Keeping it in the ground might be a problem if the soil ever gets to wet. I have seen empty tanks float out of the ground.

 

This is gonna be the main problem. You're essentially burying a boat. If it's full of water (or sewage) it sinks. If it's full of air, it floats.

 

Concrete septic tanks cost about the same money as plastic. You don't have to worry about driving over them, and they sure ain't gonna float. The problem is they have a wall across them about 2/3 of the height of the tank. Pretty inconveniant. Maybe you could talk a builder into making you one without the wall.

The plastic cisterns I have installed also want you to fill them with water as you backfill around them, so they don't cave in from the pressure. After they have been backfilled it is OK to empty them, but you would have to deal with getting 1500 gallons of water out of the tank and out of your yard.

I have done both plastic and concrete cisterns and septic tanks. The concrete septic tanks I used as cisterns leaked till I sealed them. I am thinking about the same thing for a cellar, I haven't decided which I will use, probably concrete if I can get someone to make one without the interior wall. There is no possibility of a plastic tank floating where I live.

 

The tanks I have seen float were concrete. Concrete as it cures also can cause a co2 buildup in the tank creating a confined space entry problem.

 

Once you have it in the ground...Fill it up! Put in shelving - if you need weight use cinderblocks and 2x4 wood. Don't let it sit empty during the rainy season - for that matter don't install it then either...

 

How would you seal a doorway to the plastic tank? Also it would be difficult to mount shelves in the plastic tanks I've seen because there corrugated. IMHO blocks would be better.

 

There are advantages and disadvantages to each of the choices discussed. There are also differences in construction of each by different companies, and for different applications.

Some polymer tanks are divided, and some concrete tanks are not divided.

For the situation in the OP, I agree that the polymer tank would be the right choice under the circumstances stated. I am not sure using a manhole entrance for a root cellar is the best choice. However, having one near one end, the end that is hardest to get to or happens to require deeper cover. At the other end, or possibly in one side or the other, I would suggest using a step and ramp entrance.

For initial installation, burying the entire thing so it is 'finished' might be a good idea, and installing the additional entrance later when more money and time is available. If it has to have a manhole entrance, I would strongly suggest installing a hoisting system for the goods. Either inside and extend it up to use, or outside and mobile so it can it be kept out of sight unless in use.

Either way, especially if the alternate entrance is cut in, I would suggest bracing the tank side to side, for sure, and depending on the depth of overhead cover, top to bottom. Run a 2x4 or 2x6 along the sides and on the bottom, with one ready for the top. Stand up 2x4 or 2x6s and place the top 2x in place. Drive wedges to lock them in place.

Then you can support the side-to-side braces on the verticals and wedge the side braces into place.

Once shelving is installed and things are going inside, the bracing can be changed so it is part of the shelving structure and out of the way. But without being filled with liquid almost any tank, no matter of material, if not in a solid rock vault, will need bracing. And if there is any chance at all of water entering the hole after the tank is put in the hole, before it goes into use, some type of weight has to be inside.

The tank WILL float out if water enters the hole. Even two inches of clearance between the hole wall and the tank is more than enough for water saturated dirt to float a tank out. I have personally seen sever sewer pumping stations float out of the ground.

One of these was only five feet wide, seven feet long, and five feet deep, with a foot of cover earth, everything packed in well, and the backhoe parked on top it. We told the guy it was going to float out, since it was going to rain.

He literally told us that he was a lot smarter than we were, and we did not know what we were talking about. The next morning he went to finish up the installation and found his backhoe on its side, and a bit more than two feet of the tank above ground level.

I think it is a great plan, and hope you get to do it. But do consider a better entrance, consider bracing the tank, and consider weighting it down until in full use.

Just my opinion.

 

I can buy a brand-new 1500-gallon polyethylene septic tank for $1500.

I've been pricing large water tanks lately for my own project and that seems very expensive. Right now I'm seeing 1000 gallon tanks for around $600 here at a local farm/industrial supply store.

In any case, something else to consider, is an above grade, super insulated building. I built what is essentially an above ground rootcellar which stays above freezing in the winter, and no more than 65-70 in the summer. 2x8 walls and door. 2x10 ceiling, and shaded by a storage building built over it.

The ground is basically always 55º so during the summer, the exposed concrete floor is always sucking away heat, and in the winter, its always radiating it. Not as good as real root cellar of course but pretty cheap and easy in comparison.

 

How we built one of our big walk in BOL root cellars was we used an excavator to dig a long trench into a sidehill. Then built the equivalent of a free standing 8 ft wide 40 ft long timbered tunnel in the trench. Then, covered it with 45 Mil EDPM pond liner. Then covered the pond liner with 4x8ft X 1 inch thick blue rigid foam insulation sheet boards & tarps over that.

