The question, before you get into the rest of this textual abomination: with all of the context, objectives, and design features mentioned below, is an ultra-heavy tank a resource-efficient means of killing zombies in an CBRN (nuclear/biological/chemical/radiological contamination) environment inhospitable to human life?
Let's say that, in the near future, there's a zombie apocalypse in the vein of World War Z: perpetual-motion, fast-moving, and hyper-aggressive zombies (long story).
Important note: they form chain swarms when they hear loud noises. This is relevant later.
People have retreated, re-established their industrial and technological base, and have mostly systemically exterminated the zombies infesting their country. They came up with the concept of the resource-to-kill ratio; essentially, how do we minimize the resource and/or dollar cost of killing an individual zombie? Their solution to that was to use infantry with bolt-action rifles, as that's about as efficient as you can get without endangering your soldiers: one bullet, one kill.
However, there's a massive area of the country that was sterilized with every nuclear, biological, chemical, and radiological weapon known to mankind in an attempt to destroy the zombie horde in it. Unfortunately, it didn't destroy many of them. There are still millions of the things left.
Normally, the society in question would not want to take back such a godforsaken wasteland, but the sheer number of zombies in it means that it's a threat to anything and anyone nearby, as hordes constantly march out of it to attack the living.
Building a wall around the place and making the zombies pay for it was dismissed as a waste of resources, given the scale of such a project, and the fact that the zombies climb walls like ants.
They can't be met outside the wasteland in a resource-efficient way, either - doing so means mass civilian evacuations in the path of the horde, as well as pulling hundreds of thousands off-duty (say, building new homes, or performing mass burials, or having babies - people who would generally be rebuilding after an apocalypse) in order to meet it, as well as to operate the supply lines keeping the front line fighting.
Since these zombies are both extremely fast and extremely coordinated (for instance, look at this example of World War Z zombies climbing a wall like a horde of ants), sending in armed personnel into the zone in bulky hazmat suits, CBRN-proof vehicles, and mobile decontamination bases means that they're not mobile enough to avoid getting overrun and torn to shreds. A specific vulnerability is that suit or base breaches are guaranteed death even if the zombies don't get you; the nightmarish, radioactive chemical soup the area is saturated with will kill you soon after a suit breach.
Now, fighting the zombies from within an CBRN-sealed armored vehicle helps negate the problem of a suit or base breach: the entire thing is armored, and zombie bites and/or environmental debris will not get through it.
However, fighting the zombies from inside a CBRN-sealed armored vehicle fails due to two major and two minor factors:
These zombies pile up on you - potentially by the thousand. Even if you somehow manage to kill a shit-ton of them, your vehicle is going to be buried in their corpses. The odds are that its wheels just aren't going to get enough grip to be able to move. Now, you might negate that by replacing wheels with tracks, but those get clogged with zombie parts relatively easily. Hell, they might even be able to smother your engine.
It's hard to live inside a tank. Yes, I recognize that people have done it in the past, but if you're on a patrol in the Northeastern American Dead Zone for a month on end inside a sealed metal can with nothing but death around it, you're going to go nuts eventually. Even submariners have more room per person than there is per person inside, say, an Abrams or a Bradley; the only occupation I can think of that involves less room to move around is being an astronaut on the earliest space capsules.
You're going to run out of fuel eventually. Obstinately, you can go back to base for more, but that's only if you can avoid getting bogged down by the hordes of sprinting, dog-piling zombies - and if you run out of fuel, tough luck.
Eventually, you will run out of ammunition; there is only so much room for the stuff inside a tank or an armored personnel carrier. Again, you can technically go back for more, but, again, if you run out you're hosed, and it's not like it's easy to fit an airlock onto a tank.
Moreover, going back for fuel and ammunition limits the amount an extermination team can penetrate into this massive dead zone; even if they set up fuel dumps inside it, they are liable to be contaminated and/or overrun.
Enter my solution: the ultra-heavy tank. If you want an example of a vehicle on such a scale, look at the Nazi Landkreuzer P-1000 Ratte, or NASA's crawler-transporters. I'm not saying "build a Ratte" or "build a crawler-transporter"; I'm just providing an example of what vehicles on this scale look like.
Now, there are many glaring flaws with ultra-heavy tanks that preclude their use against human enemies:
They require truly gargantuan amounts of fuel.
They're basically a gigantic "kick me" sign for aircraft and artillery.
