It’s a pretty thick wall. Could you design it? Well, I certainly couldn’t. I’ve learned my share of math, I’ve been at a technical high school, I’ve even calculated simple building statics at school, but this is not simple at all. And even if I’d know the math, it would all depend on the materials.
There will certainly be steel in the wall, will it? What steel? How do you make it? How do you know that you’ve managed to create the desired quality? Where do you get the iron ore? How do you mine it? How do you even know what iron ore is? Do you know what it looks like? Do you know where to find it, how to extract it? How do you transport it and how will you create the temperatures necessary to melt it? And then? Cast iron is brittle. A smith would know how to make steel. Do you? Do you know any smith? Even if, we don’t talk about ploughs or swords, we talk about uniform bars with certain stability-enhancing profiles and lengths of tens of meters. Thousands of them, each weighing a ton, all having reliable quality. You can’t just make that with hammer and anvil 😄
We’ve only talked about a small part of necessary materials yet. What about building the wall? Where do you get the machines? How do you power them? Can you safely build such a wall without sacrificing thousands of workers? Do you have proper incentives to make workers work voluntarily and without forcing them?
Such a dam creates a lot of electricity at times when it is needed: in winter, at night. It is important to have a few of them, in order to maintain a steady supply of energy. There is a tremendous social value in having a steady supply of energy, but it is kind of an abstract goal to build a dam for a storage lake. It’s not like building one’s own house. Do you have a population, that is educated enough to strive for such shared goals? Finance them?
Even in the best of cases: constructing such a building will involve certain dangers. Some people will get hurt. Do you have the ambulance cars or helicopters to drive or fly them to the hospital? Do you even have hospitals? Medical doctors? Emergency rooms? Antibiotics?
That’s what the freedom- and gun-loving preppers don’t get: when civilizations go down, all knowledge goes down. So does technology. After the apocalypse, you can’t just rebuild.
Knowledge will be gone, skills will be gone, machines will be gone, power supply will be gone, heating will be gone and so will be cooling. Most food will be rotten in weeks.
We will lack the materials, we will lack the knowledge to use them, and we will lack the knowledge and infrastructure to re-acquire what we’ve lost. That’s why we’ll lose more, that’s why the fall will be so deep.