Then backfilled the top & sides with the trench excavated fill. Then built a lagged post & beam front entrance & thick insulated front door & masonry steps. Once built/sealed we laid down landing mats for a floor.

Its stood the test of time well over a decade.

Lagging (our logs, cut at a small local gypo mill)

Landing mats

 

Above ground root celler

I once helped build an above ground root celler out of straw bales. It was built directly on the ground with a dirt floor. We plastered the walls inside and out and built the roof system one bale thick between celling and floor.

This was built in the early nineties so I can't recall what the winter to summer temp spread was, but do remember that it was vary satisfactory.

Straw bale constrution is quite laber-intensive, so it cost about the same to buld as super-insulated standard build, but lots of fun to build.

elgin

 

Straw bale constrution is quite laber-intensive, so it cost about the same to buld as super-insulated standard build, but lots of fun to build.

Yup. I live in a straw bale house, but I build my above ground root cellar conventionally because I wanted it done faster than I wanted it cheap.

 

I finished a very long article on root cellars, plus information on several other topics relating to long-term food supplies for another board. I thought I would go ahead and put it up here in case it can help.

Just my opinions on the subjects.

 

A manhole entrance would certainly be inconvienient but would also be much easier to hide. Remember that the gvm't has given itself the power to take what you have if they decide you are a hoarder. The way the leftests are trending I could see that happening if they ever steal power back.

There is a way to tie tanks down to prevent floating by putting concrete and straps around the outside before you backfill.

You might want to get a book called the Cellaring Handbook. They have lots of plans for differing levels of expertise. I remember my dad built his septic tank out of concrete and blocks. Didn't seem like that big of a deal at the time.

 

Thanks, everybody, for all the thoughts and information.

Definitely given me something solid to chew on.

 

Sometimes they have cracked concrete units cheaper (could be patched/coated inside and out, IMHO). I have considered dropping one in off the hillside of my driveway. The other option for my root cellar is sandbag and stucco unit out back on the house flat.

I also looked at backhoe a hole in the hill above the driveway. Pour a foundation, then stack mortar rock or use the sandbags/stucco.

 

The biggest problem I see with one entrance manhole is: What do you do when I put something heavy on the cover. Now what is your plan to get out.

 

I didnt read all the replys.
Poly tanks can not be left empty. They are suposto be filled before backfilling even. You need to get a concrete tank....some of those need to be filled also. Ask a septic supplier for a tank that can be left empty. They may not even make one.

Around here anything under ground is damp and cold. Not something i like to store things in. My property has ground water about 6ft down. In the spring higher im even up on a slight hill.

You should really do some exploring before you go do this project. A root cellar may be cool and and make you feel good but whats the plan if it just ends up being a damp mold hole in the ground that cost you thousands?

 

If your water table is the problem, just build a berm root cellar. It would be a helluva lot easier...

 

You might consider a storm cellar. They have a large entrance and steps down to the bottom. Around here where I live they come in several sizes and start at about $2,500.00 installed. We've had one everywhere we have lived (tornado country) and have never heard of one floating up.

 

Crops as they sit respirate they give off co2 and water vapor a poly tank will allow the vapor to condense this will increase spoilage rates.

 

Good point... +1

 

Here in New Hampshire the local concrete casting company makes their own tanks and make underground structures for large water wells, man hole over well for pulling pump, etc, and a set of stairs set up as a bulk head. Not real cheap but wide open in design. They also make underground water tanks to meet fire protection sprinkler codes, up to about 50,000 gallons, that are cast off site. If the water table is right, they have all the equipment to make and install the structures. Friend built a structure using poured concrete walls cast in forms, and covered with sections of precast bridge decks that had failed government tests. Made a beautiful structure, but have to be in the right place at the right time. Has anyone looked into what they use to build the parking garages? That style of form would seem to make a good roof for an underground structure.

 

Parking garages usually use through posts. Not a good system for earthquakes or blast waves.

From my prep manual:
The weight of debris on a basement roof should not be much of a factor since most will be blown off and away. It is the actual blast pressure which could cause the most damage to basement roofs. Older buildings used the best system of basement roof structure consisting of a 4 to 5 inch reinforced concrete slab with a 2 to 3 inch drop panel where the round reinforced concrete columns used flared round capitals to distribute the weight. This type of construction was usually rated at one hundred pounds per square foot live load. In tests conducted under Civil Defense authorization, this type of basement roof construction began to partially fail at 12 psi overpressure. However, at this level of pressure, most of the concrete pieces were held together by the rebar with only some falling to the basement floor.

Since the 1950’s many large buildings have an eight to ten inch reinforced concrete slab ceiling, but the posts go straight through this slab, concentrating all of the force on a small area highly susceptible to sheer forces. The whole roof slab of the basement can fail at 6 to 7 psi over pressure.

Just my opinion.