't know what kind of zombie you're running. D&D zombies, you just hack 'em 'til they drop. Some zombies, I hear you have to shoot them in the head. Still others just don't die. If you're working with a virus, I think it would be interesting to see zombies you have to kill with fire or antivirus gas. Whatever the case, I expect the feds could quickly develop a point-defense turret with sensors to spot the weak point, fire a shot, and end the target. There would also be an alarm system to tell where & when the zombies arrive.

I really like Durmacar's trenches. I'd also put downward spikes on the inside on my side. That'll be an additional security measure to stop things from crawling or jumping out. If they're really into it, maybe put compactors in the trenches to smash the zombies and make room for more. You could also have triplines by the edge to make zombies more likely to fall in. You can also maybe take the line idea further by having very strong and thin cords that can cut through zombies on the way down. That'd take quite an advanced cord, but I believe it's possible.

You could maybe also develop some kind of mind-affecting gas that makes the zombies get really mad and turn on each other.

Answer 7

Got lemons? Make lemonade.

I drew you a new wall. I overlaid part of your very large old wall on top of it to orient people.

It is not actually a wall, but a deep ditch. It will be much easier to build than a wall because it is already there. You will note the ditch follows the path of the DC Metro. You will unroof the metro and walla (ditcha?): your defensive ditch!

Ditch is great against zombies. They fall in and wander around. Your guards name the main ones.

Ditch is great against bandits. Ditch is full of zombies! Which are the lemons, if you catch my drift.