Their ground pressure is off the charts, and they destroy any bridges they cross over, meaning that follow-up forces can't use them.
However, in a near-future war against World War Z-style zombies set in a chemical hellscape, these problems are negated.
Replacing gasoline-powered engines with small modular reactors, batteries, and electrically-driven motors negates the fuel problem - fuel-wise, the thing can operate for years on end.
Zombies have neither aircraft nor artillery.
We'll get to my way around the ground-pressure problem eventually. Moreover, destroying bridges is not a problem, because:
~ the odds are that they're gone already
~ it's not like anyone will be using them anyway; they're in a toxic, irradiated moonscape unsuitable for human life
Being slow is not a problem, for reasons I will get into.
Moreover, such a vehicle has several advantages as part of a near-future war against World War Z-style zombies set in a chemical hellscape:
Despite its low speed, its great mass gives it incredible kinetic energy once it starts moving. It can flatten houses, fences, most land vehicles - actually, most obstacles that would impede a lesser vehicle in general, of which there are going to be a lot of post-apocalypse - and woe betide the pile of zombies that tries to climb onto it: the thing just keeps moving. It doesn't have to move quickly, like it would against a human adversary who's attempting to get bombs into it - it just needs to avoid getting piled up on, so that it can constantly bring its weapons to bear on the zombies.
Unlike a pack of lighter vehicles and semi-mobile, pre-fabricated bases, an ultra-heavy tank is one gigantic thing. There are no vulnerable airlock tubes in-between vehicles, no lengthy decontamination procedures, and no need to climb into a hazmat suit to go to the messroom, or go to the toilet, or to load new munitions into the thing: everything is already inside the vehicle. The Ratte, for instance, was designed with a motorcycle bay, an infirmary, several storage rooms, and its own self-contained lavatory system. Essentially, such massive vehicles have interior supply lines on a tiny scale.
Unlike lesser vehicles, an ultra-heavy tank is large enough to carry that nuclear reactor I mentioned previously, meaning that fuel is a non-problem for years.
It's a propaganda symbol: what's better for the spirits of a crushed and depleted people:
~ a bunch of hazmat-suited, faceless goons with rifles, in the same tanks that failed to stop the zombies the first time
~ a majestic, invincible land battleship and its brave, intrepid crew
Ammunition is less of a problem, because the square-cube law works in the favor of ammunition storage here; since the tank has more inside volume, it can carry a lot more ammunition.
The crew go insane less quickly; since this thing is, in terms of accommodations, more comparable to a submarine than a tank, longer zombie-extermination trips can be undertaken.
However, there are a number of features that could be incorporated in order to make the thing more efficient:
No armor. Zombies don't have armor-penetrating weapons, meaning armor is as much of an instance of dead weight as the zombies.
No artillery weapons. A high-explosive shell is not the most resource-efficient way of killing zombies. Instead, replace the main turret and the entire top of the hull with a superstructure; in it are armored, CBRN-sealed ports with integrated semi-automatic rifles, so that the crew can pop the heads off of zombies one at a time.
Instead of two tracks, add as many as possible - preferably, mounted next to one another, and using the space in between where the tracks would be on a conventional tank. This ensures redundancy, as well as reducing the vehicle's ground pressure. While this would make it very difficult for the vehicle to turn, turning is not really an issue, since it can be accomplished over long periods of time.
Add a plow on the front, for getting obstacles out of the way of the tracks.
Give it cheap, disposable drone scout support for finding and navigating towards large hordes of zombies.
Add mine flails between the ground and the firing ports, so that zombies attempting to climb it get mulched.
Add airlocks on the dorsal superstructure, so that cheap, expendable cargo drones can slowly but steadily resupply the vehicle's ammunition reserves.
The standard modus operandi for one of these vehicles would be:
Drone locates zombie horde.
Vehicle drives to horde to intercept it before it exits the dead zone.
Vehicle begins playing loud music, horns, whatever have you, forming a chain swarm
Chain swarm follows vehicle as vehicle leads chain swarm away from populated areas
As (4) is carried out, sharpshooters inside vehicle terminate zombies.
Eventually, they'll run out of swarms near the dead zone border, and begin pushing further into it. Other than that, no major changes in that plan, other than a return for crew change-out/repairs every 6 months.
Again, the question: with all of the context, objectives, and design features mentioned above, is an ultra-heavy tank a resource-efficient means of killing zombies in an CBRN (nuclear/biological/chemical/radiological contamination) environment inhospitable to human life?