We're considering installing a 2000G cistern. But, I'm concerned as to how the area *above* the cistern will be impacted. E.g., I'm sure we won't be able to park a *car* over that area as the cistern would have compromised the ability of the "soil" to support that concentrated mass.
The question boils down to how *usable* that area above the cistern will be going forward: walking on it, small groups of people (party-goers) *standing* on it, etc.
I assume the actual access port will be relatively easy to protect?
What is the nature of your planned cistern? There are cisterns all over San Francisco in the middle of intersections with traffic running over them. Ditto in Europe.
Grease interceptors are a form of cistern, and are located in parking lots behind restaurants, with traffic and parking over them.
Given this is alt.HOME.repair, I wouldn't assume I was looking to install something in a roadway! :>
... or commercial establishment! The fact that I'm concerned about supporting any significant mass suggests I'm not keen on building a STRUCTURE to support such a mass.
We want to store irrigation (non potable) water. We don't want a big, ugly
250 cu ft container (plastic, stone, etc.) sitting out for all to see (and wasting space in the yard).
We don't want to have to convert the area above it to a "patio" just to carry any pedestrian loads, either. Ideally, we want to bury it (them?) and not have them visibly noticeable *or* have to cordon off the area telling friends "Don't walk over there because you'll fall *into* the cistern."
If you buy a cistern from a reputable source, they will have reliable documented engineering answers for you.
If you buy from McChinaHarborFrightBigBoxSupermart, maybe you should offer Coast Guard approved life vests to your party-guests when they go near your new cistern.
For their 1000 gallon tank, they say it can withstand 400 psf.
It can be buried up to 28" deep, so let's say two feet.
One cu. ft. of soil weighs about 78 pounds, so 156 psf for two feet of dirt fill.
400 psf - 156 psf = 244 psf.
Assuming a big guy weighs 250 pounds and his footprint is 1 sq. ft., that guy is right about at the limit. But the tank surely is overengineered for the 400 psf specification, the tank is likely to not be empty, and there is lateral spread of the guy's weight over the 2 foot depth to the tank. And the big guy is not going to be doing jumping jacks (little people may, and no problem). More people - not a problem, as there is a bigger footprint for their weight (more feet).
I don't see a problem, but you might put heavier foot traffic off to the side, and use the cistern area as an ancillary area. But kids or people walking on it, etc. should not be a problem.
I don't know the price difference between plastic and concrete, or maintenance/lifespan comparisons, but obviously the concrete would be more robust.
Oh, maybe you don't want the local cheerleading squad to be practicing their human pyramids over the plastic cistern area.
Soil weight varies with composition. The soil *removed* from the ground is typically over 100 pounds per cubic foot (high clay content). Replacing it with a sandier mix can make things even worse.
The tank is likely to be BONE DRY for 10 months out of the year.
Ever have a scrimmage in your yard? Or, are you a couch potato? What sort of impact that 250 pound guy make when another 250 pound guy pounces on him and they both fall to the ground (the "load" isn't evenly distributed over the length of their bodies)? Or, the next guy who "piles on" just for fun?
What if a picnic bench is positioned there? Four guys sitting on it. All that weight conveyed to the four narrow bench legs.
Or, a swing set with its *dynamic* load conveyed to four ~3" poles that support it?
Go with a concrete cistern. You value your sleep, don't you.
Seriously, if this is to be put on a small parcel where the cistern area is to receive heavy use, buy yourself the peace of mind now and over the longer term.
A 2000G prefab concrete tank weighs more than 10T. A plastic one weighs ~600lbs. Have you considered the shipping costs for the two options? And, the crane that would be required to lift the concrete tank into the back yard?
Alternatively, you could build one on-site -- a lot more time and labor, inspections, etc.
I don't know where you are (hopefully quite distant), but my experience has been to call up, have a precast Jensen unit brought over to the hole that is ready and waiting, and they drop it in with their crane that is mounted on their delivery truck. No fuss no muss. I don't know the cost comparison (figure it out per year of life), and I don't care about the weight - the crane handled it just fine. And I do not worry about anything - cars and trucks (yes, TRUCKS!) drive and park over it. Done.
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