Underground tanks with a manhole in Arizona is a bad idea.
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Arizona has a law that regulates "all" in-ground tanks. Even in ground water tanks are inspected and licensed. The government money mill gets worse every year.
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To make matters worse, OSHA requires certification to maintain in-ground tanks as "enclosed areas". This means that there are precautions that must be observed and safety equipment required when entering, working in an in ground tank.
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All this means - "You had better be storing what you claim you are storing in the tank".
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The best option is a 40'x60' Tarp "Party Tent" that you just leave up.
You can dig with a simple hydraulic ditch witch...
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Lay the foundation and retaining wall.. Don't forget to lay in Pond lined as a moisture/vapor barrier - and plan for a perimeter drain tile to carry ground water away or into a collection point where it can be pumped out and away).
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Then cover it up...
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Move the tent across the yard (because you don't want to kill all your precious grass - oh yea, use white poly tarps - then you won't kill your grass because a white poly tarp not only lasts a long time, it passes enough sunlight to keep the grass growing).
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Once you get the tent moved - Build the foundation and retaining wall on the opposite side of the "initial" shelter. (remeber to lay the pond liner and groudwater collection pipe)
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Then the good part.
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Excavate for a patio (just dig the patio foundation really deep and lay a slab about 12-16 feet below grade.). Remember to put in the rebar reinforced beams and slab rebar. Lay the shelter concrete cap. Then cover it up (make sure you tamp the soil really well - you are going to lay the real patio about 2-3 feet above the concrete roof). Then lay in the rebar and footings for the patio.
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Lay (pour the concrete for) your new patio (doesn't even have to be as big as the shelter underneath, just believable).
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Make sure you installed an access that is like behind a raised planter/water fountain feature. The actual entrance can be an actual door that goes into a fountain pump room (only the pump room has a door on the other side, and for some reason the pump room is really roomy - hey its a big fountain pond, I thought I would need more room...)
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Now you can dig out the dirt between the walls and under the ceiling of the shelter.
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Dispose of the dirt in whatever way you can
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Idea: Start composting. Mix dirt with compost and offer enriched soil to neighbors. (this would go a long way towards explaining why you are coming and going with that dirt trailer). Besides, telling the neighbors that you read about a guy that made a fortune composting and making garden soil will go a long way towards convincing the neighbors that you are into some **** and mammey "get rich Scheme". They will pity your poor wife for putting up with you....
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When you get the basic shelter excavated - just put in a sand/gravel base with a water collection drain along the perimeter that drains into a sump (you may or may not need it now, or you may need it later - either way plan for it..) Now lay down pond liner under the floor slab, put in rebar and pour the slab floor...
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Now you have an underground get away - right? Not quite...
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Things you need to plan for:
1) Air supply with input air filtration. Plan and install the equipment to supply fresh air - remember that raised flower bed - flowers make nice smelling air...
2) Water supply - Pump or city water: You will need a water tank. Too big to go through the finished door - bury it while you are excavating, you're going to dig the dirt out later anyway.
3) Don't forget water filtration/purification....
4) Plumbing for fixtures (sinks/toilets/shower...).
5) Don't forget the sanitary drain. You gotta collect shower water (gray water - can be reused to water plants, etc...) and Toilet waste. Toilet waste should go into a collection tank with a sewage lift pump (below the floor, with an air tight access plate to clean/service the sewage pump). The sewage will have to be pumped out, and since you cannot water your yard - you will have to pump it out and into the public sewer. Note: You can put in a septic system for shelter use only, but people get really ancey about neighbors having septic systems, and if not done right can become problematic and revealing really quick. The best option is to use the public sanitary sewer and put an emergency septic system in just in case the sanitary sewer quits (which it probably will when TSHTF).
5) Generator (this needs it's own room/shelter. I would also suggest that the generator shelter be a separate shelter with at least 2 feet of sand and gravel/soil between the generator shelter and main shelter walls. Generators are noisy while running, even if you only run it two to three hours a day - it is still noisy two or three hours a day.) Plan for the Generator Exhaust. In fact, make sure you put in a BBQ pit on your new patio - Pipe the generator exhaust into the BBQ Exhaust (I like that word) Chimney. When you install the generator get one with a good/decent muffler. then add a second muffler that is 3 times as big to muffle the generator to a whisper (as heard at the BBQ Patio). You can make a muffler with a couple of 55 Gallon drums, some sheet metal, a lot of steel wool and some exhaust pipe (a welder would be a necessity - ya can't bolt everything together, and duct tape, no matter how much I/you like it - just won't work here).
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Now all you need is your Emergency Stash, some cots/beds and furniture. Don't forget that arcade game (I suggest a centipede, Gallaga/Mrs Pac Man and Mr Do - wanna get crazy, include a Dig Dug game. Hey, ya gotta have entertainment. Or, Just get a couple of decks of cards...).
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The only problem here is that you are doing this in "Secret" that means you can't really brag about it or show your best buddies what you have done... Dah,,,, Best buddies now - later snitching you out to the code nazis....
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Good Luck...
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Whether you build in secret or not - Good Luck.
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If you need any help or assistance - I'm available. I don't care about your shelter, I just want you to be safe. I have tons of resources, literally for anything you may or may not need, but want.
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Want help? eMail me....
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Dave
Phoenix, AZ