Opsec would be my biggest concern. Do you have others around you that might figure out what you're doing by constantly going into your septic tank? Even if you're in a remote area, away from prying eyes, your traffic patterns on the ground would be a giveaway in a heartbeat after SHTF.

Whether or not it would pop out of the ground due to water pressure depends on where you are. Not an issue in Arizona, but a headache in Florida.

I wouldn't store anything in it that needs to be rotated out on a regular basis (like food). Otherwise, you'll start to be constantly going into it to keep it updated.

 

Sure why not, but...besides floating, if you only use an inch or two of soil over the tank, it will becomes obvious that something is buried there. The grass will be different, in color, texture, etc. Filling with rain water or ground water would be another big worry. Maybe purchase a few extra gaskets for the cover. I sup[pose you could build an insert to drop into the hole to disguise the hole to make it look as if it's full of water or worse.

 

Not a septic tank expert here, but I'll give my two cents. Dig it out 2ft deeper then get a rock drill in there and drill a 4" hole into the bedrock if applicable and use 4" pipe to come up to grade staying 1ft or so into the rock and concrete it in, then use big truck straps to hold it down. Use 3" stone on top of rock construction fabric for drainage. In my opinion you'll save cost of concrete, save space. Depending on where the rock ( bedrock) is a good driller could have it done in an hour not even. You could even implement the use of kick holes for stronger anchor points.

Another idea is use the area a little, have some firewood cut and split, make it look like there's a common core reason you're there.

My only concern would be someone feeling it under them. Two inches sounds like a good idea until 1; the rain washes it off or 2; you step on it and hear something plastic.

 

Thank you all for the great ideas! I will mull these suggestions over tonight when I wake up at 3am for my nightly hour of worrying :^)

 

Toilet wax is waterproof, sticky and works well for quite a few things.

 

I've been in some underground tanks and they become humid and the air was always stale, other then that you have a good plan and you should be able to make the area concealed.

 

I'm too old to climb in and out of that tank, especially carrying an arm load of goods. I would want stairs or a flat, ground level entrance. A hole in the top of a tank would be a no-go for me.

 

You might have a problem with condensation. Moist air and cool plastic is a bad combination. I have seen swimming pools pop out of the ground after they were pumped dry for repairs. Undetected access would be a problem. People are always watching!

 

I like your idea and have had a similar one; so here is my criticisms...
1. You need a moister absorber for the general area.
2. Consider using the septic tank as an emergency shelter, kind of an improvised bunker.
3. While you are digging stuff up, move into the tree line and plant some bushes and small trees around the hatch as camouflage.
4. Plant a ladder with the tank, makes getting in and out easier.
5. Don't pour concrete into a tank, just add lots of supplies. They only float cause they are full of air.
6. Actual size of the tank would be a good idea to figure out solutions?

 

So I'm here to poke holes in your plan in hopes you find solutions to get around them.

Yes tanks will float, cement or plastic. a 1500 Gallon tank will need roughly 19,000lb of ballast to not float, a ploy 1500 gallon is roughly 500lb or so. being roughly 11' long 4.5' wide and 5 3/4' tall. 1600 Gallon cement tank is roughly 14,000lb. So you need to be looking for land that's topographically higher then the general area to combat this issue. add some drainage around it headed to lower ground.

Shallow burial will show in how the grass and weeds grow on top of it. Consider putting it deeper and using risers to get up to the ground level making you have a much smaller area of bad growth.

Just buying the tanks could be a problem on its own, and anyone seeing them head to the locations will recognize then they are pretty distinct lol Something to look at are the poly tanks used by farmers for chemicals and water likely MUCH simpler to get without questions, and still can store a boatload of stuff.

OPSEC will severely depend on the area and how its inhabitants deal/watch the newcomers. You buy land and show up with equipment to dig holes someone will be wandering on that land when you are gone for sure, especially if you dig and work but it shows nothing of it. Give them something to look at, flatten up an area and stake it like your wanting to build something.
Now if this is an area that's fully out of sight from any other land you don't own, you might get away with it.

Personally i'd do smaller caches spread around, on my own land and elsewhere. If it gets found its a tiny part of the total not a large chunk of it. Say SHTF and the world breaks, you head out to the location to find a large group of people using your nice clearing to setup their new home. Now you are screwed, the only way you keep that stuff without issues is to end them.... Even if they haven't found the stuff, would you expect them let you in to dig up the jackpot and take it all from them? If i was that group you'd be screwed, and now that the interest in the location arose i'd be looking to find out why. My faith in most is zero when its crunch time, i expect it to be brutal.

That's just my simpleton take on it, your millage will be different I'm sure.
Cheers

 

Sounds like a very expensive, labour / machinery intensive project. Me personally, I would bury a smaller water tank and a few plastic 55 gal drums. 3-4 acres is a fair size lot but burying a few septic tanks will probably not go unnoticed. As menioned earlier moisture, condensation plus humidity gives me cause for concern.

Godspeed.